XVII THE EVOLUTION OF PHYSICAL CHARACTERS

1. Having discussed what is given in the germinal constitution, we are now in a position to inquire into the question of changes in the germinal constitution. Such changes may affect physical or mental characters. In this chapter we are concerned with physical characters alone. Compared with the evolution of mental characters the evolution of physical characters is a subsidiary matter; as this evolution, however, has gone hand in hand with the evolution of mental characters and furthermore possesses an importance of its own, all reference to it cannot be omitted though the subject will be treated very briefly. As a preliminary to this inquiry we may note certain facts about the selection of physical characters in general.

We know practically nothing as to the cause of mutations. We have, therefore, to take mutations for granted, and to inquire how certain stocks with particular predispositions are favoured and others eliminated. In this process both lethal selection and reproductive selection play a part. Among species in a state of nature lethal selection is more important than reproductive selection; among men the importance of reproductive selection tends to increase until in the latest period it assumes an importance approaching that of lethal selection.

Hitherto we have only referred in passing to the existence of tradition among men. Tradition will be found to form a serious complication when we come to deal with the selection of mental characters. With regard to physical characters it also complicates the position, but only to a relatively unimportant degree. In the first place it makes the environment very varied, especially in industrial societies where men living next door to one another may be subject to very different conditions in their daily occupations. Secondly, the fact that men protect themselves against external conditions—for instance, against cold—may lead to a group of men who are protected surviving, where a less well-protected group perishes, though the former may conceivably be 369naturally less resistent to cold than the latter. In the broad and rapid survey that we shall make these complications can be disregarded; the existence of tradition does not introduce, so far as physical characters are concerned, that peculiar complication which is the cause of much difficulty when we come to deal with mental characters—namely, the combination of what is acquired through tradition with the underlying character itself in such a way that the manifestation of the character is connected in a varying degree with its innate strength. We shall thus, when dealing with mental characters, have to attempt to strip off the acquirements. When dealing with physical characters we can always get down to the character at once. There is no difficulty in separating the arm from the tool which it employs, and we thus get directly at the character which has developed as the result of the influence of certain stimuli upon a given predisposition. It is, on the other hand, difficult to separate the intellectual characters from all those traditional elements which combine with them in their outward manifestations—to measure, for example, the strength of the instinct of curiosity, which involves discounting all those elements in the tradition which may inhibit or emphasize its expression.

With regard to the strength of selection there is no exact knowledge, with the exception of some work which has been done upon statistics for modern communities. The occurrence of selection in the past is in fact merely a deduction from what we know regarding innate predispositions and regarding elimination and differential fertility. It has been shown, however, that lethal selection does occur at the present day. Professor Karl Pearson has calculated that selection accounts for a very large percentage—perhaps 60 per cent.—of the deaths at the present day,^[1603] and Mr. Snow, summing up the results of an inquiry into this subject, states that ‘natural selection in the form of a selective death-rate, is strongly operative in man in the early years of life’.^[1604] As it is universally agreed that, if anything, the intensity of natural selection has decreased with civilization, we may take it as certain that it was operative to as great, or to a greater, extent during 370the emergence of man and during what we have called the first and second periods of his history.

2. During the intermediate period, which was of relatively immense length, the greater part of human bodily evolution was accomplished. Only two or three worn and fragmentary remains have as yet been discovered from this period. It is at least apparent from them that the amount of physical evolution which has been accomplished since the end of that period is slight compared with what was accomplished within that period. We may assume that lethal selection was strongly at work. Changes in mode of life—the assumption, for instance, of a terrestrial for an arboreal existence and the adoption of an upright posture—must have involved lethal selection. These changes of habit must also have brought ancestral man into contact with new enemies. The spreading of man into new climatic zones was doubtless followed by selection, and must again have involved contact with new enemies. So, too, selection followed upon the first great steps in the acquirement of skill—the making of clothes, the use of fire, and so on. In addition to lethal selection, differential fertility acting through polygamy must have been at work. In part no doubt differential fertility merely reinforced lethal selection; but in part also it may have taken the form of sexual selection and have favoured other types.

