Read Ebook: Darwin and After Darwin Volume 3 of 3 Post-Darwinian Questions: Isolation and Physiological Selection by Romanes George John

Font size: 10 px 15 px 20 px 25 px 30 px 35 px

Background color: BlackWhiteGrayLight GrayDark GrayDim Gray

Text color: BlackWhiteGrayLight GrayDark GrayDim Gray

Apply/Save settings

Ebook has 358 lines and 60439 words, and 8 pages

GENERAL CONCLUSIONS 144

ISOLATION.

The importance of isolation as against dissimilar forms has always been fully appreciated by breeders, fanciers, horticulturists, &c., who are therefore most careful to prevent their pedigree productions from intercrossing with any other stock. Isolation is indeed, as Darwin has observed, "the corner-stone of the breeder's art." And similarly with plants and animals in a state of nature: unless intercrossing with allied forms is prevented, the principle of heredity is bound to work for uniformity, by blending the dissimilar types in one: only when there is exclusive breeding of similarly modified forms can the principle of heredity work in the direction of change--i.e. of evolution.

In order that we may set out with clearer views upon this matter, I will make one or two preliminary remarks on the more general facts of isolation as these are found to occur in nature.

With the exception of Mr. Gulick, I cannot find that any other writer has hitherto stated this supremely important distinction between isolation as discriminate and indiscriminate. But he has fully as well as independently stated it, and shown in a masterly way its far-reaching consequences. Indiscriminate isolation he calls Separate Breeding, while discriminate isolation he calls Segregate Breeding. For the sake, however, of securing more descriptive terms, I will coin the words Apogamy and Homogamy. Apogamy, of course, answers to indiscriminate isolation, or separate breeding. Homogamy, on the other hand, answers to discriminate isolation, or segregate breeding: only individuals belonging to the same variety or kind are allowed to propagate. Isolation, then, is a genus, of which Apogamy and Homogamy are species.

Now, in order to appreciate the unsurpassed importance of isolation as one of the three basal principles of organic evolution, let us begin by considering the discriminate species of it, or Homogamy.

To state the case in the most general terms, we may say that if the other two basal principles are given in heredity and variability, the whole theory of organic evolution becomes neither more nor less than a theory of homogamy--that is, a theory of the causes which lead to discriminate isolation, or the breeding of like with like to the exclusion of unlike. For the more we believe in heredity and variability as basal principles of organic evolution, the stronger must become our persuasion that discriminate breeding leads to divergence of type, while indiscriminate breeding leads to uniformity. This, in fact, is securely based on what we know from the experience supplied by artificial selection, which consists in the intentional mating of like with like to the exclusion of unlike.

The point, then, which in the first instance must be firmly fastened in our minds is this:--so long as there is free intercrossing, heredity cancels variability, and makes in favour of fixity of type. Only when assisted by some form of discriminate isolation, which determines the exclusive breeding of like with like, can heredity make in favour of change of type, or lead to what we understand by organic evolution.

Now the forms of discriminate isolation, or homogamy, are very numerous. When, for example, any section of a species adopts somewhat different habits of life, or occupies a somewhat different station in the economy of nature, homogamy arises within that section. There are forms of homogamy on which Darwin has laid great stress, as we shall presently find. Again, when for these or any other reasons a section of a species becomes in any small degree modified as to form or colour, if the species happens to be one where any psychological preference in pairing can be exercised--as is very generally the case among the higher animals--exclusive breeding is apt to ensue as a result of such preference; for there is abundant evidence to show that, both in birds and mammals, sexual selection is usually opposed to the intercrossing of dissimilar varieties. Once more, in the case of plants, intercrossing of dissimilar varieties may be prevented by any slight difference in their seasons of flowering, of topographical stations, or even, in the case of flowers which depend on insects for their fertilization, by differences in the instincts and preferences of their visitors.

But, without at present going into detail with regard to these different forms of discriminate isolation, there are still two others, both of which are of much greater importance than any that I have hitherto named. Indeed, these two forms are of such immeasurable importance, that were it not for their virtually ubiquitous operation, the process of organic evolution could never have begun, nor, having begun, continued.

Natural selection, then, is thus unquestionably a form of isolation of the discriminate kind; and therefore, notwithstanding its unique importance in certain respects, considered as a principle of organic evolution it is less fundamental--and also less extensive--than the principle of isolation in general. In other words, it is but a part of a much larger whole. It is but a particular form of a general principle, which, as just shown, presents many other forms, not only of the discriminate, but likewise of the indiscriminate kind. Or, reverting to the terminology of logic, it is a sub-species of the species Homogamy, which in its turn is but a constituent part of the genus Isolation.

