We know far too little of this phenomenon as yet to be able to understand its significance, but I suppose we may anticipate with some confidence that it will be found to be a manifestation of dissimilarity between the male and female gametes of the same individual, comparable with that first seen in the Stocks (Matthiola) which throw doubles—a state of things in all likelihood to be found widely spread among hermaphrodite organisms. Whether the incompatibility between species is to be associated with that of the self-steriles also cannot be positively asserted, though it seems not unreasonable to expect that such an association will be discovered.
The case of the apple and the pear is an impressive illustration of this possibility. The two species are of course exceedingly alike in all outward respects, but nevertheless the pollen of each is entirely without effect on the other. Presumably we shouldinterpret this fact as meaning not so much that the apple and the pear are in reality very wide apart, but rather that either, each is lacking in one of two complementary elements, or that each possesses a factor with an inhibitory effect. Their incompatibility may well be of the same nature as that of the classes inCardamine pratensis.
Returning now to the problem of inter-specific sterility; we note, as I have said, the absence of contemporary evidence that variation can confer on a variety the power to form a sterile hybrid with the parent species. The considerations based on this want of evidence have for a long while been familiar to all who have discussed evolutionary theories, and it is worth observing the exact reason why the difficulty strikes us now with a new and special force. In pre-Mendelian times all that was known was that some forms could freely interbreed without diminution of fertility in the product, while others could not. But now we find that, by virtue of segregation, from one and the same pair of parents, or even, in the case of hermaphrodites, from one and the same individual, offspring commonly arises showing among themselves exactly such differences as distinguish species—and very good species too. This we see happening again and again. But to forms capable of arising as brethren in one family the title species has never been meant to apply, and if we are going to use the term in application to fraternal groups we must definitely recognise that by "specific" difference is to be understood simplydifference, without any immediate or even ulterior physiological limitation whatever. Naturally, therefore, we begin to think of the appearance of sterility in crosses as something apart, and as a manifestation which distinguishes certain kinds of unions in a very special way.
I am perfectly aware that there are gradations in the sterility of hybrids as in every other characteristic upon which it has been proposed to base specific definitions; but, as also so often happens in the matter of defining intergrading categories, the difficulty in practice is not often such as to lead to actual ambiguity. I am speaking of course of those examples which are amenable to genetic experiment. As to the rest there is complete and permanentuncertainty. But the experience of the practical breeder does, I think, on the whole, support the contention to which systematists have so steadily clung under all the assaults of evolutionary philosophers, that, though we cannot strictly define species, they yet have properties which varieties have not, and that the distinction is not merely a matter of degree.
The first step is to discover the nature of the factors which by their complementary action inhibit the critical divisions and so cause the sterility of the hybrid. Thus expressed, we see the problem of inter-specific sterility in its right place; and the question why we do not now find contemporary instances of varieties lately arisen in domestication, which when crossed back with their parents, or with their coderivatives, can produce sterile products, is perceived to be only a special case of a problem which in its more general form is that of the origin of new and additional factors.
For the requisite evidence no comprehensive search has been made, but perhaps it will yet be found. All that we can say at the present time is that the incidence both of hybrid sterility, and of incompatibility also, is most capricious; and provided that two forms have such features in common that a cross between them seems not altogether out of the question, no one can predict without experiment whether such a cross is feasible, and if feasible whether the product will be fertile, or sterile more or less completely. For instance, though probably all the British and some Foreign Finches (Fringillidae) have been crossed together, and some of these crosses, as for instance, the various Canary-mules have been made in thousands, I believe no quite clear example of a fertile hybrid can be produced. Many species of Anatidae cross readily and produce fertile hybrids: others give results uniformly sterile. Though most of the Equidae can be crossed and some of the hybrids are among the commonest of domesticated animals there is no certain record of a fertile mule. Among the Canidae the dogs, wolves and jackals all give fertile hybrids, but there is no clearly authenticated instance of a cross between any of these forms and the European fox. In spite of their close anatomical resemblance it is doubtful if the rabbitand the hare have ever interbred. Many of the wild species ofBoshave been crossed and recrossed both with each other and with many domesticated races, but I understand that no cross with the Indian buffalo (Bos bubalus) has yet been successful even in producing a live calf.[8]In the genusPrimulamany hybrids are known and several of them occur in nature, but hitherto no certain hybrid betweenP. sinensisand any other species has been made, in spite of repeated attempts.
