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[235] numerous instances to show how excessively sensitive to various influences this system is. He says:[236] "Sterility is independent of general health, and is often accompanied by excess of size, or {234} 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."[237] 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.[238] 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."[239] Mr. Darwin also says: "There is reason to believe that insects are affected by confinement like the higher animals," and he gives examples.[240] 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.[241] 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 {235} 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.