How, then, are we to account for the fact that Darwin attributed to natural selection the power to cause divergence of character? The answer is sufficiently simple. _He does so by tacitly invoking the aid of some other form of h.o.m.ogamy in every case._ If we carefully read pp. 86-97 of the _Origin of Species_, where this subject is under consideration, we shall find that in every one of the arguments and ill.u.s.trations which are adduced to prove the power of natural selection to effect "divergence of character," he either pre-supposes or actually names some other form of h.o.m.ogamy as the originating cause of the diversity that is afterwards presented to natural selection for further intensification. To give only one example. At the starting-point of the whole discussion the priority of such other forms of h.o.m.ogamy is a.s.sumed in the following words:--
But how, it may be asked, can any a.n.a.logous principle [to that of diversity caused by artificial selection] apply in nature? I believe it can and does apply most efficiently (though it was a long time before I saw how), from the simple circ.u.mstance that the more diversified the descendants from any one species become in structure, const.i.tution, 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.
Now, without question, so soon as segregate breeding in two or more lines of h.o.m.ogamy has been in any sufficient degree determined by some "change of structure, const.i.tution, or habits," natural selection will forthwith proceed to increase the divergence in as many different lines as there are thus yielded discriminately isolated sections of the species. And this fact it must have been that Darwin really had before his mind when he argued that diversification of character is caused by natural selection, through the benefit gained by the diversified forms being thus "enabled to increase in number." Nevertheless he does not expressly state the essential point, that although diversification of character, _when once begun_, is thus _promoted_ by natural selection, which forthwith proceeds to cultivate each of the resulting branches, yet diversification of character can never be _originated_ by natural selection. The change of "structure," of "const.i.tution," of "habits," of "station," of geographical area, of reciprocal fertility, and so on--this change, _whatever_ it may have been, must clearly have been antecedent to any operation of natural selection through the benefit which arose from the change. Therefore the change must in all cases have been due, in the first instance, to some other form of isolation than the superadded form which afterwards arose from superior fitness in the possession of superior benefit--although, so long as the prior form of isolation endured, or continued to furnish the necessary condition to the co-operation of survival of the fittest, survival of the fittest would have continued to increase the divergence of character in as many ramifying lines as there were thus given to its action separate cases of isolation by other means.
In short, as divergence of character must in all cases be due to a prevention of intercrossing, and as in the process of natural selection there is, _ex hypothesi_, nothing to prevent the intercrossing until the divergence has already arisen, to suppose that natural selection alone can have caused the divergence, is to suppose that natural selection can have caused the conditions of its own activity, which is absurd.
Seeing, then, that even in cases where any "benefit" arises from divergence of character, such benefit can arise only after the divergence has already commenced, and seeing that on this as on other accounts previously mentioned it is plainly impossible to attribute the origin of such divergence to natural selection, we find that natural selection must be in all cases a.s.sisted by some other form of isolation, if it is to be concerned in polytypic as distinguished from monotypic evolution. But this does not hinder that, when it is so a.s.sisted, natural selection may become--and, I believe, does become--the most efficient of all the forms of isolation in promoting divergence of character. For, in the first place, of all the forms of isolation natural selection is probably the most energetic in promoting monotypic evolution; so that under the influence of such isolation monotypic evolution probably advances more rapidly than it does under any other form of isolation. In the second place, when polytypic evolution has been begun by any of these other forms of isolation, and natural selection then sets to work on each of the resulting branches, although natural selection is thus engaged in as many different acts of monotypic evolution as there are thus separate cases supplied to it by these other forms of isolation, the joint result of all these different acts is to hurry on the polytypic evolution which was originally started by the other forms of isolation. So to speak, natural selection is the forcing heat, acting simultaneously on each of the separate branches which has been induced to sprout by other means; and in thus rapidly advancing the growth of all the branches, it is still ent.i.tled to be regarded as the most important _single_ cause of diversification in organic nature, although we must henceforth cease to regard it as in any instance the _originating_ cause--or even so much as the _sustaining_ cause.
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 const.i.tute 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 s.e.x, 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.