But when we come to details we find that we are ignorant regarding the causes of even the largest changes. Some guesses have been hazarded. The increase in the capacity of the skull is connected with the evolution of the intellect and may be left for consideration in the next chapter. The decrease in the size of the jaw and the corresponding decrease in the size of the teeth were perhaps connected with a change in diet. The loss of the hairy covering may have been connected with sexual selection or it may have been favoured because it removed a lodging-place for parasites.

3. As we pass from the intermediate to the first period of human history we reach a region of less uncertainty. We are ignorant as to the physical characters of man at the close of the intermediate period, but we know that in the earlier part of the Upper Palaeolithic there was existing one variety—the Grimaldi race—which bears certain resemblances to the negroid type, and that in the latter part of the same period there were existing 371several varieties closely resembling types of modern European man. It seems, therefore, that, so far as this period is concerned, we have to account not merely for the evolution of the main types but also for the evolution of the less easily distinguishable varieties of man. The splitting up may have begun in the former period but it probably did not go far. As to how this evolution took place certain conclusions may first be drawn from what we know must have been the general conditions of life. Secondly, from what we know as to the position among races of the first group, rather more definite conclusions may be added concerning the nature and direction of selection.

It has already been remarked that in all probability man spread into various climatic zones before the beginning of the first period of history. The consequences of this spreading of man must have been twofold. Mankind became segregated into groups, the surroundings of which differed, first in that they were subject to different climatic environments, and secondly in that they were forced to pursue different modes of life. It is clear that, whereas in some tropical regions man would be supporting himself by hunting and collecting in such a way as to require a certain type of bodily exertion, in a temperate region he would be supporting himself by a very different form of exertion. This difference between the needs of daily life would result in the favouring of different types in the two regions. The type best adapted to gain a living in one region would be different to that best adapted to gain a living in another region.^[1605]

Of far more importance in the production of varieties of men than the factors already noted are differences in climate to which man has become exposed. How great these differences are is familiar and need not be laboured. We know little with regard to the manner in which different types of man are suited to different types of climatic zones. But it is clear that in general the races of men are innately adapted to different climates. There is no historical reason why in all parts of North and South America the European races should not have ousted the American Indian as they have done in the United States. In Mexico and other regions, however, where the climate differs most markedly from that in Europe, Europeans have not succeeded in establishing 372themselves as they have elsewhere in the Continent. In Mexico Indian blood largely predominates over European, and the explanation must be that Europeans are not so well physically adapted to the climate as are the original inhabitants. In this there is nothing surprising. We can understand how certain types of respiratory, excretory, and circulatory organs might be better adapted than others to certain conditions of temperature, moisture, actinic rays, and so on, though we may be ignorant as to what types of circulatory and other organs are best fitted to any particular climatic zone. It would be very remarkable if it were not so, if, that is to say, any type of constitution was equally well fitted to any kind of climate. In this manner to differences in habits and in climate we may attribute the origin of the varieties of mankind.

Nevertheless, differences between races are far from being wholly due to selection on these grounds. Sexual selection is of importance. There grows up within each race a more or less clearly defined type of physical beauty, and differential fertility working through polygamy would evidently favour such types. That differences in colour to some extent represent adaptations to climate is clear; in part such differences are probably to be explained as due to varying ideals of beauty, but there are probably yet other factors entering into the evolution of racial types.

