[Ill.u.s.tration: HAND OF THE POTTO (PERODICTICUS), FROM LIFE.]
Again, to antic.i.p.ate somewhat, the great group of whales (Cetacea) was fully developed at the deposition of the Eocene strata. On the other hand, we may pretty safely conclude that these animals were absent as late as the latest secondary rocks, so that their development could not have been so very slow, unless geological time is (although we shall presently see there are grounds to believe it is not) practically infinite. It is quite true that it is, in general, very unsafe to infer the absence of any animal forms during a certain geological period, because no remains of them {106} have as yet been found in the strata then deposited: but in the case of the Cetacea it is safe to do so; for, as Sir Charles Lyell remarks,[100] they are animals, the remains of which are singularly likely to have been preserved had they existed, in the same way that the remains were preserved of the Ichthyosauri and Plesiosauri, which appear to have represented the Cetacea during the secondary geological period.
[Ill.u.s.tration: SKELETON OF A PLESIOSAURUS.]
As another example, let us take the origin of wings, such as exist in birds. Here we find an arm, the bones of the hand of which are atrophied and reduced in number, as compared with those of most other Vertebrates.
Now, if the wing arose from a terrestrial or subaerial organ, this abortion of the bones could hardly have been serviceable--hardly have preserved individuals in the struggle for life. If it arose from an aquatic organ, like the wing of the penguin, we have then a singular divergence from the ordinary vertebrate fin-limb. In the ichthyosaurus, in the plesiosaurus, in the whales, in the porpoises, in the seals, and in others, we have shortening of the bones, but no reduction in the number either of the fingers or of their joints, which are, on the contrary, multiplied in Cetacea and the ichthyosaurus. And even in the turtles we have eight carpal bones and five digits, while no finger has less than two phalanges. It{107} is difficult, then, to believe that the Avian limb was developed in any other way than by a comparatively sudden modification of a marked and important kind.
[Ill.u.s.tration: SKELETON OF AN ICHTHYOSAURUS.]
How, once more, can we conceive the peculiar actions of the tendrils of some climbing plants to have been produced by minute modifications? These, according to Mr. Darwin,[101] oscillate till they touch an object, and then embrace it. It is stated by that observer, "that a thread weighing no more than the thirty-second of a grain, if placed on the tendril of the _Pa.s.siflora gracilis_, will cause it to bend; and merely to touch the tendril with a twig causes it to bend; but if the twig is at once removed, the tendril soon straightens itself. But the contact of other tendrils of the plant, or of the falling of drops of rain, do not produce these effects."[102] But some of the zoological and anatomical discoveries of late years tend rather to diminish than to augment the evidence in favour of minute and gradual modification. Thus all naturalists now admit that certain animals, which were at one time supposed to be connecting links between groups, belong altogether to one group, and not at all to the other. For example, the aye-aye[103] (_Chiromys Madagascariensis_). {108} was till lately considered to be allied to the squirrels, and was often cla.s.sed with them in the rodent order, princ.i.p.ally on account of its dent.i.tion; at the same time that its affinities to the lemurs and apes were admitted. The thorough investigation into its anatomy that has now been made, demonstrates that it has no more essential affinity to rodents than any other lemurine creature has.
[Ill.u.s.tration: THE AYE-AYE.]
Bats were, by the earliest observers, naturally supposed to have a close relationship to birds, and cetaceans to fishes. It is almost superfluous to observe that all now agree that these mammals make not even an approach to either one or other of the two inferior cla.s.ses.
{109} In the same way it has been recently supposed that those extinct flying saurians, the pterodactyles, had an affinity with birds more marked than any other known animals. Now, however, as has been said earlier, it is contended that not only had they no such close affinity, but that other extinct reptiles had a far closer one.
The _amphibia_ (_i.e._ frogs, toads, and efts) were long considered (and are so still by some) to be reptiles, showing an affinity to fishes. It now appears that they form with the latter one great group--the ichthyopsida of Professor Huxley--which differs widely from reptiles; while its two component cla.s.ses (fishes and amphibians) are difficult to separate from each other in a thoroughly satisfactory manner.