Well, although I have no wish at all to disparage the biological value of these actual measurements of the range of individual variation, I must point out that they are without any value at all in the connexion which Mr. Wallace adduces them. We did not require these measurements to tell us the broad and patent fact that "no being on this earthly ball is like another all in all"--or, in less Tennysonian words, that as regards every specific structure there is a certain amount of individual variability round an average mean. Indeed, in my own paper on _Physiological Selection_--against which Mr. Wallace is here specially arguing--I expressly said, as previously remarked, "that a specific type may be regarded as _the average mean of all individual variations_." The fact of such individual variability round a specific mean has always been well known to anatomists; it const.i.tutes one of the basal pillars of the whole Darwinian theory; and is besides a matter of universal recognition as regards human stature, features, and so forth. The value of Mr. Allen"s work consists in accurately measuring the _amount_ or _range_ of individual variation; but the question of its amount or range is without relevancy in the present connexion. For the desirability of isolation as an aid to natural selection even where monotypic evolution is concerned, does not arise with any reference to the amount or range of variation: it arises with reference to the _number_ of variations which are--or are not--_similar_ and _simultaneous_. If there be a sufficient number which are both similar and simultaneous, the desirability of any co-operating form of isolation is correspondingly removed, because natural selection may then have sufficient material wherewith to overcome the adverse influence of free intercrossing, and so of itself to produce monotypic evolution. Now, variations may be numerous, similar, and simultaneous, either on account of some common cause acting on many individuals at the same time, or on account of the structures in question being more or less variable round a specific mean. In the latter case--which is the only case that Mr. Allen"s measurements have to do with--the law of averages will of course determine that half the whole number of variations in any given structure, in any given generation, will be above the mean line. But, equally of course, no one has ever denied that where, for either of these reasons, natural selection is provided with sufficient material, it is correspondingly capable of improving the specific type without the a.s.sistance of any other form of h.o.m.ogamy; so to speak, they protect themselves by their very numbers, and their superiority over others leads to their survival and acc.u.mulation. But what is the result? _The result can only be monotypic evolution._ No matter how great the number, or how great the range, of variations round an average specific mean, out of such material natural selection can never produce _polytypic_ evolution: it may _change_ the type to any extent during successive generations, and in a single line of change; but it cannot _branch_ the type, unless some other form of h.o.m.ogamy intervenes. Therefore, when Mr.
Wallace adduces the well-known fact that all structures vary more or less round a specific mean as proof that natural selection need not be incommoded by free intercrossing, but can of itself produce all the known phenomena of specific evolution, he fails to perceive that his argument refers only to one aspect of such evolution (viz. the transformation of species in time), and does not apply to the aspect with which alone my paper on _Physiological Selection_ was concerned (viz. the multiplication of species in s.p.a.ce).
The same thing may be shown in this way. It is perfectly obvious that where the improvement of type in a linear series is concerned (monotypic evolution), free intercrossing, far from being a hindrance to the process, _is the very means by which the process is accomplished_.
Improvement here ascends by successive steps, in successive generations, simply _because_ of the general intercrossing of the generally most fit with the result that the species, _as a whole_, gradually becomes transformed into another species, _as a whole_. Therefore, it would be mere fatuity in any one to adduce free intercrossing as a "difficulty"
against natural selection alone being competent to produce evolution of this kind. But where the kind of evolution is that whereby the species is _differentiated_--where it is required, for instance, to produce different structures in different portions of the species, such as the commencement of a fighting spur on the wing of a duck, or _novel_ characters of any sort in different groups of the species--free intercrossing is no longer a condition to, but an absolute preventive of, the process; and, therefore, unless checked as between each portion of the species by some form of h.o.m.ogamy other than natural selection, it must effectually inhibit any _segregation_ of specific types, or divergence of character.
Hence it is that, while no Darwinian has ever questioned the power of unaided selection to cause _improvement of character in successive generations_, in common now with not a few other Darwinians I have emphatically denied so much as the abstract possibility of selection alone causing a _divergence of character in two or more simultaneous lines of change_.
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.
The second difficulty which I have to mention as at first sight attaching to the views of Mr. Gulick and myself on the subject of Isolation is, that in an isolated section of a species Mr. Francis Galton"s law of regression in the average character of offspring to the typical character of the group through reversion or atavism (_Natural Inheritance_, p. 97) must have the effect of neutralizing the segregative influence of mere apogamy. That such, however, cannot be the case has been well shown by Mr. Gulick in his paper on _Intensive Segregation_. Without at all disputing the validity of Mr. Galton"s law, he proves that "it can hold in full force only where there is free crossing, otherwise no divergent race could ever be formed by any amount of selection and independent breeding[14]." This is so self-evident that I need not quote his demonstration of the point.
[14] _Zool. Journal Lin. Soc._, vol. xxiii. p. 313.
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 h.o.m.ogamy, 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.
CHAPTER III.
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[15].
[15] _See Nineteenth Century_, January, 1887, pp. 61, 62.
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 (if able at all) 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 circ.u.mstances 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.
