APPENDIX C. Some Extracts from the Author's Note-books.

Bearing of Weismannism on Physiological Selection.—If in view of other considerations I could fully accept Professor Weismann's theory of heredity, it would appear to me in no small measure to strengthen my own theory of physiological selection. For Weismann's theory supposes that all changes of specific type must have their origin in variations of a continuous germ-plasm. But the more the origin of species is referred directly to variations arising in the sexual elements, the greater is the play given to the principles of physiological selection[67]; while, on the other hand, the less standing-ground is furnished to the theory that cross-infertility between allied species is due to "external conditions of life," "prolonged exposure to uniform change of conditions," "structural modifications re-acting on the sexual functions"; or, in short, that "somatogenetic" changes of any kind can of themselves induce the "blastogenetic" change of cross-infertility between progeny of the same parental stock.

Cross-infertility and Diversity of Life.—Observe that one great consequence of duly recognizing the importance of intercrossing is indefinitely to raise our estimate of the part played by the principle of cross-infertility in diversifying organic nature. For whenever in any line of descent the bar of sterility arises, there the condition is given for a new crop of departures (species of a genus); and when genera are formed by the occurrence of this bar, there natural selection and all other equilibrating causes are supplied with new material for carrying on adaptational changes in new directions. Thus, owing to cross-infertility, all these causes are enabled to work out numberless adaptations in many directions (i. e. lines of descent) simultaneously.

Cross-infertility and Stability.—The importance of sterility as a diagnostic feature is obvious if we consider that more than any other feature it serves to give stability to the type; and unless a type is stable or constant, it cannot be ranked as a species. That Darwin himself attributes the highest importance to this feature as diagnostic, see Forms of Flowers, pp. 58, 64.

Cross-infertility and Specific Differentiation.—In their elaborate work on the many species of the genus Hieracium, Nägeli and Peter are led to the general conclusion that the best defined species are always those which display absolute sterility inter se; while the species which present most difficulty to the systematist are always those which most easily hybridize. Moreover, they find, as another general rule applicable to the whole genus, that there is a constant correlation between inability to hybridize and absence of intermediate varieties, and, conversely, between ability to hybridize and the presence of such varieties.

Cross-infertility in Domesticated Cattle.—Mr. J. W. Crompton, who has had a large experience as a professional cattle-breeder, writes to me (March 2, 1887)—

"That form of barrenness, very common in some districts, which makes heifers become what are called 'bullers'—that is, irregularly in 'season,' wild, and failing to conceive—is certainly produced by excess of iron in their drinking-water, and I suspect also by a deficiency of potash in the soil."

He also informs me that pure white beasts of either sex are so well known by experienced breeders to be comparatively infertile together, that they are never used for breeding purposes, so that "in some parts of the country, where a tendency to sterility had become so confirmed in the white race that they utterly died out," only the coloured breeds are now to be found. He goes on to say that if "a lot of white heifers were put to a lot of white bulls, I think you would probably get a fertile breed of pure white cattle.... I think, in short, that domestication has produced just what your theory suggests, a new variety inclined to prove sterile with its parent stock."

Commenting on the origin of domesticated cattle, Professor Oscar Schmidt remarks (Doctrine of Descent, p. 139)—

"Rütimeyer's minute researches on domestic cattle have shown that, in Europe at least, three well-defined species of the diluvial period have contributed to their formation—Bos primigenius, longifrons, and frontosus. These species once lived geographically separate, but contemporaneously; and they and their specific peculiarities have perished, to rise again in our domestic races. These races breed together with unqualified fertility. In the form of skull and horns they recall one or other of the extinct species; but collectively they constitute a new main species. That from their various breeds, the three or any one of the aboriginal species would ever emerge in a state of pristine purity, would be an utterly ludicrous assertion."

Now, seeing that these "aboriginal species," although living "contemporaneously," were "geographically separate," we can well understand that their divergence of type from a common ancestor did not require, as a condition to their divergence, that any cross-sterility should have arisen between them. The geographical isolation was enough to secure immunity from mutual intercrossing, and therefore, as our present theory would have expected as probable, morphological divergence occurred without any corresponding physiological divergence, as must almost certainly have been the case if such polytypic evolution had occurred on a common area. Indeed, one of the two lines of experimental verification of our theory consists in selecting cases where nearly allied species are separated by geographical barriers, and proving that, in such cases, there is no cross-sterility.

Fertility of Domesticated Varieties.—Some writers have sought to explain the contrast between domesticated varieties and natural species in respect of fertility when crossed, by the consideration that it is only those natural species which have proved themselves so far flexible as to continue fertile under changed conditions of life that can have ever allowed themselves to become domesticated. But although this condition may well serve to explain the unimpaired fertility under domestication of such species as for this very reason have ever become domesticated, I fail to see how it explains the further and altogether different fact, that this fertility continues unimpaired between all the newly differentiated morphological types which have been derived from the original specific type. It is one thing that this type should continue fertile after domestication: it is quite another thing that fertility should continue as between all its modified descendants, even although the amount of modification may extend much further than that which usually obtains between different natural species.