If we admit the hypothesis of gradual and minute modification, the succession of organisms on this planet must have been a progress from the more general to the more special, and no doubt this has been the case in the majority of instances. Yet it cannot be denied that some of the most recently formed fossils show a structure singularly more generalized than any exhibited by older forms; while others are more specialized than are any allied creatures of the existing creation.
A notable example of the former circ.u.mstance is offered by macrauchenia--a hoofed animal, which was at first supposed to be a kind of great llama (whence its name)--the llama being a ruminant, which, like all the rest, has two toes to each foot. Now hoofed animals are divisible into two very distinct series, according as the number of functional toes on each hind foot is odd or even. And many other characters are found to go with this obvious one. Even the very earliest Ungulata show this distinction, which is completely developed and marked even in the Eocene palaeotherium and anoplotherium found in Paris by Cuvier. The former of these has the toes odd (perissodactyle), the other has them even (artiodactyle).
Now, the macrauchenia, from the first relics of it which were found, {110} was thought to belong, as has been said, to the even-toed division.
Subsequent discoveries, however, seemed to give it an equal claim to rank amongst the perissodactyle forms. Others again inclined the balance of probability towards the artiodactyle. Finally, it appears that this very recently extinct beast presents a highly generalized type of structure, uniting in one organic form both artiodactyle and perissodactyle characters, and that in a manner not similarly found in any other known creature living, or fossil. At the same time the differentiation of artiodactyle and perissodactyle forms existed as long ago as in the period of the Eocene ungulata, and that in a marked degree, as has been before observed.
Again, no armadillo _now living_ presents nearly so remarkable a speciality of structure as was possessed by the _extinct_ glyptodon. In that singular animal the spinal column had most of its joints fused together, forming a rigid cylindrical rod, a modification, as far as yet known, absolutely peculiar to it.
[Ill.u.s.tration: DENt.i.tION OF THE SABRE-TOOTHED TIGER (MACHAIRODUS).]
In a similar way the _extinct_ machairodus, or sabre-toothed tiger, is characterized by a more highly differentiated and specially carnivorous dent.i.tion than is shown by any predacious beast of the _present day_. {111} The specialization is of this kind. The grinding teeth (or molars) of beasts are divided into premolars and true molars. The premolars are molars which have deciduous vertical predecessors (or milk teeth), and any which are in front of such, _i.e._ between such and the canine tooth. The true molars are those placed behind the molars having deciduous vertical predecessors. Now, as a dent.i.tion becomes more distinctly carnivorous, so the hindmost molars and the foremost premolars disappear. In the existing cats this process is carried so far that in the upper jaw only one true molar is left on each side. In the machairodus there is no upper true molar at all, while the premolars are reduced to two, there being only these two teeth above, on each side, behind the canine.
Now, with regard to these instances of early specialization, as also with regard to the changed estimate of the degrees of affinity between forms, it is not pretended for a moment that such facts are irreconcilable with "Natural Selection." Nevertheless, they point in an opposite direction. Of course not only is it conceivable that certain antique types arrived at a high degree of specialization and then disappeared; but it is manifest they did do so. Still the fact of this early degree of excessive specialization tells to a certain, however small, extent against a progress through excessively minute steps, whether fortuitous or not; as also does the distinctness of forms formerly supposed to const.i.tute connecting links.
For, it must not be forgotten, that if species have manifested themselves generally by gradual and minute modifications, then the absence, not in one but in _all cases_, of such connecting links, is a phenomenon which remains to be accounted for.
It appears then that, apart from fortuitous changes, there are certain difficulties in the way of accepting extremely minute modifications of any kind, although these difficulties may not be insuperable. Something, at all events, is to be said in favour of the opinion that sudden and {112} appreciable changes have from time to time occurred, however they may have been induced. Marked _races_ have undoubtedly so arisen (some striking instances having been here recorded), and it is at least conceivable that such may be the mode of _specific_ manifestation generally, the possible conditions as to which will be considered in a later chapter. [Page 113]
CHAPTER V.
