CHAPTER X.

USE OR DISUSE OF PARTS AND ADAPTATIONS OF ORGANS.

ACCORDING to Darwin, the use and disuse. of parts, and the habits due thereto, with their consequences, are transmitted by parents to their offspring, and thus what were at first slight differences accumulated from generation to generation until they ultimately become specific variations; and conversely, by disuse organs once perfect degenerate, and come to be represented in some cases only by fragmentary remains.

It is not questioned that differences in expression of type, limited in extent, do arise from various causes. Every organ has its allotted share of the life-force of an animal, and with normal use develops normally. Experience teaches that an organ may be developed abnormally by excessive use-apparently by transfer to it of part of the life-force of some other part of the or

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ganism; for while the organ thus reinforced develops abnormal size, strength, keenness, or sensibility, as the case may be, the organs despoiled fail to attain normal development. The energy of every life-force is limited, and if one organ benefits by appropriating a portion of the energy properly belonging to another, the advantage is not a clear gain to the animal, but merely a transference of energy from one organ to another. Athletes sometimes break down under excessive training: they have by excessive use exhausted their strength or the elasticity of the muscles excessively used.

When only certain parts of the body are developed by excessive use, other parts are abnormally weak. The senses of touch and hearing become highly developed in the blind, by transference of vital energy from the disused organs to those called into excessive use. If a cow yields milk largely, she will not put on flesh, and hence, as a rule, good milch cows are thin. On the other hand, the arm of the Indian fakir becomes withered by persistent disuse.

But all such modifications are differences only in degree, and do not disclose any tendency towards the specific variation that constitutes a new species. Moreover, the

means adopted to bring about these differences are only applicable to man, or to animals under his control.

No specific modification arises in the organs of wild animals, when brought under what seem to be more favourable conditions of existence, and such trifling modifications in expression of type as do arise, come within a limited space of time. When an animal is first exposed to an adverse climate, its protection from the weather will develop; but the development will probably be as much in the first season as it ever will be: it is not indefinite, and there is no tendency to specific variation.

Darwin attempts to prove the existence of a general law of Nature, that use and disuse of parts are attended by modifications that lead to specific variation; but although use and disuse do produce slight differences in man, and in animals under his influence, the cases among wild animals where use has not caused development, nor disuse degeneration, are sufficiently numerous to prove that there is no general law of Nature such as he seeks to establish.

Certain fishes found in caves are blind, and in some cases have only fragments of eyes; but then reptiles, and even rats, living

in the same caves, have fully developed eyes that can see after becoming accustomed to light.

According to Darwin, the inability of the domestic duck to fly is the result of disuse through domestication; but then the loggerheaded wild duck of South America does not fly. It can only flap its wings along the surface of the water.

A large proportion of the beetles in Madeira have no wings, and Darwin argues that the reason is because beetles of indolent habits or with imperfect wings flew least, and therefore were not so often carried to sea as those of more active habits: thus partly by disuse and partly by selection many genera of beetles in Madeira became wingless.

"But," Darwin continues, "the wings of beetles that must use them to live are not reduced but even enlarged, and this is compatible with natural selection.” 1

Those beetles that continued to battle with the wind had, it would appear, their wings enlarged, and survived, although this seems at variance with the preceding argument, and with the practice of the sailor, who takes in a reef when the wind becomes too strong for the safety of his craft.

1 Origin of Species, Ed. vi., p. 109.

Upland geese, that do not frequent water, have webbed feet although they never swim. The water - hen lives habitually in water, but has not developed webbed feet like the duck, neither has the water-ousel, although it gets its food by diving. The ostrich frequently uses its wings, but has not developed the faculty of flight, and Darwin explains the failure in these words: "A moment's reflection will show what an enormous supply of food would be necessary to give this huge bird of the desert force to move its huge body through the air"; that is to say, in this case use did not develop cause failed of its normal effectbecause of the consequence. But this is an argument in favour of design and against Darwin's theory.

These examples of use and disuse to bring about modifications of structure conclusively prove that there is no general law that use or disuse of parts develops specific variation.

Acquired Habits.

Darwin does not give any examples of animals acquiring habits.

He assumes that certain habits have been