XIV THE INFLUENCE OF THE ENVIRONMENT AMONG ANIMALS AND PLANTS

1. A brief inquiry into the part played by the environment among animals and plants forms a necessary introduction to a similar inquiry as regards man such as we shall undertake in the next chapter.

In the second chapter something was said as to fertilization and as to the development of the zygote or fertilized egg to the adult form. It has been shown by Herbst that, if this process of development is to result in a normal adult, all the factors comprising the normal environment must be present. Herbst ascertained the exact composition of sea-water at Naples.^[1529] Using as his material the larvae of sea-urchins, he changed slightly in many different ways the composition of the water. The experiments were very exhaustive. At every stage in the development he observed the results of abstracting one or more of the constituents of sea-water—the normal environment of the larvae. Commenting upon the results of these experiments, Jenkinson says: ‘Whatever may be the ultimate explanation of the facts, there can be no doubt whatever that the most complete demonstration has been given of the absolute necessity of many of the elements occurring in ordinary sea-water, its normal environment, for the proper growth and differentiation of the larva of the sea-urchin. Nor is this all. Some of the substances are necessary for one part or phase of development, some for another, some at the very beginning, others only later on. Thus potassium, magnesium and some degree of alkalinity are essential for fertilization, chlorine and sodium for segmentation, calcium for the adequate cohesion of the blastomeres, potassium, calcium and the hydroxyl-ion for securing the internal osmotic pressure necessary for growth, while without the sulph-ion and magnesium the due differentiation of the alimentary tract and the proper 326formation of the skeleton cannot occur; the secretion of the skeleton depends on the presence of some sulphate and alkalinity, the skeleton requires calcium carbonate, cilia will only beat in an alkaline medium containing potassium and magnesium, and muscle will only contract when potassium and calcium are there.’^[1530]

Summing up the results of these and other experiments, Jenkinson says: ‘Every factor, or nearly every factor [of the environment], is necessary for this or that phase or part of the process, some for the whole. Light of a certain wave length will accelerate development; light of another kind, or, in some instances, darkness, will retard it, or will stop it altogether; a certain degree of heat is indispensable; oxygen is required for respiration, water for growth; some eggs demand constant agitation, others complete rest; fertilization or segmentation or gastrulation, or some one or other of the later phases of development may depend absolutely on the presence of some particular chemical element; remove the factor in question, whatever it may be, and that particular process will not occur, and the specific typical end which is reached in normal development will not be attained.’^[1531]

Putting aside until later the question as to what is implied by the term normal environment, we may now ask what the relation is between heredity and environment. To regard a developing organism as subject to the action of two forces, one tending to thrust it in one direction and the other in another, is to view the matter in a wholly false light. Heredity and environment are, as is clear from the evidence given above, complementary one to the other; without a germinal constitution there can be no organism; without appropriate stimuli an organism cannot develop. Similarly it is misleading to speak of the relative importance of the two factors unless the terms are very clearly defined. To see that careful definition is necessary, we have only to remember that, because, with the exception of the anaerobic bacteria, all organisms require free oxygen, environment might be held to be all important. Here, however, we are not concerned with the problem as to the conditions under which we might speak of the relative importance of the two factors; we have merely to note that they are essentially complementary.

2. We may now observe that, just as a normal adult only 327develops under normal stimuli, abnormal stimuli experimentally induced may be followed by every kind of result from the most extreme to the most insignificant changes both of form and of life-cycle. We may note the results of a few of the vast number of experiments which have been made. With regard to plants it has been shown that ‘each developmental stage depends upon special external conditions, and in cases where our knowledge is sufficient, a particular stage may be obtained at will. In the Green Algae, as in the case of the Fungi, we may classify the stages of development into purely vegetative growth (growth, cell-division, branching), asexual reproduction (formation of zöospores, conidia), and sexual process (formation of male and female sexual organs). By modifying the external conditions, it is possible to induce algae or fungi ... to grow continuously for several years, or, in the course of a few days, to die after an enormous production of asexual or sexual cells. In some instances even an almost complete stoppage of growth may be caused, reproductive cells scarcely being formed before the organism is again compelled to resort to reproduction. Then again the sequence of different stages of development may be modified as we desire.’^[1532]