AS TO SPECIFIC STABILITY.
What is meant by the phrase "specific stability;" such stability to be expected _a priori_, or else considerable changes at once.--Rapidly increasing difficulty of intensifying race characters; alleged causes of this phenomenon; probably an internal cause co-operates.--A certain definiteness in variations.--Mr. Darwin admits the principle of specific stability in certain cases of unequal variability.--The goose.--The peac.o.c.k.--The guinea fowl.--Exceptional causes of variation under domestication.--Alleged tendency to reversion.--Instances.--Sterility of hybrids.--Prepotency of pollen of same species, but of different race.--Mortality in young gallinaceous hybrids.--A bar to intermixture exists somewhere.--Guinea-pigs.--Summary and conclusion.
As was observed in the preceding chapters, arguments may yet be advanced in favour of the opinion that species are stable (at least in the intervals of their comparatively sudden successive manifestations); that the organic world consists, according to Mr. Galton"s before-mentioned conception, of many facetted spheroids, each of which can repose upon any one facet, but, when too much disturbed, rolls over till it finds repose in stable equilibrium upon another and distinct facet. Something, it is here contended, may be urged, in favour of the existence of such facets--of such intermitting conditions of stable equilibrium.
A view as to the stability of species, in the intervals of change, has been well expressed in an able article, before quoted from, as follows:[104]--"A given animal or plant appears to be contained, as it were, within a {114} sphere of variation: one individual lies near one portion of the surface; another individual, of the same species, near another part of the surface; the average animal at the centre. Any individual may produce descendants varying in any direction, but is more likely to produce descendants varying towards the centre of the sphere, and the variations in that direction will be greater in amount than the variations towards the surface." This might be taken as the representation of the normal condition of species (_i.e._ during the periods of repose of the several facets of the spheroids), on that view which, as before said, may yet be defended.
Judging the organic world from the inorganic, we might expect, _a priori_, that each species of the former, like crystallized species, would have an approximate limit of form, and even of size, and at the same time that the organic, like the inorganic forms, would present modifications in correspondence with surrounding conditions; but that these modifications would be, not minute and insignificant, but definite and appreciable, equivalent to the shifting of the spheroid on to another facet for support.
Mr. Murphy says,[105] "Crystalline formation is also dependent in a very remarkable way on the medium in which it takes place." "Beudant has found that common salt crystallizing from pure water forms cubes, but if the water contains a little boracic acid, the angles of the cubes are truncated. And the Rev. E. Craig has found that carbonate of copper, crystallizing from a solution containing sulphuric acid, forms hexagonal tubular prisms; but if a little ammonia is added, the form changes to that of a long rectangular prism, with secondary planes in the angles. If a little more ammonia is added, several varieties of rhombic octahedra appear; if a little nitric acid is added, the rectangular prism appears again. The changes take place not by the addition of new crystals, but by changing the growth of the original ones." These, however, may be said{115} to be the same species, after all; but recent researches by Dr. H.
Charlton-Bastian seem to show that modifications in the conditions may result in the evolution of forms so diverse as to const.i.tute different organic species.
Mr. Murphy observes[106] that "it is scarcely possible to doubt that the various forms of fungi which are characteristic of particular situations are not really distinct species, but that the same germ will develop into different forms, according to the soil on which it falls;" but it is possible to interpret the facts differently, and it may be that these are the manifestations of really different and distinct species, developed according to the different and distinct circ.u.mstances in which each is placed. Mr. Murphy quotes Dr. Carpenter[107] to the effect that "No _Puccinia_ but the _Puccinia rosae_ is found upon rose bushes, and this is seen nowhere else; _Omygena exigua_ is said to be never seen but on the hoof of a dead horse; and _Isaria felina_ has only been observed upon the dung of cats, deposited in humid and obscure situations." He adds, "We can scarcely believe that the air is full of the germs of distinct species of fungi, of which one never vegetates until it falls on the hoof of a dead horse, and another till it falls on cat"s dung in a damp and dark place."