Now we must remember that physiological isolation is not like those other forms of isolation (e.g. geographical) which depend for their occurrence on accidents of the environment, and which may therefore take place suddenly in a full degree of completeness throughout a large section of a species. Physiological isolation depends upon distinctive characters belonging to organisms themselves; and it would be opposed to the whole theory of descent with progressive modification to imagine that absolute sterility usually arises, in a single generation between two sections of a perfectly fertile species. Therefore evolutionists must believe that in most, if not in all cases--could we trace the history, say of any two species, which having sprung from a single parent stock on a common area, are now absolutely sterile with one another--we should find that this mutual sterility had been itself a product of gradual evolution. Starting from complete fertility within the limits of a single parent species, the infertility between derivative or divergent species, _at whatever stage in their evolution this began to occur_, must usually at first have been well-nigh imperceptible, and thenceforth have proceeded to increase stage by stage.
But, if it be true that physiological isolation between genetically allied groups must usually itself have been the product of a gradual evolution; and if, when fully evolved, it const.i.tutes a condition of the first importance to any further differentiation of these groups (by preventing fusion again into one group, more or less resembling the original parent form), do we not perceive at least a strong probability that in the lower stages of its evolution such mutual infertility must have acted as a segregating influence between the diverging types, in a degree proportional to its own development? The importance of mutual sterility as a condition to divergent evolution is not denied, _when this sterility is already present in an absolute degree_; and we have just seen that, before it can have attained to this absolute degree _it must presumably, and as a rule, itself have been the subject of a gradual development_. Does it not therefore become, on merely antecedent grounds, in a high degree probable, that from the moment of its inception this isolating agency must have played the part of a segregating cause, in a degree proportional to that of its completeness as a physiological character?
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[16], clearly enunciated in its main features by Mr. Catchpool in 1884[17], 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[18].
[16] _Nicaragua_, p. 207.
[17] _Nature_, vol. x.x.xi. p. 4.
[18] _Zool. Journal, Lin. Soc._, vol. xix. pp. 337-411 (1886); and for Mr. Gulick"s papers, _ibid._, vol. xx. pp. 189-274 (1887), vol.
xxiii. pp. 312-380 (1889). Mr. Gulick has recently drawn my attention, in a private letter, to the fact that as early as 1872 a paper of his was read at the British a.s.sociation, bearing the t.i.tle _Diversity of Evolution under one set of External Conditions_, and that here the principle of physiological segregation is stated.
Although it does not appear that Mr. Gulick then appreciated the great importance of this principle, it ent.i.tles him to claim priority.
I must next proceed to state some of the leading features of physiological selection in further detail.
It has already been shown that Darwin clearly perceived that the very general occurrence of some degree of infertility between allied species cannot possibly be attributed to the _direct_ agency of natural selection. His explanation was that the slight structural modifications entailed by the transformation of one specific type into another, so react upon the highly delicate reproductive system of the changing type as to render it in some degree infertile with its parent type. Now the theory of physiological selection begins by traversing this view. It does not, however, deny that in _some_ cases the morphological may be the prior change; but it strenuously denies that this must be so in _all_ cases. Indeed, according to my statement in 1886, the theory inclines to the view that, _as a rule_, the physiological change is prior. At the same time, the theory, as I have always stated it, maintains that it is immaterial whether, "in the majority of instances,"
the physiological change has been prior to the morphological, or vice versa; since in either case the physiological change will equally make for divergence of character.
To show this clearly the best way will be to consider the two cases separately, taking first that in which the physiological change has priority. In this case our theory regards any morphological changes which afterwards supervene as due to the independent variability which will sooner or later arise under the physiological isolation thus secured. But to whatever causes the subsequent morphological changes may be due, the point to notice is that they are as a general rule, consequent upon the physiological change. For in whatever _degree_ such infertility arises between two sections of a species occupying the same area, in that _degree_ is their interbreeding prevented, and, therefore, opportunity is given for a subsequent divergence of type, whether by the influence of independent variability alone, or also by that of natural selection, as now acting more or less independently on each of the partially separated groups. In short, all that was said in the foregoing chapters with respect to isolation in general, here applies to physiological isolation in particular; and by supposing such isolation to have been the prior change, we can as well understand the subsequent appearance of morphological divergence on continuous areas, as in other forms of isolation we can understand such divergence on discontinuous areas, seeing that even a moderate degree of cross-infertility may be as effectual for purposes of isolation as a high mountain-chain, or a thousand miles of ocean.
Here, then, are two sharply-defined theories to explain the very general fact of there being some greater or less degree of cross-infertility between allied species. The older, and hitherto current theory, supposes the cross-infertility to be but an _accident_ of specific divergence, which, therefore, has nothing to do with _causing_ the divergence. The newer theory, on the other hand, supposes the cross-infertility to have often been a necessary _condition_ to the divergence having begun at all. Let us now consider which theory has most evidence in its favour.