InNicotianamany—doubtless all—the various forms ofN. tabacumcan be crossed together without diminution of fertility, though some are very distinct in appearance, but crosses betweentabacumandsylvestrisare highly sterile (in my experience totally sterile[9]), though the distinctions between them are not to outward observation nearly so great as those which can be found between the various races ofPrimula sinensis.
Recently some remarkable experiments bearing closely on these questions have been published by F. Rosen.[10]They concern the forms ofErophila (Draba) verna, celebrated in the history of evolutionary theory as the plants especially chosen by Alexis Jordan for the exposition of his views on these subjects.
The "species" contains a profusion of forms dissimilar in many structural characters, such as the size and shape of leaves, flowers, fruits, etc. Of these forms many grow in association. Jordan found, on experiment, that each, to the number of some two hundred, bred true, and that therefore, the conventional assumption that polymorphism of this kind must mean great contemporary variability had no foundation in fact. So farindeed is the evidence from favouring the belief that such forms are in any way transitional or indeterminate, that, as is well known, Jordan used it with every plausibility to support the doctrine of the fixity of species. To certain aspects of Jordan's work we will return later in this chapter, but the matter is in the present connection of especial interest for the reason that Rosen has lately found by experiment that some of these presumably very closely allied forms, crossed together, gave hybrids more or less sterile. In the case of the offspring of one pair of forms only (E. cochleataandstricta) was the fertility undiminished, and the various degrees of sterility found in the other crosses ranged up to the extreme infertility of the hybrids betweenE. stricta×elata. From this cross ten plants were bred. Of these the four strongest were chosen to breed from, but two of the four proved totally sterile; one had only bad seeds; and from the fourth a single seedling was raised which in its turn proved to be sterile. From the less sterile hybrids F2families were raised, with the usual experience that in this and subsequent generations the sterility diminished among extracted forms, new and true-breeding types with complete fertility being thus derived from the original cross.[11]
The production of sterility as a consequence of crossing plants so nearly approaching each other as theseErophila"species" do is not a little interesting, and the fact well exemplifies the futility of the various attempts to frame general expressions as to specific properties or behaviour. Commenting on his results Rosen argues that the polymorphic group commonly called by systematistsErophila (Draba) vernamay now be regarded as having arisen by crossing, as did his own types mentioned above. The question, however,whatspecies were the original progenitors of the group cannot be answered. Rosen considers that no form which he knows satisfies the requirements,and that it or they must be supposed to be lost. This conclusion will recall the similar problem raised by theOenotheramutants (Chap. V); and unsatisfactory as it may be to have recourse to such hypotheses we must remember the possibility that as a consequence of hybridisation, subsequent segregation and recombination of factors, species may have thus actually, as we may say, exploded, and left nothing but a polymorphic group of miscellaneous types to represent them in posterity. If this way of regarding the phenomena be a true one, the sterility now seen when some of the group are re-crossed, becomes analogous to that "reversion or crossing" which we now so well understand to be a consequence of the recombination of characters separated at some previous point in the history of descent. In the partial sterility of the contemporary hybrid we see this character reappearing, formed now as it was on the occasion of the original cross, by the meeting of complementary factors.
