Genesis of Species
We are logically driven to this positive conception if we do not accept the view that there is no force but volition, and that all phenomena whatever are the immediate results of the action of intelligent and self-conscious will.
We have seen that the notion of sudden changes—saltatory actions in nature—has received countenance from Professor Huxley. We must conceive that these jumps are orderly, and according to law, inasmuch as the whole cosmos is such. Such orderly evolution harmonizes with a teleology derived, not indeed from external nature directly, but from the mind of man. On this point, however, more will be said in the next chapter. But, once more, if new species are not manifested by the action of external conditions upon minute indefinite individual differences, in what precise way may we conceive that manifestation to have taken place?
Are new species now evolving, as they have been from time to time evolved? If so, in what way and by what conceivable means?
In the first place, they must be produced by natural action in pre-existing material, or by supernatural action.
For reasons to be given in the next chapter, the second hypothesis need not be considered.
If, then, new species are and have been evolved from pre-existing material, must that material have been organic or inorganic?
As before said, additional arguments have lately been brought forward to show that individual organisms do arise from a basis of in-organic material only. As, however, this at the most appears to be the case, if at all, only with the lowest and most minute organisms exclusively, the process cannot be observed, though it may perhaps be fairly inferred.
We may therefore, if for no other reason, dismiss the notion that highly organized animals and plants can be suddenly or gradually built up by any combination of physical forces and natural powers acting externally and internally upon and in merely inorganic material as a base.
But the question is, how have the highest kinds of animals and plants arisen? It seems impossible that they can have appeared otherwise than by the agency of antecedent organisms not greatly different from them.
A multitude of facts, ever increasing in number and importance, all point to such a mode of specific manifestation.
One very good example has been adduced by Professor Flower in the introductory lecture of his first Hunterian Course. It is the reduction in size, to a greater or less degree, of the second and third digits of the foot in Australian marsupials, and this, in spite of the very different form and function of the foot in different groups of those animals.
A similarly significant evidence of relationship is afforded by processes of the zygomatic region of the skull in certain edentates existing and extinct.
Again, the relation between existing and recent faunas of the different regions of the world, and the predominating (though by no means exclusive) march of organization, from the more general to the more special, point in the same direction.
Almost all the facts brought forward by the patient industry of Mr. Darwin in support of his theory of “Natural Selection,” are of course available as evidence in favour of the agency of pre-existing and similar animals in specific evolution.
Now the new forms must be produced by changes taking place in organisms in, after or before their birth, either in their embryonic, or towards or in their adult, condition.
Examples of strange births are sufficiently common, and they may arise either from direct embryonic modifications or apparently from some obscure change in the parental action. To the former category belong the hosts of instances of malformation through arrest of development, and perhaps generally monstrosities of some sort are the result of such affections of the embryo. To the second category belong all cases of hybridism, of cross breed, and in all probability the new varieties and forms, such as the memorable one of the black-shouldered peacock. In all these cases we do not have abortions or monstrosities, but more or less harmonious forms often of great functional activity, endowed with marked viability and generative prepotency, except in the case of hybrids, when we often find even a more marked generative impotency.
It seems probable therefore that new species may arise from some constitutional affection of parental forms—an affection mainly, if not exclusively, of their generative system. Mr. Darwin has carefully collected numerous instances to show how excessively sensitive to various influences this system is. He says: “Sterility is independent of general health, and is often accompanied by excess of size, or great luxuriance,” and, “No one can tell, till he tries, whether any particular animal will breed under confinement, or any exotic plant seed freely under culture.” Again, “When a new character arises, whatever its nature may be, it generally tends to be inherited, at least in a temporary and sometimes in a most persistent manner.” Yet the obscure action of conditions will alter characters long inherited, as the grandchildren of Aylesbury ducks, removed to a distant part of England, completely lost their early habit of incubation, and hatched their eggs at the same time with the common ducks of the same place.