Recently Professor Keith has made an interesting contribution to the problem of racial differences. Reference has already been made to the ductless glands. It is now known that variations in the functioning of these glands have a profound influence upon the bodily organs. Keith has remarked that, if the peculiarities characteristic of the chief racial types of man are considered together, it appears that they are attributable to different degrees of development of these glands in different races. It should be observed that this suggestion does not involve the conclusion that racial differences are of the nature of modifications. It is merely supposed that the innate development of these glands is different in different races—that there is a predisposition in one race to the peculiar development of one gland and in another race to the peculiar development of another gland. The value of the suggestion lies in the fact that it shows how many racial peculiarities, for which there are at least no obvious explanations, may be merely the accompaniments of a difference in the development 373of one or more of these glands. It must be supposed that one or more of the results of the developments of a gland are of direct value in facing the peculiar conditions, whether climatic or otherwise, to which the race is subject, and that the other consequences of the development of the gland are in any case not prejudicial. These other consequences may possibly take the form of noticeable peculiarities of colour or of bodily structure which thus turn out in themselves not to be of survival value but merely to be, so to speak, accidental peculiarities.

Some quotations from Professor Keith’s exposition of this suggestion may make the matter more clear. ‘When we compare’, he says, ‘the three chief racial types of humanity—the Negro, the Mongol, and the Caucasian or European—we can recognize in the last named a greater predominance of the pituitary than in the other two. The sharp pronounced nasalization of the face, the tendency to strong eyebrow ridges, the prominent chin, the tendency to bulk of body and height of stature in the majority of Europeans, is best explained, so far as the present state of our knowledge goes, in terms of pituitary functions.’^[1606] After remarking that the interstitial glands are largely the cause of secondary sexual differences he goes on to say: ‘I am of opinion that the sexual differentiation—the robust manifestation of the male characters—is more emphatic in the Caucasian than in either the Mongol or Negroid racial types. In both Mongol and Negro, in their most representative form, we find a beardless face and an almost hairless body, and in certain negro types, especially in Nilotic tribes, with their long, stork-like legs, we seem to have a manifestation of the abeyance in the action of the interstitial glands. At the close of sexual life we often see the features of a woman assume a coarser and more masculine appearance.’^[1607] Later he remarks that the evidence points to the original human colouring as black. Now, the supra-renal bodies cause a clearing away of pigment and ‘there can be no doubt that the supra-renal bodies constitute an important part of the mechanism which regulates the development and growth of the human body and helps in determining the racial characters of mankind. We know that certain races come more quickly to sexual maturity than others, and that races vary in development of hair and of pigment, and it is therefore reasonable to expect a satisfactory explanation 374of these characters when we have come by a more or less complete knowledge of the supra-renal mechanism.’^[1608] The thyroid acts directly upon the skin and the hair and also upon the skeleton. ‘This is particularly the case as regards the base of the skull and the nose, the arrest of growth falls mainly upon the basal parts of the skull with the result that the root of the nose appears to be flattened and drawn backwards between the eyes, the upper forehead appears projecting or bulging, the face appears flattened, and the bony scaffolding of the nose particularly when compared with the prominence of the jaw is greatly reduced. Now these facial features which I have enumerated give the Mongolian face its characteristic aspect, and, to a lesser degree, they are also to be traced in the features of the negro. Indeed, in one aberrant branch of the negro race—the Bushman of South Africa—the thyroid facies is even more emphatically brought out than in the most typical Mongol. You will observe that, in my opinion, the thyroid—or a reduction or alteration in the activity of the thyroid—has been a factor in determining some of the racial characteristics of the Mongol and Negro races. I know of a telling piece of evidence which supports this thesis. Some years ago there died in the East End of London a Chinese giant—the subject one must suppose of an excessive action of the pituitary glands—the gland which I regard as playing a predominant part in shaping the face and bodily form of the European. The skeleton of this giant was prepared and placed in the museum of the London Hospital Medical College by Col. T. H. Openshaw, and any one inspecting that skeleton can see that, although certain Chinese features are still recognizable, the nasal region and the supraorbital ridges of the face have assumed the more prominent European type.’^[1609]

4. If we turn now and look at the conditions of life among primitive races, we find that natural selection and differential fertility tend to work towards the preservation of existing types rather than towards further evolution. The Australians may have existed in their present home for many thousands of years without undergoing any considerable change of type, and we have every reason to suppose that, if they had been left untouched by white or other races, the racial type would have remained substantially the same for thousands of years to come. The same 375holds good if we consider any primitive race in its normal surroundings.