Another case that may be mentioned in this connection is that of the crosses between various culinary peas (Pisum sativum) and a peculiar form found by Mr. Arthur Sutton growing ostensibly in a wild state in Palestine. This Palestine Pea is low growing, rarely reaching 18 inches. It is in general appearance like a small and poorly grown field pea. The stems are thin and rather hard. The most obvious differences which distinguish this from other field peas are the marked serration of the stipules, and the development of pith in the pods. Such pith is often present in the pods of peas more or less, but in the Palestines it is so strongly developed as almost to form a lomentum. Curiously enough, though the flowers are purple much as those of ordinary field peas, there is no coloured spot in the axils. On the other hand, the stems have coloured stripes running up from the axils. Though this plant differs so little from domesticated peas, all crosses with them either failed, or produced hybrids quite or almost quite sterile. This was Mr. Sutton's experience, and on repeating the experiments with material kindly given by him I found the same result.[12]
In a large series of crosses some seeds died or gave rise to feeble plants. Of the plants which lived, few gave any seed. The seed, however, that was obtained from F1plants grew well enough, and the F2plants proved, as often in such cases, fertile. In these, indeed, no sign of sterility was noticeable. The experiment is being repeated in various ways, for, as the genetic behaviour of peas is comparatively well known, the subject is an exceptionally favourable one for these investigations.
Such an example shows the confusion produced the moment we attempt to harmonize conceptions of specific difference with results attained by experimental methods. It has been usual to regard the field pea (P. arvense) as a species distinct from the edible pea (P. sativum). De Candolle and others regard the field pea as derived from a form wild in Italy, but the origin of the edible pea is considered to be unknown. From breeding experiments we find no sterility whatever in the crosses between the variousarvenseandsativumtypes, nor in the crosses made between them and several other peculiar types from various countries; whereas this Palestine Pea, which only differs from a smallarvensein what might have been thought trivial characters,[13]either fails to cross altogether or gives a sterile product, whatever type be chosen as the other parent.
Examples of this kind have at least the merit that they lead to more precise delimitations of the problem. We are confronted with two distinct alternatives.
1. We may apply the term Species promiscuously to all distinct forms. If we do so it must be clearly understood that we cannot even rule out the several combinations of "presences and absences" represented by the various types whether wild or domesticated. For we may feel perfectly assured that at least all thearvenseand all thesativumtypes yet subjected to experimental tests are on precisely the same level in this respect. There is no distinction, logical or physiological, to be drawn between them. Some contain more factors, and others contain fewer. In some the re-combinations have been brought about by natural variation or crossing, while the same consequences in the others have resulted from man's interference.
2. We may follow the conventions of systematists and distinguish the outstanding or conspicuous forms such asarvense,quadratum,sativumand perhaps a few more as species, and leave the rest unheeded. If this course is followed it must be clearly understood and permitted as a piece of pure pragmatism, deliberately adopted for the convenience of cataloguers and collectors, without regard to any natural fact or system whatsoever.
But while following either the one plan or the other we shall be still awaiting the answer, which only genetic experiment can provide, to the question whether among the various types there are some which differ from the rest in a peculiar way: whether by having groups of characters linked together in especially durable combinations, or by possessing ingredients which cause greater or less disturbance in the processes of cell-division, and especially in the processes of gametic maturation, when they are united by fertilisation with complementary ingredients.
Before any but the vaguest ideas regarding the nature and significance of inter-specific sterility can be formed, a vast amount of detailed work must be done. Sterility as a result of crossing, as well as that which is alleged sometimes to arise in consequence of changed conditions, is at best a negative characteristic, and there are endless opportunities for mistake and misinterpretation in studying features of this kind. No one, I suppose, would now feel any great confidence in most of the data which from time to time are resuscitated for the purpose of such discussions. Even the best collections of evidence, such as those given by Darwin inForms of Flowers, cannot be regarded as critical when judged by present-day standards. Nothing short of the most familiar acquaintance with the habitual behaviour of individuals, and of strains kept under constant scrutiny for several years would enable the experimenter to form reliable judgments as to the value to be attached to observations of this class.
The admission must, however, be faced that nothing in recent work materially tends to diminish the surprise which has always been felt at the absence of sterility in the crosses betweenco-derivatives. We should expect such groups of forms to behave like theErophilatypes, and frequently to produce sterile products on crossing. Whatever be the explanation, the fact remains that such evidence is wanting almost completely. In spite of all that we know of variability nothing readily comparable with the power to produce a sterile hybrid on crossing with a near ally, has yet been observed spontaneously arising, though that characteristic of specificity is one of the most widely distributed in nature. It may be that the lacuna in our evidence is due merely to want of attention to this special aspect of genetic inquiry, and on the whole that is the most acceptable view which can be proposed. But seeing that naturalists are more and more driven to believe the domesticated animals and plants to be poly-phyletic in origin—the descendants, that is to say, of several wild forms—the difficulty is proportionately greater than it was formerly, when variation spontaneously occurring was regarded as a sufficient account of their diversity.