Another kind of experiment shows that foliage shoots can be converted into runners and vice versa; it is, for instance, possible to induce a germinating tuber of the potato to form foliage shoots under the influence of a higher temperature. The transfer of plants from one environment to another is often followed by remarkable changes. MacLeod, for instance, states that a species of Philodendron which has large leaves pierced with round holes was cultivated for many years in the Botanical Gardens at Ghent in a greenhouse which was rather cool and dry. The holes were found to be rare; at times it was not possible to observe a single perforation in any of the leaves of a specimen. In every other respect the plants were healthy. The plants were later transferred to a greenhouse that was warm and moist and after a few months the new leaves were found to be abundantly perforated.^[1533] Bonnier made some very interesting observations on the dandelion. He found that the plant, when sown at a high altitude in the Pyrenees, produced very short stems with hairy, dark green leaves and 328a compact flower. On the other hand, seeds gathered from such plants growing at a high altitude and sown in the neighbourhood of Paris produced after three years elongated stems with less hairy and brighter leaves, or, in other words, plants very similar to those grown from seeds obtained in the neighbourhood of Paris. The modifications acquired during a given time by a lowland plant grown at a high altitude, or by a highland plant grown at a low altitude, took about the same time to disappear on returning the plants to their original climates.^[1534] Similarly ‘Schubeler sowed seeds of various plants in different latitudes in Norway and proved that the brilliancy of the flowers increased with the latitude. So great was the difference that it was difficult to conceive that they were produced from the same batch of seeds.’^[1535] Observations on European peach-trees transported to Réunion were made by Bordage. Such trees lost their deciduous habit and became evergreen, though in some cases it took twenty years before the change was complete.^[1536] Lastly the red primrose ‘reared at a temperature of 30°–35° C. (with moisture and shade) has pure white flowers, but the same plants reared at 15°–20° C. have red flowers. If the white-bearing plants are brought into a cooler place, the flowers that are already in bloom remain white, but those that develop later in the cooler temperature are red.’^[1537]

We may also notice the results of some of the experiments upon developing animals. Stockard experimented with the fish Fundulus heteroclitus. He subjected the eggs both before cleavage began, and after the two- and four-celled stages had been reached, to solutions of magnesium salts in sea-water. The eyes of a large percentage of the embryos were abnormal. In some cases there was a single median eye; in other cases there was a median eye showing signs of a double structure.^[1538] ‘In a long series of experiments Féré has shown that monstrosities can be produced by exposing the hen’s egg to the unfavourable influence of a large variety of substances. Vapour of ether, alcohol, essential oils, nicotine, mercury and phosphorus, injection of alkaloids such as morphine, nicotine, strychnine and others, of bacterial toxins (those of tubercle and diphtheria), of peptone, dextrose, and 329glycerine, several alcohols, certain salts ... are all harmful, retarding and distorting the embryo to a greater or less extent.’^[1539] Many experiments show the effect of differences in food, temperature, and humidity upon developing organisms. Agar carried out some experiments on a small water-flea, Simocephalus. This animal is enclosed in a kind of shell composed of two valves, somewhat like a mussel. Normally the edges of the valves nearly meet, so that if a section is cut across the animal transversely the shape of the body enclosed in the valves is oval. When the food was varied in a certain way, Agar found that the edges of the valves were turned outwards so that the shape of a transverse section was no longer oval but bell-like. He also found that the length was reduced by exposure to a high temperature.^[1540] Experimenting with a beetle, Tower found that both the colour and the colour pattern could be modified by changes in temperature and humidity.^[1541] Similarly Morgan showed that a species of the fruit fly exhibited a peculiar formation of the abdomen in the presence of moisture; when raised in dry conditions the abnormality disappeared.^[1542] Poulton obtained some remarkable results when working with the larvae of moths. ‘Larvae surrounded by the leaves on which they fed, became, in the majority of species, light brown or light grey in colour. If, however, an abundance of twigs had been mixed with the leaves of the food plant, they became dark in colour. The larvae of the Peppered Moth afforded the most striking result of all, for when reared among green leaves and shoots they became bright green without exception, whilst in the presence of dark brown twigs they nearly all assumed a corresponding colour.’^[1543]