This is true, but it does not quite follow that they are necessarily the same species if, as Dr. Bastian seems to show, thoroughly different and distinct organic forms[108] can be evolved one from another by modifying the conditions. This observer has brought forward arguments and facts from which it would appear that such definite, sudden, and considerable transformations may take place in the lowest organisms. If such is really the case, we might expect, _a priori_, to find in the highest organisms a tendency (much more impeded and rare in its manifestations) to {116} similarly appreciable and sudden changes, under certain stimuli; but a tendency to continued stability, under normal and ordinary conditions. The proposition that species have, under ordinary circ.u.mstances, a definite limit to their variability, is largely supported by facts brought forward by the zealous industry of Mr. Darwin himself. It is unquestionable that the degrees of variation which have been arrived at in domestic animals have been obtained more or less readily in a moderate amount of time, but that further development in certain desired directions is in some a matter of extreme difficulty, and in others appears to be all but, if not quite, an impossibility. It is also unquestionable that the degree of divergence which has been attained in one domestic species is no criterion of the amount of divergence which has been attained in another. It is contended on the other side that we have no evidence of any limits to variation other than those imposed by physical conditions, such, _e.g._, as those which determine the greatest degree of speed possible to any animal (of a given size) moving over the earth"s surface; also it is said that the differences in degree of change shown by different domestic animals depend in great measure upon the abundance or scarcity of individuals subjected to man"s selection, together with the varying direction and amount of his attention in different cases; finally, it is said that the changes found in nature are within the limits to which the variation of domestic animals extends,--it being the case that when changes of a certain amount have occurred to a species under nature, it becomes _another species_, or sometimes _two or more other species_ by divergent variations, each of these species being able again to vary and diverge in any useful direction.
But the fact of the rapidly increasing difficulty found in producing by ever such careful selection, any further extreme in some charge already carried very far (such as the tail of the "fan-tailed pigeon" or the crop of the "pouter"), is certainly, so far as it goes, on the side of the {117} existence of definite limits to variability. It is a.s.serted in reply, that physiological conditions of health and life may bar any such further development. Thus, Mr. Wallace says[109] of these developments: "Variation seems to have reached its limits in these birds. But so it has in nature.
The fantail has not only more tail-feathers than any of the three hundred and forty existing species of pigeons, but more than any of the eight thousand known species of birds. There is, of course, some limit to the number of feathers of which a tail useful for flight can consist, and in the fantail we have probably reached that limit. Many birds have the oesophagus or the skin of the neck more or less dilatable, but in no known bird is it so dilatable as in the pouter pigeon. Here again the possible limit, compatible with a healthy existence, has probably been reached. In like manner, the differences in the size and form of the beak in the various breeds of the domestic pigeon, is greater than that between the extreme forms of beak in the various genera and sub-families of the whole pigeon tribe. From these facts, and many others of the same nature, we may fairly infer, that if rigid selection were applied to any organ, we could in a comparatively short time produce a much greater amount of change than that which occurs between species and species in a state of nature, since the differences which we do produce are often comparable with those which exist between distinct genera or distinct families."
But in a domestic bird like the fantail where Natural Selection does not come into play, the tail-feathers could hardly be limited by "utility for flight," yet two more tail-feathers could certainly exist in a fancy breed if "utility for flight" were the only obstacle. It seems probable that the real barrier is an _internal_ one in the nature of the organism, and the existence of such is just what is contended for in this chapter. As to{118} the differences between domestic races being greater than those between species or even genera, that is not enough for the argument. For upon the theory of "Natural Selection" all birds have a common origin, from which they diverged by infinitesimal changes, so that we ought to meet with sufficient changes to warrant the belief that a hornbill could be produced from a humming-bird, proportionate time being allowed.