Mr. Darwin quotes Mr. Bartlett as saying: “It is remarkable that lions breed more freely in travelling collections than in the zoological gardens; probably the constant excitement and irritation produced by moving from place to place, or change of air, may have considerable influence in the matter.”
Mr. Darwin also says: “There is reason to believe that insects are affected by confinement like the higher animals,” and he gives examples.
Again, he gives examples of change of plumage in the linnet, bunting, oriole, and other birds, and of the temporary modification of the horns of a male deer during a voyage.
Finally, he adds that these changes cannot be attributed to loss of health or vigour, “when we reflect how healthy, long-lived, and vigorous many animals are under captivity, such as parrots, and hawks when used for hawking, chetahs when used for hunting, and elephants. The reproductive organs themselves are not diseased; and the diseases from which animals in menageries usually perish, are not those which in any way affect their fertility. No domestic animal is more subject to disease than the sheep, yet it is remarkably prolific.... It would appear that any change in the habits of life, whatever these habits may be, if great enough, tends to affect in an inexplicable manner the powers of reproduction.”
Such, then, is the singular sensitiveness of the generative system.
As to the means by which that system is affected, we see that a variety of conditions affect it; but as to the modes in which they act upon it, we have as yet little if any clue.
We have also seen the singular effects (in tailed Lepidoptera, &c.) of causes connected with geographical distribution, the mode of action of which is as yet quite inexplicable; and we have also seen the innate tendency which there appears to be in certain groups (birds of paradise, &c.) to develop peculiarities of a special kind.
It is, to say the least, probable that other influences exist, terrestrial and cosmical, as yet un-noted. The gradually accumulating or diversely combining actions of all these on highly sensitive structures, which are themselves possessed of internal responsive powers and tendencies, may well result in occasional repeated productions of forms harmonious and vigorous, and differing from the parental forms in proportion to the result of the combining or conflicting action of all external and internal influences.
If, in the past history of this planet, more causes ever intervened, or intervened more energetically than at present, we might a priori expect a richer and more various evolution of forms more radically differing than any which could be produced under conditions of more perfect equilibrium. At the same time, if it be true that the last few thousand years have been a period of remarkable and exceptional uniformity as regards this planet’s astronomical relations, there are then some grounds for thinking that organic evolution may have been exceptionally depressed during the same epoch.
Now, as to the fact that sudden changes and sudden developments have occurred, and as to the probability that such changes are likely to occur, evidence was given in Chapter IV.
In Chapter V. we also saw that minerals become modified suddenly and considerably by the action of incident forces—as, e.g., the production of hexagonal tabular crystals of carbonate of copper by sulphuric acid, and of long rectangular prisms by ammonia, &c.
We have thus a certain antecedent probability that if changes are produced in specific manifestation through incident forces, these changes will be sensible and considerable, not minute and infinitesimal.
Consequently, it is probable that new species have appeared from time to time with comparative suddenness, and that they still continue so to arise if all the conditions necessary for specific evolution now obtain.
This probability will be increased if the observations of Dr. Bastian are confirmed by future investigation. According to his report, when the requisite conditions were supplied, the transformations which appeared to take place (from very low to higher organisms) were sudden, definite, and complete.
Therefore, if this is so, there must probably exist in higher forms a similar tendency to such change. That tendency may indeed be long suppressed, and ultimately modified by the action of heredity—an action which would increase in force with the increase in the perfection and complexity of the organism affected. Still we might expect that such changes as do take place would be also sudden, definite, and complete.
Moreover, as the same causes produce the same effects, several individual parent forms must often have been similarly and simultaneously affected. That they should be so affected—at least that several similarly modified individuals should simultaneously arise—has been seen to be a generally necessary circumstance for the permanent duration of such new modifications.
It is also conceivable that such new forms may be endowed with excessive constitutional strength and viability, and with generative prepotency, as was the case with the black-shouldered peacock in Sir J. Trevelyan’s flock. This flock was entirely composed of the common kind, and yet the new form rapidly developed itself “_to the extinction of the previously existing breed_.”