That this is so is clear if we glance at the chief causes of elimination. The heavy child mortality recorded of all these races is put down to neglect and exposure; it must on the whole result in the elimination of the physically weak. Such customs as the bathing of new-born babies in cold water seem designed to ensure this result. Among the adults who on the whole are but little protected against climate there must always be a certain tendency towards the elimination of those less able to withstand the relatively harder seasons. There are also many factors making for the cutting-off of the malformed and the congenitally deficient. Among those races which normally practise infanticide, deformed children are always killed, and among those races, such as the Bantu races of Africa, who never regularly practise infanticide as a custom, abnormal children are nearly always done away with. Similarly the destruction of witches tends towards the same end though they are more often marked by mental than by physical peculiarities. Again the fact that abortion and infanticide are practised to a greater degree among the less fortunate and the lower social classes of the races in the second group has the same result, as in general the stronger and more successful members of the race are those who do not need to practise these customs to the same extent. Finally, the general conditions of life among these races—more particularly among the races of the first group—is such as to bring about a continual elimination of the less physically fit. Speaking of the Seri Indians McGee remarks upon the ‘elimination of the weak and helpless’, and says further that ‘a parallel eliminative process is common among American aborigines; the wandering bands frequently undergo hard marches under the leadership of athletic warriors with whom all are expected to keep pace, which leads both to the desertion of the aged and the feeble’. He calls it ‘a merciless mechanism for improving the fit and eliminating the unfit’.^[1610]

As remarked above, the effect of polygamy must be to intensify the action of selection. In this particular case the general result of polygamy must be to preserve the average features of the race; for it will be, so to speak, the all-round man, the man best adapted to the climate, to the peculiar conditions of life and so 376on who will leave most descendants. It is thus of interest to investigate the frequency of polygamy among the races of the first two groups. This point has been inquired into by Professor Hobhouse and his fellow authors. Polygamy was distinguished by them into general and occasional polygamy, and though it is very difficult to arrive at any exact figures their general conclusions may be summed up as showing that, in all the subdivisions considered by them, general and occasional polygamy was found in from 80–95 per cent. of the cases. General polygamy increases among the races here placed in the second group, though there is no strong correlation between the degree of polygamy and the economic stage. We observe in fact a wide prevalence of polygamy among all races of any economic stage, and upon the basis of these figures we must regard the differential fertility arising from polygamy as a factor of great importance.

It seems, therefore, that within the first and second periods there were two tendencies at work—towards the evolution of varieties and towards the maintenance of the various types. The evolution of varieties was due in the first place to the spreading of man. As soon as spreading was complete, a tendency towards a cessation of progress must have set in. That further evolution took place was due chiefly to migration, with the results of which we have yet to deal. Once varieties had been evolved, apart from migration, and apart from changes in climate and progress in skill, which only slightly and at long intervals modified the surroundings, there was little basis for further evolution. Man had mastered his surroundings up to a point and had made his position secure in the circumstances under which he lived and the tendency was towards the preservation of the types which had achieved these results in different places.

5. In the third of our three periods there has been a remarkable change. We find that there has been a gradual moving away from the conditions under which in normal times there is in the races of the first and second groups a rigorous elimination of those types which depart from the mean. Lethal selection has come largely to take the form of selection through disease.^[1611] Though polygamy gradually ceases to be a factor of importance, other forms of differential fertility become prominent.

