Page images
PDF
EPUB
[graphic]

seasons, either by the males alone, or by the males and females ; but I have not space here to enter on this subject.

[ocr errors]

of life, but 1 "inmllmalgolflnrhongiament, such diffe'fiafic een mainly,caussihxusexnaiiéleaion; that is, individualnraiefli'a'i‘é had, in successive generations, some slight advantage over other males, in their weapons, means of defence, or charms; and have transmitted these» anta es ' ' g. Yet, I would not wish to attribute all such sexual differences to this agency: for we see peculiarities arising and becoming attached to ‘the male sex in our domestic animals (as the wattle in male carriers, horn-like protuberances in the cocks of certain fowls, &c.), which we cannot believe to be either useful to the males in battle, or attractive to the females. We see analogous cases under nature, for instance, the tuft of hair on the breast of the turkey-cock, which can hardly be either useful or ornamental to this bird ;—indeed, had the tuft appeared under domestication, it would have been called a monstrosity.

Illustrations of the action of Natural Selection.-—In order to make it clear how, as I believe, natural selection acts, I must beg permission to give one or two imaginary illustrations. Let us take the case of a wolf, which preys on various animals, securing some by craft, some by strength, and some by fleetness; and let us suppose

that the fleetest prey, a deer for instance, had from any.

change in the country increased in numbers, or that other prey had decreased in numbers, during that season of the year when the wolf is hardest pressed for food. Under such circumstances the swiftest and slimmest wolves would have the best chance of surviving, and so be preserved or selected,—provided always that they retained strength to master their prey at this or at some other period of the year, when they might be compelled to prey on other animals. I can see no more reason to doubt this, than that man can improve the fleetness of his greyhounds by careful and methodical selection, or by that unconscious selection which results from each man trying to' keep the best dogs without any thought of modifying the breed.

[graphic]

through variation and the modifications are accumulated by natural selection for the good of the being, cause other modifications, often of the most unexpected nature.

As we see that those variations which under domesti— cation appear at any particular period of life, tend to reappear in the offspring at the same period ;—-for instance, in the shape, size, and flavour of the seeds of the many varieties of our culinary and agricultural plants; in the caterpillar and cocoon stages of the varieties of the silkworm; in the eggs of poultry, and in the colour of the down of their chickens; in the horns of our sheep and cattle when nearly adult ;—so in a state of nature, natural selection will be enabled to act on and modify organic beings at any age,'by the accumulation of variations profitable at that age, and by their inheritance at a corresponding age. If it profit a plant to have its seeds more and more widely disseminated by the wind, I can see no greater difficulty in this being effected through natural selection, than in the cotton-planter increasing and improving by selection the down in the pods on his cotton-trees. Natural'\ selection may modify and adapt the larva of an insect .\ to a score of contingencies, wholly different from those i which concern the mature insect. These modifications will no doubt af‘iect, through the laws of correlation, the \_ structure ofthe adult; and probably in the case of those ‘\ insects which live only for a few hours, and which never ‘\ feed, a large part of their structure is merely the correlated result of successive changes in the structure of their larvae. So, conversely, modifications in the adult will probably often affect the structure of the larva; but in all cases natural selection will ensure that modifications consequent on other modifications at a different period of life, shall not be in the least degree ' injurious: for if they became so, they would cause the / extinction of the species.

Natural selection will modify the structure of the 'young in relation to the parent, and of the parent in relation to the young. In social animals it will adapt the structure of each individual‘ior the. benefit of the community; if each in consequence profits by the selected change. What natural selection cannot do, is to modify the structure of one species, without giving it any advantage, for the good of anothg species; and though statements to this effect may be found in Works

of natural history, I cannot find one case which will bear investigation. A structure used only once in an animal’s whole life, if of high importance to it, might be modified to any extent by natural selection; for instance, the great jaws possessed by certain insects, used exclusively for opening the cocoon—or the hard tip to the beak of nestling 'rds, used for breaking the egg. It has been asserted, hat of the best short-beaked tumbler-pigeons more perish in the egg than are able to get out of it; so that fanciers assist in the act of hatching. Now, if nature had' to'make the beak of a full-grown pigeon very short for“ the bird’s own advantage, the process of modification would be very slow, and there would be simultaneously the most rigorous selection of the young birds within the egg, which had the most powerful and hardest beaks, for all with weak beaks would inevitably perish: or, more delicate and more easily broken shells might be selected, the thick— ness of the shell being known to vary like every other structure.