Testing for Cross-infertility among varieties growing on the same area is a much more crucial line of verification than testing for unimpaired fertility between allied species which occupy different areas, because while in the former case we are dealing with "incipient species" with a view to ascertaining whether the divergence which they have already undergone is accompanied by physiological isolation, in the latter case we can never be sure that two allied species, which are now widely disconnected geographically, have always been so disconnected. They may both have originated on the same area; or one may have diverged from the other before it migrated from that area; or even if, when it migrated, it was unchanged, and if in its new home it afterwards split into two species by physiological selection, the newer species would probably prove infertile, not only with its parent type, but also with its grand-parent in any other part of the world.

Seebohm on Isolation.—Seebohm is so strongly influenced by the difficulty from "the swamping effects of free intercrossing," that he is driven by it to adopt Asa Gray's hypothesis of variations as teleological. Indeed, he goes as far as Wagner, for he maintains that in no case can there be divergence or multiplication of species without isolation. He makes the important statement that "the more the geographical distribution of birds is studied, the more doubtful it seems to be that any species of bird has ever been differentiated without the aid of geographical isolation" (Charadriidae, p. 17). If this is true, it makes in favour of physiological selection by showing the paramount importance of the swamping effects of intercrossing, and consequent importance of isolation. But it makes against physiological selection by showing that the geographical form of isolation is sufficient to explain all the cases of specific differentiation in birds. But I must remember that the latter point rests largely on negative inference, and that birds, owing to their highly locomotive habits, are the class of animals where physiological selection is likely to be most handicapped.

Herbert on Hybridization.—Herbert tells us that when he first astonished the Horticultural Society by laying before them the results of his experiments on hybridization, his brother botanists took serious alarm. For it appeared to them that this "intermixture of species would confuse the labours of botanists, and force them to work their way through a wilderness of uncertainty." Therefore he was bluntly told by several of these gentlemen, "I do not thank you for your mules." Now, although naturalists have travelled far and learnt much since those days, it appears to me that a modern evolutionist might still turn to the horticulturist with the same words. For assuredly he has no reason to thank the horticulturist for his mules, until he has found a satisfactory answer to the question why it is that natural species differ so profoundly as regards their capacity for hybridizing.

Advance on Herbert's Position.—- If it be said that all my work amounts to showing what Herbert said long ago—viz. that the only true or natural distinction between organic types is the sexual distinction—I answer that my work does much more than this. For it shows that the principle of sterility is the main condition to the differentiation, not merely of species and genera, but also to the evolution of adaptations everywhere, in higher as well as in lower taxonomic divisions. Moreover, even though naturalists were everywhere to consent to abandon specific designations, and, as Herbert advises, to "entrench themselves behind genera," there would still remain the facts of what are now called specific differences (of the secondary or morphological kind), and by whatever name these are called, they alike demand explanation at the hands of the evolutionist.

Fritz Müller on Cross-infertility.—Fritz Müller writes, "Every plant requires, for the production of the strongest possible and most prolific progeny, a certain amount of difference between male and female elements which unite. Fertility is diminished as well when this degree is too low (in relatives too closely allied) as when it is too high (in those too little related)." Then he adds, as a general rule, "Species which are wholly sterile with pollen of the same stock, and even with pollen of nearly allied stocks, will generally be fertilized very readily by the pollen of another species. The self-sterile species of the genus Abutilon, which are, on the other hand, so much inclined to hybridization, afford a good example of this theory, which appears to be confirmed also by Lobelia, Passiflora, and Oncidium" (American Naturalist, vol. viii, pp. 223-4, 1874).

Different groups of plants exhibit remarkable differences in the capability of their constituent species to hybridize.—In so far as these differences have reference only to first crosses, they have no bearing either for or against my theory. Only in so far as the differences extend to the production of fertile hybrids does any question arise for me. First of all, therefore, I must ascertain whether (or how far) there is any correlation between groups whose species manifest aptitude to form first crosses, and groups where first crosses manifest aptitude to produce fertile hybrids. Next, whatever the result of this inquiry should be, if I find that certain natural groups of plants exhibit comparatively well-marked tendencies to form fertile hybrids, the question will arise, Are these tendencies correlated with paucity of species? If they are, the fact would make strongly in favour of physiological selection. For the fact would mean that in these natural groups, owing to "the nature of the organisms" included under them, less opportunity is given to physiological selection in its work of differentiating specific types than is given by other natural groups where the nature of the organism renders them more prone to mutual sterility. But in prosecuting this branch of verification, I must remember to allow for possibilities of differential degrees of geographical isolation in the different groups compared.