377We may glance at the chief features of these changes. In the last section we mentioned various factors—infanticide of the deformed, customs with regard to the treatment of children, the neglect and exposure of children, the general conditions of life, and so on—all of which have as their result the cutting-off not merely of the deformed and the monstrosities but also of all departures from the type best fitted to contend with the climatic environment and the conditions of daily life. Some of these factors—such as infanticide—disappear; the action of others becomes, if anything, reversed; not only do the less well-fitted types have a better chance of survival than before, because the conditions are less rigorous, but they suffer little or no disadvantage owing to the fact that the conditions have been artificially rendered almost as favourable for the less fitted as for the better-fitted. To take two examples, not only are men with defective eyesight not eliminated, but they are by the invention of spectacles placed in as good a position as those with perfect eyesight. Similarly a woman with a narrow pelvis is, owing to the advance in surgical skill, enabled to bear children and to transmit her peculiarity to her daughters.

It should not be forgotten that, in spite of the gradual lessening of the rigour of selection, selection owing to climate and the general conditions of life still continues. But another factor has within this period come to assume a preponderating importance, and that is selection through disease. It was pointed out that diseases may be roughly distinguished into those due to the attacks of parasites and those due to structural defects. It is to diseases of the first kind that the greater part of selection, which occurs in the third period, is due. Some figures were given in an earlier chapter showing how large a proportion of deaths at the present day is due to one or other of these diseases. In the last chapter it was shown that men differ in their susceptibility to these diseases, and selection has thus very largely come to take the form of the elimination of the more susceptible, and of the favouring of the naturally immune and of those who have a power of resisting disease and of acquiring immunity. There has therefore come to be an increasingly heavy premium upon the type of constitution which can resist disease, and a strong constitution in this sense is not necessarily the same as a strong constitution among primitive races where the premium is rather upon muscular strength, 378perfect development of the senses of seeing and hearing, and upon resistance to climatic conditions. We have thus to think of the whole course of bodily evolution as changing in the third period so as to meet a new danger rather than as continuing on the former lines so as better to contend with the old difficulties.

It is noteworthy that selection in this period has not been on account of diseases of the second type due directly to structural defects. These diseases are seldom lethal until after maturity has been reached, and the tendency has rather been, as in the case of eyesight, towards the increased chance of survival of those suffering from these defects. Thus, though selection has turned in this period towards the weeding-out of those susceptible to the attacks of parasites, it has come to tolerate those who exhibit defects in structure as distinguished from the peculiarities of structure which must be presumed to constitute the physical basis of susceptibility.

All this is familiar enough. It is not so often realized that the disappearance of polygamy in what we have called the mediaeval and the modern sections of the third period works in precisely the same manner. But the disappearance of polygamy does not mean that reproductive selection ceases to be of importance. In mediaeval and modern times celibacy, postponement of marriage, and restriction of families have come to be practised in varying degrees by different stocks. There is some trace of differential fertility in the earlier periods owing to causes other than polygamy, but it is only in the mediaeval and modern periods that they become important. It is doubtful how far, if at all, the religious celibate class of the mediaeval period differed in physical characters from the average. It would appear that during this period postponement of marriage led to the producing of less children by the lower social classes than by the upper. Therefore, so far as this epoch is concerned, it is not apparent that differential fertility had any considerable effect upon bodily characters one way or the other. Possibly the net result of postponement may have been to favour the better stocks.

Within the latter part of the modern period restriction of families has assumed very great importance. It has been calculated that, as a consequence of the fact that restriction is more practised by certain sections of the population than by others, 50 per cent. 379of the married population provide 75 per cent. of the next generation. It thus becomes most important to determine whether the various sections of the population are innately different as regards their physical characters. Largely owing to our ignorance respecting the direct influence of the environment, it is not possible to arrive at present at any precise answer. But there can be little doubt that on the whole the most fertile sections of the population are the less physically fit sections.