Sexual Selection.—-Inasmuch as peculiarities often appear under domestication in one sex and become hereditarily attached to that sex, the same fact probably occurs under nature, and if so, natural selection will be able to modify one sex- in its functional relations to the other sex, or in relation to wholly different habits of life in the two sexes, as is sometimes the case with insects. And this leads me to say a few words on what I call Sexual Selection. This depends, not on a struggle for existence, but on a struggle between the males fOr possession of the females; the result is not death to the unsuccessful competitor, but few or no offspring. Sexual selection is, thereforekless rigorous than natural selec~

411ml. Generallyj‘the mdst_"vig'or6us' males, those which are best fitted for their places in nature, will leave most progeny. But in many cases, victory depends not on general vigour, but on having special weapons, confined to the male sex. A hornless stag or spurless cock would have a poor chance of leaving offspring. Sexual selection by always allowing the victor to breed might surely give indomitable courage, length to the spur, and strength to the wing to strike in the spurred leg, as well as the brutal cockfighter, who knows well that he can improve his breed by careful selection of the best

cocks. How low in the scale of nature the law of battle descends, I know not; male alligators have been described as fighting, bellowing, and whirling round, ' like Indians in a war-dance, for the possession of the females; male salmons have been seen fighting all day long; male stag-beetles sometimes bear wounds from the huge mandibles of other males. The war is, perhaps, severest between the males of polygamous animals, and \ ‘ these seem oftenest provided with special weapons. The 1 males of carnivorous animals are already well armed; (

l3 ’ though to them and to others, special means of defence ‘may be given through means of sexual selection, as the mane to the lion, the shoulder-pad to the boar, land the hooked jaw to the male salmon; for the 'shield may be as important for victory, as the sword or spear.

Amongst birds, the contest is often of a more peaceful
character. All those who have attended to the subject,
believe that there is the severest rivalry betWeen the

‘ males of many species to attract by singing the females.
3The r0ck~thrush of Guiana, birds of Paradise, and some
others, congregate; and successive males display their
gorgeous plumage and perform strange antics before the
females, which, standing by as spectators, at last choose
the most attractive partner. Those who have closely
‘ attended to birds in confinement well know that they
voften take individual preferences and dislikes: thus
Sir R. Heron has described how one pied peacock was
eminently attractive to all his hen birds. It may
appear childish to attribute any effect to such appar—
ently weak means: I cannot here enter on the details
necessary to support this view; but if man can ,5 in a
short time give elegant carriage and beauty to his
vbantams, according to his standard of beauty, I can see
no good reason to doubt that female birds, by selecting,
during thousands of generations, the most melodious
or beautiful males, according to their standardof beauty,
might produce a marked effect. I strongly suspect that
some well-known laws, with respect to the plumage of
male and female birds, in comparison with the plumage
of the young, can be explained on the view of plumage
having been chiefly modified by sexual selection, acting
when the birds have come to the breeding age or
during the breeding season; the modifications thus
produced being inherited at corresponding ages or

seasons, either by the males alone, or by the males and females ; but I have not space here to enter on this subject.

Thus it is, IMMEhekthMMd legal/eszeqxnimal havedthe' same~generaLghabits of life, but ifferiistructurengolflnrhor @ament, such différ'fifc'esrhave‘been mainlyweaursedjyjexual'ielfiion ; that is, individualmafesii'ave had, in successive generations, some slight advantage over other males, in their weapons, means of defence, or charms; and have transmittg these~ads@n_taggs,to_their_male_ofispring. Yet, would not wish to attribute all such sexual differences to this agency: for we see peculiarities arising and becoming attached to the male sex in our domestic animals (as the wattle in male carriers, horn-like protuberances in the cocks of certain fowls, &c.), which we cannot believe to be either useful to the males in battle, 0r attractive to the females. We see analogous cases under nature, for instance, the tuft of hair on the breast of the turkey-cock, which can hardly be either useful or ornamental to this bird ;~indeed, had the tuft appeared under domestication, it would have been called a monstrosity.

Illustrations of the action 0/ Natural Selection.—In order to make it clear how, as I believe, natural selection acts, I must beg permission to give one or two imaginary illustrations. Let us take the case of a wolf, which preys on various animals, securing some by craft, some by strength, and some by fleetness; and let us suppose that the fleetest prey, a deer for instance, had from any. change in the country increased in numbers, or that other prey had decreased in numbers, during that season of the year when the wolf is hardest pressed for food. Under such circumstances the swiftest and slimmest wolves would have the best chance of surviving, and so be preserved or selected,—provided always that they retained strength to master their prey at this or at some other period of the year, when they might be compelled to prey on other animals. I can see no more reason to doubt this, than that man can improve the fleetness of his greyhounds by careful and methodical selection, or by that unconscious selection which results from each man trying to‘ keep the best dogs without any thought of modifying the breed.

« PreviousContinue »