On this subject Focke writes me as follows:—"In a natural group (family, order, genus) showing considerable variability in the structure of the flower, we may expect to find [or do find] a greater number of mules than in a group whose species are only distinguished by differences in the shape of the leaves, or in growth, &c. I do not know, however, which in this connexion of things is the cause and which the effect. A useful ancestral structure of the flower may be conserved by an otherwise varying progeny, on condition that the progress of diversity be not disturbed by frequent intercrossings. [Therefore, if this condition be satisfied, the structure of the flower in different members of the group will continue constant: here the cause of constancy in the flower (however much variability there may be in the leaves, &c.) is its original inability to hybridize.] On the other hand, in species or groups ready to hybridize [or capable of hybridizing], the fixation of a new specific type will require some change in the structure of the flower, and a change considerable enough to alter the conditions of fertilization. [Here the reason of the inconstancy of the flower in different members of the group is the original aptitude of their ancestral forms to hybridize.] Perhaps there is something in this suggestion, but certainly there are other efficient physiological relations, which are at present unknown. Your theory of physiological selection may serve to explain many difficult facts."

The Importance of Prepotency.—A. Kerner shows by means of his own observations on sundry species of plants which hybridize in the wild state, that they do so very much more frequently if both, or even if only one of the parent forms be rare in the neighbourhood. This fact can only be explained by supposing that, even in species most prone to hybridizing under Nature, there is some degree of prepotency of pollen of the same species over that of the other species; so that where both species are common, it is correspondingly rare that the foreign pollen gets a chance. But if there were no prepotency, the two species would blend; and this Kerner supposes must actually take place wherever two previously separated species, thus physiologically circumstanced, happen to be brought together. (Kerner's paper is published in Oester. Bot. Zeitschrift, XXI, 1871, where he alludes to sundry other papers of his own advocating similar views.)

The relation of these observations to Jordan's espèces affines is obvious. We have only to suppose that some such slight and constant difference characterizes the sexual elements of these allied varieties as demonstrably characterizes their morphology, and we can understand how pollen-prepotency would keep the forms distinct—such forms, therefore, being so many records of such prepotency.

Both from Kerner's work, and still more from that of Jordan and Nägeli, I conclude that (at all events in plants) prepotency is the way in which physiological selection chiefly acts. That is to say, sudden and extreme variations in the way of sexual incompatibility are probably rare, as compared with some degree of prepotency. According as this degree is small or great so will be the amount of the corresponding separation. This view would show that in plants the principle of physiological selection is one of immensely widespread influence, causing (on the same areas) more or less permanent varieties much below specific rank. And when we remember on how delicate a balance of physiological conditions complete correspondency of pollen to ovules depends, we may be prepared to expect that the phenomenon of prepotency is not of uncommon occurrence.

Self-fertilization and Variability.—It occurred to Count Berg Sagnitz that, if physiological selection is a true principle in nature, vegetable species in which self-fertilization obtains ought to be more rich in constant varieties than are species in which cross-fertilization rules. For, although even in the latter case physiological isolation may occasionally arise, it cannot be of such habitual or constant occurrence as it must be in the former case. Acting on this idea, Count Berg Sagnitz applied himself to ascertain whether there is any general correlation between the habit of self-fertilization and the fact of high variability; and he says that in all the cases which he has hitherto investigated, the correlation in question is unmistakable.

Additional Hypothesis concerning Physiological Selection.—In reciprocal crosses A × B is often more fertile than B × A. If hybrid AB is more fertile with A, and hybrid BA with B, than vice versa, there would be given a good analogy on which to found the following hypothesis.

Let A and B be two intergenerating groups in which segregate fecundity is first beginning. Of the hybrids, AB will be more fertile with A, and BA with B, than vice versa. The interbreeding of AB with A will eventually modify sexual characters of A by assimilating it to those of AB, while the interbreeding of BA with B will similarly modify sexual characters of B by assimilating it to those of BA. Consequently, A will become more and more infertile with B, while B becomes more and more infertile with A. Fewer and fewer hybrids will thus be produced till mutual sterility is complete.

To sustain this hypothesis it would be needful to prove experimentally, (1) that hybrid forms AB are more fertile with A than with B, while hybrid forms BA are more fertile with B than with A [or, it may be possible that the opposite relations would be found to obtain, viz. that AB would be more fertile with B, and BA with A]; (2) that, if so, effect of intercrossing AB with A is to make progeny more fertile with A than with B, while effect of intercrossing BA with B is to make progeny more fertile with B than with A.

Such experiments had best be tried with species where there is already known to be a difference of fertility between reciprocal crosses (e.g. Matthiola annua and M. glabra, see Origin of Species, p. 244).