It is necessary, however, to view these facts in their proper perspective. Though differential fertility of this kind may have been at work among some of the ancient empires in their later years, it is chiefly a very modern phenomenon characteristic of modern European countries and their derivatives only within the last fifty years. Therefore, however important a problem it may be for modern communities, it is a factor which has had but little effect on human history as a whole. It may be noticed that restriction of increase is not the only form of reproductive selection in modern communities. Sexual selection is also at work.^[1612]

6. Such has been the direction in which, through selection and differential fertility within races, changes have taken place. But changes also take place through the conflict of races, and the nature of these changes demands some notice. Warfare plays a part in the normal existence of nearly all primitive races. It is sometimes, as among the American Indians, a bloodthirsty affair, but more often than not it is a relatively unimportant cause of elimination. It is not easy to arrive at any conclusion regarding the results of warfare as an agent of selection. Upon the whole, among primitive races, so far as physical characters are concerned, there is probably a tendency towards the preservation of the physically strong and fit. But it has to be remembered that missile weapons were early introduced, and that, as Thucydides remarked, missile weapons kill the strong man in the prime of life as well as the weak. Again, success in warfare very largely depends upon characters other than physical—such as the possession of skill. All that can be said with certainty is that in the first and second periods warfare was not a cause of elimination of the fit which it has come to be in the third period. This latter fact has been lately brought home to the civilized nations 380of the world in so obvious a manner that it need not be further discussed.^[1613]

Warfare following upon migration may exterminate whole races. The Bushmen were apparently on the verge of extinction at the hands of the invading Bantu peoples. In this manner a peculiar physical type may be lost and another may become of relatively greater prevalence. Migration may also be followed by any degree of racial mixture. Many rash statements have been made by historians as to the biological results of crossing. The consequences actually observed to follow upon a mixture of races are to so large a degree the result of a conflict of traditions—a subject with which we shall deal in a later chapter—that the purely biological effects of crossing may be very largely obscured. Bearing this in mind we may briefly refer to the conclusions to be derived from recent biological work which bear on the problem. Roughly speaking there are two possible kinds of crosses between races. First there are crosses between the most clearly distinguished varieties such as white and black. Heterosis, or hybrid vigour, will be exhibited in a marked fashion in the first generation. Heterosis, the underlying cause of which has only recently become apparent, is always at its height in the first cross. The increase of vigour, however, is not long maintained in subsequent generations. Further, each type, such as those which we are considering, has a series of character complexes, built up through ages of selection and compatible with one another, and by crossing such complexes are broken apart. The chance of gain, on the other hand, through the favourable re-combination of characters is small. On the average, therefore, the result of such a cross is unfavourable. There may also be crosses between races exhibiting less differences. Again, heterosis will be visible on crossing. But in distinction to the results of the former kind of cross, the other results may not be unfavourable. Great variability may follow such a cross and this is on the whole advantageous. Valuable character re-combinations may also come to light. Thus we may say that, so long as there is not too great a difference between the races which cross, the results are 381usually genetically favourable; there will be the advantage of hybrid vigour, though this is always temporary, and there may very possibly be the advantage of valuable character re-combinations.^[1614]

Migration is important not only because it is connected with war and racial crossing but also because of the selection which follows upon transfer to new climatic conditions. To the nature of these changes allusion has already been made; it has been pointed out that races adapted to maintain themselves in one environment cannot as a general rule maintain themselves in a very different environment. To new surroundings, which only differ slightly from the old, they may become adapted by selection, and much selection on these lines must have taken place as a result of racial movements in Europe. It is probably now taking place among Europeans who have migrated to America. Dublin and Baker, for instance, have shown that the death-rate varies considerably among the different racial elements who have immigrated, some elements being probably better adapted to the new environment than others.^[1615] Similarly selection may be going on within modern races owing to the rapid urbanization of industrial countries. Urban conditions may be more favourable to some types than to others. But it has not yet been shown in what direction these changes, if they are in progress, are taking.

Summing up our conclusions we may say that the great changes in human bodily form were accomplished in the intermediate period when the splitting up into varieties may also have begun. This splitting was continued in the first period and the modern types were formed before the end of that period. The chief characteristic, however, of the first and second periods was the maintenance of the types evolved, though this was interfered with by migration which brought about elimination of certain types, racial mixture, and further climatic adaptation. Finally, in modern times there has been a lessening of the stringency of selection, which previously tended to maintain the existing 382varieties, and a turning of lethal selection to the building up of immunity against disease.