The many converging lines of evidence point so clearly to the central fact of the origin of the forms of life by an evolutionary process that we are compelled to accept this deduction, but as to almost all the essential features, whether of cause or mode, by which specific diversity has become what we perceive it to be, we have to confess an ignorance nearly total. The transformation of masses of population by imperceptible steps guided by selection, is, as most of us now see, so inapplicable to the facts, whether of variation or of specificity, that we can only marvel both at the want of penetration displayed by the advocates of such a proposition, and at the forensic skill by which it was made to appear acceptable even for a time.
In place of this doctrine we have little teaching of a positive kind to offer. We have direct perception that new forms of life may arise sporadically, and that they differ from their progenitors quite sufficiently to pass for species. By the success and maintenance of such sporadically arising forms, moreover, there is no reasonable doubt that innumerable strains, whether in isolationor in community with their co-derivatives, have as a fact arisen, which now pass in the lists of systematists as species. For an excellent account of typical illustrations I would refer the reader to the book lately published by R. E. Lloyd[14]on the rat-population of India. The observations there recorded are typical of the state of things disclosed whenever the variations of large numbers of individuals are closely investigated, whether in domestication or in natural conditions.
Guided by such clues we may get a good way into the problem. We see the origin of colourable species in abundance. Then, however, doubt arises whether though these new forms are as good species as many which are accepted as such by even cautious systematists, there may not be a stricter physiological sense in which the term species can be consistently used, which would exclude the whole mass of thesepetites espèces.
If further we find that we have, with certain somewhat doubtful exceptions, never seen the contemporary origin of a dominant factor, or of inter-racial sterility between indubitable co-derivatives, it needs no elaboration of argument to show that the root of the matter has not been reached.
Examination of the inter-relations of unquestionably distinct species nearly allied, such as the two common species ofLychnis, leads to the same disquieting conclusion, and the best suggestion we can make as to their origin is thatconceivablythey may have arisen as two re-combinations of factors brought together by the crossing of parent species, one or both of which must be supposed to be lost.
All this is, as need hardly be said, an unsatisfying conclusion. To those permanently engaged in systematics it may well bring despair. The best course for them is once for all to recognise that whether or no specific distinction may prove hereafter to have any actual physiological meaning, it is impossible for the systematist with the means at his disposal to form a judgment of value in any given case. Their business is purely that of the cataloguer, and beyond that they cannot go. They will serve science best by giving names freely and by describing everythingto which their successors may possibly want to refer, and generally by subdividing their material into as many species as they can induce any responsible society or journal to publish. Between Jordan with his 200 odd species forErophila, and Grenier and Godron with one, there is no hesitation possible. Jordan's view, as he again and again declares with vehemence, is at least a view of natural facts, whereas the collective species is a mere abstraction, convenient indeed for librarians and beginners, but an insidious misrepresentation of natural truth, perhaps more than any other the source of the plausible fallacies regarding evolution that have so long obstructed progress.
Nevertheless though we have been compelled to retreat from the speculative position to which scientific opinion had rashly advanced, the prospect of permanent progress is greatly better than it was. With the development of genetic research clear conceptions have at length been formed of the kind of knowledge required and of the methods by which it is to be attained. If we no longer see how varieties give rise to species, we may feel confident that a minute study of genetic physiology of varieties and species is the necessary beginning of any critical perception of their inter-relations. It is little more than a century since no valid distinction between a mechanical mixture and a chemical combination could be perceived, and in regard to the forms of life we may well be in a somewhat similar confusion.
As yet the genetic behaviour of animals and plants has only been sampled. When the work has been done on a scale so large as to provide generalisations, we may be in a position to declare whether specific difference is or is not a physiological reality.