A Japanese experimenter exercised rats for 90 to 180 days; he found that this long-continued exercise markedly increased the weight of the heart, kidneys, and other organs on an average to about 20 per cent.^[1544] A thickening has been observed in the stomach of a gull fed on grain for a year and this change is said to take place under natural conditions in the herring gull, which feeds in winter on fish and in summer on grain. Conversely if graminivorous birds are fed on a carnivorous diet, the gizzard assumes the form of a carnivorous bird’s stomach. Cuvier found the length of the intestine of the wild boar is as 9 to 1 and of 330the domestic boar as 13·5 to 1, a difference which is in part probably due to diet.^[1545] The colour of birds’ plumage is affected by their diet. Hempseed causes bullfinches and other birds to become black. Cayenne pepper causes yellow to change to orange red. In the New York Zoological Gardens it has been shown that some birds, such as the bobolink, may be so dieted that they keep their breeding plumage throughout the year and will sing their spring song in mid-winter.^[1546] Warren produced marked changes in the common water-flea, Daphnia, by keeping them in a confined space for many generations. ‘Semper and de Varigny found that when fresh-water snails were reared in vessels of a shape that allowed them abundant water but very little surface in which to take exercise they developed into dwarf forms. Every precaution was taken to procure abundant food, perfect aeration and thorough removal of waste products. De Varigny’s experiments were particularly careful and point convincingly to the conclusion that the condition of dwarfing was the restricted area for exercise.’^[1547] Lastly it may be remembered that, as is well known, parasites may induce remarkable changes in their hosts.

3. From what was said in the third chapter it follows that the results of artificial subjection to abnormal stimuli just described are to be interpreted as examples of the fact that different responses are given to different stimuli by the same or approximately the same germinal constitution. But large and conspicuous changes of this nature are not visible only as the result of artificial circumstances. Many species are normally subject to more or less definite and sudden changes in the surroundings, and some of them exhibit more or less definite responses to such changes. This is especially obvious in the case of sessile organisms the outward form of which is clearly modified by the surroundings. Thus ‘the water ranunculus, when growing submerged beneath the surface of a pond, produces leaves the blades of which are cut up into a number of fine thread-like segments. As soon as the top of the plant reaches the surface of the water, those leaf-rudiments which are just commencing their existence, proceed to develop in a totally different fashion. The leaves to which they give rise possess a wide and unpointed blade, which floats 331upon the surface of the water. The two sets of leaves are as utterly different in their appearance as it is possible for leaves to be. Yet the effect of the external conditions upon the young leaf-rudiment determines which of the kinds is to appear.’^[1548]

Examples may also be found in the life-history of free-living organisms. One is afforded by the common honey-bee. As is well known, a queen bee differs markedly from a worker bee in shape. Both queens and workers arise from fertilized eggs; whether a queen or a worker develops from any one egg appears to depend wholly on the environment—a larva that gives rise to a queen receiving in the first place different and presumably more nutritious food than that received by a larva giving rise to a worker, and, in the second place, inhabiting a cell which differs in size and shape from that inhabited by a worker larva. Another is afforded by the life-history of the plant lice (Aphids). At certain seasons of the year winged forms appear. It has long been suspected that the appearance of winged forms depends on some environmental stimulus. That this is so has been rendered practically certain by the work of Shinji, who has shown that aphids reared on plants watered with certain solutions are winged almost without exception.^[1549]

Among mammals the assumption of a winter coat by the lemming, ptarmigan, and variable hare is a similar phenomenon. Sir John Ross has told how a Hudson Bay lemming was protected from the cold on board his ship by keeping it in the cabin. It retained its normal summer coat during the winter. On exposing it in a cage on deck to a temperature of 30° below zero, the fur on the cheeks and a patch on each shoulder became perfectly white during the first night. After another day’s exposure ‘the patches on each shoulder had extended considerably, and the posterior part of the body and the flanks had turned a dirty white.... At the end of a week it was entirely white except in a dark band across the shoulders prolonged posteriorly down the middle of the back.’^[1550]

A curious example of the importance of the environment before birth is afforded by the difference between the mule and the jennet. The former is the product of a cross between a stallion and a she-ass, the latter between a jackass and a mare. There 332is every reason to believe that it is not the fact that horse-ancestry is in one case traced through the father and in the other case through the mother that gives rise to the difference; the difference can only be attributed to the fact that in one case the period before birth is passed within a mother of one species and in the other case is passed within a mother of another species.

4. We are now in a position to discuss what is meant by the term ‘normal environment’. There is a more or less definite range of variations of the environmental stimuli to which species in a state of nature are subject. The range may differ greatly from species to species, but remains more or less constant for any species. So long as the variations fall within this range, the environment may be described as normal. Such variations will be followed by different responses on the part of similar germinal constitutions, and the variations among the members of any species under a normal environment are due to the combined influence of differences in the environment and of differences in the germinal constitutions. From time to time in a state of nature organisms are subject to variations of the environmental stimuli which fall outside the normal range, and there are thus produced extreme modifications similar to those which can be experimentally induced. Thus Gemmill on examining a large number of fish embryos found monsters with a single eye very similar to those experimentally produced by Stockard.^[1551]

It may next be observed that there is a marked difference in the degree to which sessile organisms on the one hand and free-living organisms on the other hand respond to differences in the normal environment. The former are much more susceptible to differences—or at least to certain differences. Sessile organisms, for instance, differ from one another very greatly in form and such variations are known to be chiefly due to differences in the environmental stimuli. Free-living organisms do not vary in this manner. The reason is fairly clear. All species are adapted to a particular niche in nature, and among the former the mode of adaptation of necessity takes the form of susceptibility to surrounding conditions; a tree or a sponge must be able to adapt itself to its actual surroundings. A free-living organism is, on the other hand, adapted by its specific form to its niche in nature 333and, in order that it may reach and maintain this form, it must exhibit a relative lack of susceptibility to surrounding conditions.