7. There are one or two other points connected with germinal change about which a word may be said. Of the origin of mutations we know nothing, but since in all probability the ultimate cause of mutations may have to be sought in some kind of environmental change, it should be borne in mind that, as man is subject to an immense variety of environmental stimuli, mutations may be more frequent in man than in any species in a state of nature. It has been suggested that the germinal constitution may be adversely affected by certain factors, especially by the use of alcohol. Of this there is no certain evidence.^[1616] It has been supposed that there may be some connexion between the difference in age of the parents on the one hand and the germinal constitution of the offspring on the other. It has been supposed, for instance, that the offspring are innately more vigorous when the parents are at a certain age. So far as investigation has gone at present, it has not been established that there is any such connexion—of sufficient importance at least to deserve consideration here. In other words, the fact that among certain races young men have wives of considerably greater age than themselves and older men young wives, does not to any noteworthy extent affect the germinal constitution of the race. Again it has been stated that the first-born are innately inferior to the later-born children.^[1617] This conclusion has been criticized.^[1618] As, however, the preponderance of first-born children among the offspring as a whole would not appear to be markedly greater at any one period of history than at another, the inferiority of the first-born, if it exists, would make no difference between men of different races at the same or at different times.

The correlation of characters one with another should not be forgotten. The favouring of one character may involve the favouring of quite other characters. Of this what was said regarding the ductless glands is an example. It has been supposed that liability to disease is correlated with pigmentation and that, owing to selection through disease, a change in the average pigmentation 383is taking place. Attempts to establish this correlation have, however, failed. It is of particular interest to notice that no correlations have been found between physical and mental characters; this is so even as regards intellect and size of head.

8. A survey of the facts regarding the bodily evolution of man, and of the conditions under which it has taken place, shows that there is at least no more difficulty in understanding how it has come about than in understanding how the varieties of any other species have come into existence. Given mutations, the action of selection and of differential fertility provides an explanation which is in general satisfactory, though we may be a very long way from understanding how any one particular change came about. There are in fact greater difficulties met with from time to time in trying to reach a satisfactory explanation of evolution among other animals than when dealing with man. The evolution along particular lines of organization, the evolution of teeth and horns among mammals, for instance, raises a difficulty which seems in the present state of our knowledge perhaps to require the assumption that at times there is a tendency for the continual occurrence of mutations in the same direction. Such a difficulty does not occur in the case of human bodily evolution.

It has been suggested that the facts regarding the course which the evolution of living organisms has taken requires the assumption that large mutations have occurred from time to time and have established themselves as varieties. There is no doubt that large mutations do occur and have established themselves in the course of the evolution of the varieties of animal types known to us, and they may have done so in the case of man. There is, however, no reason to imagine that the origin of any particular change in man was due to a large mutation. It has been stated that the number of chromosomes is different in the white man and in the negro, and upon the basis of this statement it has been suggested that one variety had arisen as a mutation from the other. This statement, however, was apparently based upon erroneous observation. Again, it has been suggested that the Pygmy type arose as a mutation. But it has been observed that some Ba Twa pygmies of the Congo who three generations ago left their forests, settled near the Bushongo, and took to agriculture have become markedly taller. Intermarriage is said to be out of the question and therefore we are led to suppose in the new environment 384a taller type has been favoured.^[1619] If this is so, if a taller type can be selected by degrees in one environment, the shorter type may equally well have been selected in another environment.

So far, therefore, as the history of man outlined in the fifth chapter is the history of the development of his bodily structure, so far the explanation of history is to be sought in changes in the germinal constitution, and the changes undergone are strictly comparable with the changes among species in a state of nature, though owing to the more varied environment outward differences in the case of man may be rather more often due to environmental influences than in the case of species closely related to man.

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