But not only does it appear that there are barriers which oppose change in certain directions, but that there are positive tendencies to development along certain special lines. In a bird which has been kept and studied like the pigeon, it is difficult to believe that any remarkable spontaneous variations would pa.s.s unnoticed by breeders, or that they would fail to be attended to and developed by some one fancier or other. On the hypothesis of _indefinite_ variability, it is then hard to say why pigeons with bills like toucans, or with certain feathers lengthened like those of trogans, or those of birds of paradise, have never been produced. This, however, is a question which may be settled by experiment. Let a pigeon be bred with a bill like a toucan"s, and with the two middle tail-feathers lengthened like those of the king bird of paradise, or even let individuals be produced which exhibit any marked tendency of the kind, and indefinite variability shall be at once conceded.
As yet all the changes which have taken place in pigeons are of a few definite kinds only, such as may be well conceived to be compatible with a species possessed of a certain inherent capacity for considerable yet definite variation, a capacity for the ready production of certain degrees of abnormality, which then cannot be further increased.
Mr. Darwin himself has already acquiesced in the proposition here maintained, inasmuch as he distinctly affirms the existence of a marked internal barrier to change in certain cases. And if this is admitted in one case, the _principle_ is conceded, and it immediately becomes probable that such internal barriers exist in all, although enclosing a much larger {119} field for variation in some cases than in others. Mr. Darwin abundantly demonstrates the variability of dogs, horses, fowls, and pigeons, but he none the less shows clearly the _very small_ extent to which the goose, the peac.o.c.k, and the guinea-fowl have varied.[110] Mr. Darwin attempts to explain this fact as regards the goose by the animal being valued only for food and feathers, and from no pleasure having been felt in it on other accounts. He adds, however, at the end the striking remark,[111] which concedes the whole position, "but the goose seems to have _a singularly inflexible organization_." This is not the only place in which such expressions are used. He elsewhere makes use of phrases which quite harmonize with the conception of a normal specific constancy, but varying greatly and suddenly at intervals. Thus he speaks[112] of a _whole organization seeming to have become plastic, and tending to depart from the parental type_. That different organisms should have different degrees of variability, is only what might have been expected _a priori_ from the existence of parallel differences in inorganic species, some of these having but a single form, and others being polymorphic.
To return to the goose, however, it may be remarked that it is at least as probable that its fixity of character is the cause of the neglect, as the reverse. It is by no means unfair to a.s.sume that _had_ the goose shown a tendency to vary similar in degree to the tendency to variation of the fowl or pigeon, it would have received attention at once on that account.
As to the peac.o.c.k it is excused on the pleas (1), that the individuals maintained are so few in number, and (2) that its beauty is so great it can hardly be improved. But the individuals maintained _have not been too few_ for the independent origin of the black-shouldered form, or for the supplanting of the commoner one by it. As to any neglect in selection,{120} it can hardly be imagined that with regard to this bird (kept as it is all but exclusively for its beauty), any spontaneous beautiful variation in colour or form would have been neglected. On the contrary, it would have been seized upon with avidity and preserved with anxious care. Yet apart from the black-shouldered and white varieties, no tendency to change has been known to show itself. As to its being too beautiful for improvement, that is a proposition which can hardly be maintained. Many consider the Javan bird as much handsomer than the common peac.o.c.k, and it would be easy to suggest a score of improvements as regards either species.
The guinea-fowl is excused, as being "no general favourite, and scarcely more common than the peac.o.c.k;" but Mr. Darwin himself shows and admits that it is a noteworthy instance of constancy under very varied conditions.
These instances alone (and there are yet others) seem sufficient to establish the a.s.sertion, that degree of change is different in different domestic animals. It is, then, somewhat unwarrantable in any Darwinian to a.s.sume that _all_ wild animals have a capacity for change similar to that existing in _some_ of the domestic ones. It seems more reasonable to a.s.sert the opposite, namely, that if, as Mr. Darwin says, the capacity for change is different in different domestic animals, it must surely be limited in those which have it least, and _a fortiori_ limited in wild animals.
Indeed, it cannot be reasonably maintained that wild species certainly vary as much as do domestic races; it is possible that they may do so, but at least this has not been yet shown. Indeed, the much greater degree of variation amongst domestic animals than amongst wild ones is a.s.serted over and over again by Mr. Darwin, and his a.s.sertions are supported by an overwhelming ma.s.s of facts and instances.