First of all we have to notice the very general occurrence of the fact in question. For when we include the infertility of hybrids, as well as first crosses, the occurrence of some degree of infertility between allied species is so usual that Mr. Wallace recommends experiments to ascertain whether careful observation might not prove, even of species which hybridize, "that such species, when crossed with their near allies, do always produce offspring which are more or less sterile _inter se_[19]." This seems going too far, but nevertheless it is the testimony of a highly competent naturalist to the very general occurrence of an a.s.sociation between the morphological differentiation of species and the fact of a physiological isolation. Now I regard it as little short of self-evident that this general a.s.sociation between mutual infertility and innumerable secondary, or relatively variable morphological distinctions, is due to the former having been an original and a necessary condition to the occurrence of the latter, in cases where intercrossing has not been otherwise prevented.
[19] _Darwinism_, p. 169.
The importance of physiological isolation, _when once fully developed_, cannot be denied, for it is evident that if such isolation could be suddenly destroyed between two allied species occupying a common area, they would sooner or later become fused into a common type--supposing, of course, no other form of isolation to be present. The necessity then for this physiological form of isolation in _maintaining_ a specific differentiation which has been already _attained_ cannot be disputed.
Yet it has been regarded as "Darwinian heresy" to suggest that it can have been of any important service _during the process of attainment_, or while the specific differentiation is being advanced, and this notwithstanding that the physiological change must presumably have developed _pari pa.s.su_ with the morphological, and notwithstanding that in countless cases the former is a.s.sociated with every conceivable variety of the latter.
Again, why should the physiological change be thus a.s.sociated with _every conceivable variety_ of morphological change? Throughout the length and breadth of both vegetable and animal kingdoms we find this a.s.sociation, in the great majority of cases, where new species arise.
Therefore, on the supposition that in all such cases the physiological change has been advent.i.tiously induced by the morphological changes, we have to face an apparently unanswerable question--Why should the reproductive mechanism of all organic beings have been thus arranged, as it were, to change in immediate response to the very slightest alteration in the complex harmony of "somatic" processes, which now more than ever is recognized as exercising so comparatively little influence on the _hereditary_ endowments of this mechanism? Consider the difference between a worm and the bird that is eating it, an oak tree and the gall-insect that is piercing it: are we to suppose that in all cases, no matter how greatly the types differ, they must agree in this, that when any parts of these complex structures change, ever so slightly, the reproductive system is almost certain to be advent.i.tiously affected, yet always thus affected in the same peculiar way?
If it be answered that the reproductive system is known to be very sensitive to slight changes in the external conditions of life, the answer proves too much. For though this is true, yet our opponents must acknowledge that the reproductive system is not so sensitive, _in this particular respect_, as their interpretation of the origin of specific infertility requires. The proof of this point is overwhelming, for there is the evidence from the entire range of our domesticated productions, both vegetable and animal. Here the amount of structural change, which has been slowly acc.u.mulated by artificial selection, is often much greater in amount, and incomparably more rapid, than that which has been induced between allied species by natural selection; and yet there is scarcely any indication of the reproductive system having been affected in the particular way that our opponents" theory requires. There are many instances of its having been affected in sundry other ways (chiefly, however, without any accompanying morphological change); but among all the thousands of our more or less enormously modified artificial types, there is scarcely one instance of such a peculiar s.e.xual relation between the modified descendants of a common type as so usually obtains between allied species in nature. Yet in all other respects evolutionists are bound to believe that the process of modification has been in both cases strictly a.n.a.logous. Why then this conspicuous difference with respect to the reproductive system?
The answer is simple. It has never been the object of breeders or of horticulturists to select variations in the direction of cross-infertility, for the swamping effects of intercrossing are much more easily and rapidly prevented by artificial isolation. Consequently, although they have been able to modify natural types in so many directions and in such high degrees with regard to _morphology_, there has been no accompanying physiological modification of the kind required. But in nature there is no such thing as artificial, i.e.
intentional, isolation. Consequently, on common areas it must usually happen that those changes of morphology which are a.s.sociated with cross-infertility are the only ones which can arise. Hence the very remarkable contrast between our domesticated varieties and natural species with regard to cross-infertility is just what the present theory would expect, or, indeed, require. But on any other theory it has. .h.i.therto remained inexplicable.
In particular, the contrast in question has const.i.tuted one of the main difficulties with which the theory of natural selection has. .h.i.therto 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:--