So much then for homogamy, or isolation of the discriminate order. Passing on now to apogamy, or isolation of the indiscriminate kind, we may well be disposed, at first sight, to conclude that this kind of isolation can count for nothing in the process of evolution. For if the fundamental importance of isolation in the production of organic forms be due to its segregation of like with like, does it not follow that any form of isolation which is indiscriminate must fail to supply the very condition on which all the forms of discriminate isolation depend for their efficacy in the causing of organic evolution? Or, to return to our concrete example, is it not self-evident that the farmer who separated his stock into two or more parts indiscriminately, would not effect any more change in his stock than if he had left them all to breed together?

As a matter of fact, we find that no one individual "is like another all in all"; which is another way of saying that a specific type may be regarded as the average mean of all its individual variations, any considerable departure from this average being, however, checked by intercrossing.... Consequently, if from any cause a section of a species is prevented from intercrossing with the rest of its species, we might expect that new varieties should arise within that section, and that in time these varieties should pass into new species. And this is just what we do find.

e. g. p. 81.

For, in point of fact, Mr. Gulick was led to his recognition of the principle in question, not by any deductive reasoning from general principles, but by his own particular and detailed observations of the land mollusca of the Sandwich Islands. Here there are an immense number of varieties belonging to several genera; but every variety is restricted, not merely to the same island, but actually to the same valley. Moreover, on tracing this fauna from valley to valley, it is apparent that a slight variation in the occupants of valley 2 as compared with those of the adjacent valley 1, becomes more pronounced in the next--valley 3, still more so in 4, &c., &c. Thus it was possible, as Mr. Gulick says, roughly to estimate the amount of divergence between the occupants of any two given valleys by measuring the number of miles between them.

As already stated, I have myself examined his wonderful collection of shells, together with a topographical map of the district; and therefore I am in a position to testify to the great value of Mr. Gulick's work in this connexion, as in that of the utility question previously considered. The variations, which affect scores of species, and themselves eventually run into fully specific distinctions, are all more or less finely graduated as they pass from one isolated region to the next; and they have reference to changes of form and colour, which in no one case presents any appearance of utility. Therefore--and especially in view of the fact that, as far as he could ascertain, the environment in the different valleys was essentially the same--no one who examines this collection can wonder that Mr. Gulick attributes the results which he has observed to the influence of apogamy alone, without any reference to utility or natural selection.

I have laid special stress on this particular case of the Sandwich Islands' mollusca, because the fifteen years of labour which Mr. Gulick has devoted to their exhaustive working out have yielded results more complete and suggestive than any which so far have been forthcoming with regard to the effects of isolation in divergent evolution. But, if space permitted, it would be easy to present abundance of additional facts from other sources, all bearing to the same conclusion--namely, that as a matter of direct observation, no less than of general reasoning, any unprejudiced mind will concede to the principle of indiscriminate isolation an important share in the origination of organic types. For as indiscriminate isolation is thus seen sooner or later to become discriminate, and as we have already seen that discriminate isolation is a necessary condition to all or any modification, we can only conclude that isolation in both its kinds takes rank with heredity and variability as one of the three basal principles of organic evolution.

Having got thus far in the way of generalities, we must next observe sundry further matters of comparative detail.

This term may here be taken as equivalent to Isolation.

I may here quote from Mr. Gulick's paper three propositions, serving to state three large and general bodies of observable fact, which severally and collectively go to verify, with an overwhelming mass of evidence, the conclusion previously reached on grounds of general reasoning.

The facts of geographical distribution seem to me to justify the following statements:--

A species exposed to different conditions in the different parts of the area over which it is distributed, is not represented by divergent forms when free interbreeding exists between the inhabitants of the different districts. In other words, Diversity of Natural Selection without Separation does not produce divergent evolution.

We find many cases in which areas, corresponding in the character of the environment, but separated from each other by important barriers, are the homes of divergent forms of the same or allied species.

In cases where the separation has been long continued, and the external conditions are the most diverse in points that involve diversity of adaptation, there we find the most decided divergences in the organic forms. That is, where Separation and Divergent Selection have long acted, the results are found to be the greatest.

The 1st and 3rd of these propositions will probably be disputed by few, if by any. The proof of the 2nd is found wherever a set of closely allied organisms is so distributed over a territory that each species and variety occupies its own narrow district, within which it is shut by barriers that restrain its distribution while each species of the environing types is distributed over the whole territory. The distribution of terrestrial molluscs on the Sandwich Islands presents a great body of facts of this kind.