5. The broad features of the situation among species in a state of nature are now fairly clear. By outward inspection it cannot be ascertained whether any particular characteristic is of the nature of a modification or of a mutation. But we know that among sessile organisms outward form is largely of the nature of modification, whereas among free-living organisms, which interest us more closely because conditions are more nearly comparable to those obtaining among men, modifications play a much smaller part. A large number of measurements has been made of certain features of particular species in a state of nature as, for instance, by Allen for the squirrel and Weldon for the common shrimp. Though on general grounds we may have reason to suspect how far the variations recorded are of the nature of modifications, we can arrive at no certain answer. Observations made on members of a species in a state of nature, some of which are subject to conditions that differ from those to which the rest of the members are subject, are of more assistance for our present purpose. Thus the tiger ranges from tropical to temperate regions. Tigers from the former regions exhibit certain differences from those from the latter regions in respect of the condition of the coat. It is possible that these differences are purely environmental. Again certain marine molluscs from cold waters exhibit differences when compared with members of the same species from warmer waters; these differences may again be purely environmental. In those cases in which a species has within recent years spread to a new environment, it is often found the degree of variation has increased. Thus Bumpus found that the egg of the common sparrow is more variable in the United States than in England;^[1552] it has also been noticed that the variation of the common periwinkle is greater in America than in England—both these species having been recently introduced into America.^[1553] It is also of interest to observe that Montgomery found greater variations among migratory than among non-migratory species of birds and the greatest variation among those that had the widest range.^[1554] Though, until the matter has been put to the test of experiment, nothing can be affirmed as to the 334nature of these differences, it is probable that in part at least these differences are environmental. Such observations help to exhibit the degree of importance that we are led to attribute to modifications among free-living animals in a state of nature. It must also be borne in mind that departures from the normal range of variations of the environmental stimuli are not so very infrequent and that more or less extreme modifications, as result, for instance, from the attacks of parasites, do occur.

Lastly we may go a step farther and ask what happens when the environment changes. This inquiry, if pursued, would lead us beyond the scope of this chapter. A reference, however, to this problem may assist to render the relation of a species under natural conditions to its environment rather more clear. Change of environment may be due either to the migration of some or all the members of a species to a locality where the environment is different, or to an actual change in the environment in the same locality. If the change is at all marked, then there will be a different response on the part of the germinal constitution to the new conditions. Let us suppose that the diet is changed and that some members of a species of bird take to a diet of grain having previously existed on a diet of fish. There will be a different response in that the stomach will assume a different form. But it is most unlikely that the old germinal constitution will be that which gives the best response to the new conditions. Almost certainly a somewhat different type of germinal constitution will be that which will give the best response to the new conditions, will be that, in other words, from which there will arise the form of stomach most suited to dealing with grain. If and when, therefore, a mutation arises exhibiting this changed type of germinal constitution, it will be favoured, and in this manner a new variety and ultimately a new species may be formed. It has thus to be remembered that, although the germinal constitution to some extent responds differently to different stimuli, it is very unlikely that any average type of germinal constitution will give the best response to any other environment than that to which the species is now subject. It does not therefore follow that, because the responses of the germinal constitution are various, there are not factors making for a change in the germinal constitution when the environment changes.

Summing up what we have said, we have to think of every 335species as living under a more or less clearly defined environment into which very many elements enter; whether the variations in these elements are great or small, there is an average condition, and to that average condition a certain type of germinal constitution gives the best response. In the case of a free-living animal in the adult form the germinal constitution does not respond readily to ordinary variations from the normal, though it has to be remembered that extreme influences such as those caused by parasites may cause marked reactions. Further, it has always to be borne in mind that during development all animals and plants are particularly susceptible to environmental changes. It is not possible to make any precise statement as to how far the differences we see under natural conditions are modifications and how far mutations. All that we can say is that the part played by the environment in producing modifications is on the whole smaller in the case of free-living animals than in the case of sessile animals and of plants.

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