Of course, it may be a.s.serted that a tendency to indefinite change exists in all cases, and that it is only the circ.u.mstances and conditions of {121} life which modify the effects of this tendency to change so as to produce such different results in different cases. But a.s.sertion is not proof, and this a.s.sertion has not been proved. Indeed, it may be equally a.s.serted (and the statement is more consonant with some of the facts given), that domestication in certain animals induces and occasions a capacity for change which is wanting in wild animals--the introduction of new causes occasioning new effects. For, though a certain degree of variability (normally, in all probability, only oscillation) exists in all organisms, yet domestic ones are exposed to new and different causes of variability, resulting in such striking divergencies as have been observed. Not even in this latter case, however, is it necessary to believe that the variability is indefinite, but only that the small oscillations become in certain instances intensified into large and conspicuous ones. Moreover, it is possible that some of our domestic animals have been in part chosen and domesticated through possessing variability in an eminent degree.
That each species exhibits certain oscillations of structure is admitted on all hands. Mr. Darwin a.s.serts that this is the exhibition of a tendency to vary which is absolutely indefinite. If this indefinite variability _does_ exist, of course no more need be said. But we have seen that there are arguments _a priori_ and _a posteriori_ against it, while the occurrence of variations in certain domestic animals greater in degree than the differences between many wild species, is no argument in favour of its existence, until it can be shown that the causes of variability in the one case are the same as in the other. An argument against it, however, may be drawn from the fact, that certain animals, though placed under the influence of those exceptional causes of variation to which domestic animals are subject, have yet never been known to vary, even in a degree equal to that in which certain wild kinds have been ascertained to vary.
In addition to this immutability of character in some animals, it is {122} undeniable, that domestic varieties have little stability, and much tendency to reversion, whatever be the true explanation of such phenomena.
In controverting the generally received opinion as to "reversion," Mr.
Darwin has shown that it is not all breeds which in a few years revert to the original form; but he has shown no more. Thus, the feral rabbits of Porto Santo, Jamaica, and the Falkland Islands, have not yet so reverted in those several localities.[113] Nevertheless, a Porto Santo rabbit brought to England reverted in a manner the most striking, recovering the proper colour of its fur "in rather less than four years."[114] Again, the white silk fowl, in our climate, "reverts to the ordinary colour of the common fowl in its skin and bones, due care having been taken to prevent any cross."[115] This reversion taking place in spite of careful selection, is very remarkable.
Numerous other instances of reversion are given by Mr. Darwin, both as regards plants and animals; amongst others, the singular fact of bud reversion.[116] The curiously recurring development of black sheep, in spite of the most careful breeding, may also be mentioned, though, perhaps, reversion has no part in the phenomenon.
These facts seem certainly to tell in favour of limited variability, while the cases of non-reversion do not contradict it, as it is not contended that all species have the same tendency to revert, but rather that their capacities in this respect, as well as for change, are different in different kinds, so that often reversion may only show itself at the end of very long periods indeed.
Yet some of the instances given as probable or possible causes of reversion by Mr. Darwin, can hardly be such. He cites, for example, the occasional presence of supernumerary digits in man.[117] For this notion, however, he is not responsible, as he rests his remark on the authority of a {123} pa.s.sage published by Professor Owen. Again, he refers[118] to "the greater frequency of a monster proboscis in the pig than in any other animal." But with the exception of the peculiar muzzle of the Saiga (or European antelope), the only known proboscidian Ungulates are the elephants and tapirs, and to neither of these has the pig any close affinity. It is rather in the horse than in the pig that we might look for the appearance of a reversionary proboscis, as both the elephants and the tapirs have the toes of the hind foot of an odd number. It is true that the elephants are generally considered to form a group apart from both the odd and the even-toed Ungulata. But of the two, their affinities with the odd-toed division are more marked.[119]
Another argument in favour of the, at least intermitting, constancy of specific forms and of sudden modification, may be drawn from the absence of minute transitional forms, but this will be considered in the next chapter.
It remains now to notice in favour of specific stability, that the objection drawn from physiological difference between "species" and "races"