ISOLATION .

I will now recapitulate the main doctrines which have been set forth in the foregoing chapter, and then proceed to consider the objections which have been advanced against them.

It must be remembered that by isolation I mean exactly what Mr. Gulick does by "Segregation," and approximately what Professor Weismann does by "Amixia "--i. e. the prevention of intercrossing.

Isolation occurs in very many forms besides the geographical, as will be more fully shown at the end of this chapter; and in all its forms it admits of degrees.

It also occurs in two very different species or kinds--namely, discriminate and indiscriminate. These I have called respectively Homogamy and Apogamy. This all-important distinction has been clearly recognized by Mr. Gulick, as a result of his own thought and observation, independently of anything that I have published upon the subject.

In view of this distinction Isolation takes rank with Heredity and Variability as one of the most fundamental principles of organic evolution. For, if these other two principles be granted, the whole theory of descent resolves itself into an inquiry touching the causes, forms, and degrees of Homogamy.

But how, it may be asked, can any analogous principle apply in nature? I believe it can and does apply most efficiently , from the simple circumstance that the more diversified the descendants from any one species become in structure, constitution, and habits, by so much will they be better enabled to seize on many and widely diversified places in the polity of nature, and so be enabled to increase in numbers.

So much by way of summary and recapitulation. I will now briefly consider the only objections which, so far as I can see, admit of being brought against the foregoing doctrine of Isolation as held by Mr. Gulick and myself. These possible objections are but two in number--although but one of them has been hitherto adduced. This, therefore, I will take first.

Mr. Wallace, with his customary desire to show that natural selection is everywhere of itself capable of causing organic evolution, seeks to minimize the swamping effects of free intercrossing, and the consequent importance of other forms of isolation. His argument is as follows.

Alluding to the researches of Mr. J. A. Allen, and others, on the amount of variation presented by individuals of a species in a state of nature, Mr. Wallace shows that, as regards any given part of the animal under consideration, there is always to be found a considerable range of individual variation round the average mean which goes to constitute the specific character of the type. Thus, for example, Mr. Allen says of American birds, "that a variation of from fifteen to twenty per cent. in general size, and an equal degree of variation in the relative size of different parts, may be ordinarily expected among specimens from the same species and sex, taken at the same locality, while in some cases the variation is even greater than this." Now, Mr. Wallace is under the impression that these facts obviate the difficulty which arises from the presence of free intercrossing--the difficulty, that is, against the theory of natural selection when natural selection is supposed to have been the exclusive means of modification. For, as he says, "if less size of body would be beneficial, then, as half the variations in size are above and half below the mean or existing standard of the species, there would be ample beneficial variations"; and similarly with regard to longer or shorter legs, wings, tails, &c., darker or lighter colour, and so on through all the parts of any given organism.

And, although these opposite views cannot be reconciled, I am under the impression that they do admit of being explained. For I take them to indicate a continued failure to perceive the all-important distinction between evolution as monotypic and polytypic. Unless one has fully grasped this distinction, and constantly holds it in mind, he is not in a position to understand the "difficulty" in question; nor can he avoid playing fast and loose with natural selection as possibly the sole cause of evolution, and as necessarily requiring the co-operation of some other cause. But if he once clearly perceives that "evolution" is a logical genus, of which the monotypic and the polytypic forms are species, he will immediately escape from his confusion, and find that while the monotypic form may be caused by natural selection alone the polytypic form can never be so caused.

In conclusion, then, and having regard to the principle of isolation as a whole, or in all the many and varied forms in which this principle obtains, I trust that I have redeemed the promise with which I set out--viz. to show that in relation to the theory of descent this principle is of an importance second to no other, not even excepting heredity, variability, and the struggle for existence. This has now been fully shown, inasmuch as we have clearly seen that the importance of the struggle for existence, and consequent survival of the fittest, arises just because survival of the fittest is a form, and a very stringent form, of isolation; while, as regards both heredity and variability, we are now in a position to see that the more fully we recognize their supreme importance as principles concerned in organic evolution, the more must we also recognize that any rational theory of such evolution becomes, in the last resort, a theory of the different modes in which efficient isolation can be secured. For, in whatever degree the process of organic evolution has been dependent upon heredity with variability, in that degree must it also have been dependent upon the means of securing homogamy, whereby alone the force of heredity can be made to expend itself in the innumerable directions of progressive change, instead of continually neutralizing the force of variability by promiscuous intercrossing.

PHYSIOLOGICAL SELECTION.

So far we have been concerned with the principle of Isolation in general. We have now to consider that form of isolation which arises in consequence of mutual infertility between the members of any group of organisms and those of all other similarly isolated groups occupying simultaneously the same area.

Against the view that natural selection is a sufficient explanation of the origin of species, there are two fatal difficulties: one, the contrast between natural species and domesticated varieties in respect of cross-sterility; the other, the fact that natural selection cannot possibly give rise to polytypic as distinguished from monotypic evolution. Now it is my belief that the theory of physiological selection fully meets both these difficulties. Indeed I hold this to be undeniable in a formal or logical sense: the only question is as to the evidence which can be adduced for the theory in a practical or biological sense. Therefore in this chapter, where the theory has first of all to be stated, I shall restrict the exposition as much as possible to the former, leaving for subsequent consideration the biological side.

The following is a brief outline sketch of this theory.

Of all parts of those variable objects which we call organisms, the most variable is the reproductive system; and the variations may carry with them functional changes, which may be either in the direction of increased or of diminished fertility. Consequently variations in the way of greater or less fertility frequently take place, both in plants and animals; and probably, if we had adequate means of observing this point, we should find that there is no one variation more common. But of course where infertility arises--whether as a result of changed conditions of life, or, as we say, spontaneously--it immediately becomes extinguished, seeing that the individuals which it affects are less able to propagate and to hand on the variation. If, however, the variant, while showing some degree of infertility with the parent form, continues to be as fertile as before when mated with similar variants, under these circumstances there is no reason why such differential fertility should not be perpetuated.

Stated in another form this suggestion enables us to regard many, if not most, species as the records of variations in the reproductive systems of their ancestors. When variations of a non-useful kind occur in any of the other systems or parts of organisms, they are, as a rule, immediately extinguished by intercrossing. But whenever they arise in the reproductive system in the way here suggested, they tend to be preserved as new natural varieties, or incipient species. At first the difference would only be in respect of the reproductive systems; but eventually, on account of independent variation, other differences would supervene, and the variety would take rank as a true species.

Whoever answers this question in the affirmative will have gone most of the way towards accepting, on merely antecedent grounds, the theory of physiological selection. And therefore it is that I have begun this statement of the theory by introducing it upon these grounds, thereby hoping to show how extremely simple--how almost self-evident--is the theory which it will now be my endeavour to substantiate. I may here add that the theory was foreshadowed by Mr. Belt in 1874, clearly enunciated in its main features by Mr. Catchpool in 1884, and very fully thought out by Mr. Gulick during a period of about fifteen years, although he did not publish until a year after the appearance of my own paper in 1886.

I must next proceed to state some of the leading features of physiological selection in further detail.

In particular, the contrast in question has constituted one of the main difficulties with which the theory of natural selection has hitherto had to contend, not only in the popular mind, but also in the judgement of naturalists, including the joint-authors of the theory themselves. Thus Darwin says:--

The fertility of varieties is, with reference to my theory, of equal importance with the sterility of species, for it seems to make a broad and clear distinction between varieties and species.

And Mr. Wallace says:--

One of the greatest, or perhaps we may say the greatest, of all the difficulties in the way of accepting the theory of natural selection as a complete explanation of the origin of species, has been the remarkable difference between varieties and species in respect of fertility when crossed.

Mr. Wallace meets the difficulty by arguing that sterility between allied species may have been brought about by the direct influence of natural selection. But, as previously remarked, this view is expressly opposed to that of Darwin, who held that Wallace's contention is erroneous.

It will be seen, then, that both Darwin, and Wallace, fully recognize the necessity of finding some explanation of the infertility of allied species, over and above the mere reaction of morphological differentiation on the physiology of the reproductive system, and they both agree in suggesting additional causes, though they entirely disagree as to what these causes are. Now, the theory of physiological selection likewise suggests an additional cause--or, rather, a new explanation--and one which is surely the most probable. For what is to be explained? The very general association of a certain physiological peculiarity with that amount of morphological change which distinguishes species from species, of whatever kind the change may be, and in whatever family of the animal or vegetable kingdom it may occur. Well, the theory of physiological selection explains this very general association by the simple supposition that, at least in a large number of cases, it was the physiological peculiarity which first of all led to the morphological divergence, by interposing the bar of sterility between two sections of a previously uniform species; and by thus isolating the two sections one from another, started each upon a subsequently independent course of divergent evolution.

Share on: WhatsApp @Hash Facebook Tweet