DISUSE. It is familiar knowledge that continued lack of use of any particular muscles in our body by any one of us tends to make these muscles flabby and weak; in a word, these disused muscles tend to degenerate. This is not only true of muscles but of many other tissues and organs, and is not only true of parts of our body, but of body parts of other animals.
It was largely on the basis of a knowledge of this fact, and of the other related one, that muscles and most other body parts which are much used become larger and better developed, and of an assumption as a further fact that these changes in the structure and functioning of the parts of an individual acquired during the individual's lifetime through use and dis use, are directly inherited by the offspring of the individual, that Lamarck founded his famous theory of evolution. Sec articles EvotunoN, HISTORY OF and EVOLUTION, THEORIES OF.
Many animals that live in dark caves, or at great depths in the sea, or in underground burrows, have rudimentary eyes or no eyes at all, although their nearest relations above ground have well-developed eyes. Species or races of winged animals that have given up, or are giving up, flying, under conditions of do mestication, where flying is no longer necessary, as in the case of the barnyard fowl and the moths of the silkworm, have degenerating wings. In man there is a host of rudimentary, or better, degenerating, organs, such as the vermiform appendix, the skin muscles of the scalp, the wisdom teeth, etc., whose functions are no longer necessary to us, and which are consequently in a state of developing back ward toward total disappearance rather than forward toward better condition. Lamarck's explanation of this is that although these organs were useful to our ancestors of far remote times, they have gradually become less needed because of changing conditions, and hence have, through disuse and the inheritance of the degenerating effects of this disuse, come gradually to be rudimentary in the species. Oc casionally an individual is met who can use the skin muscles of scalp or the muscles of his outer ears. He represents an ancestral con dition. .
Lamarck's explanation is simple: It is plau sible. But it lacks an all-important element,
and that is the proof of the assumption that characters or changes, acquired by an individual in his lifetime, due to use or disuse, or the influence of the environment, are actually in herited. As a matter of fact, this has not only not been proved, but has been apparently disproved. Darwin, who, many years after Lamarck's theory was offered, proposed another explana tion to account for rudimentary or degenerat ing organs, accepted in some measure Lamarck's assumption of the inheritance of acquired char acters. But the Neo-Darwinians, coming after Darwin, rejected Lamarck's assumption en tirely, maintaining that there was no evidence to support it and much evidence against it. It was August Weisman, an eminent German evolutionist, who took the lead in the attack and final practical overthrow of Lamarck's evo lution explanation in general and of his ex planation of degenerating organs in particular.
The other principal explanation that has been offered of the presence in animal bodies of rudimentary organs and their increasing degeneration even to actual disappearance is in cluded in Darwin's general explanation of evolu tion, namely his theory of natural selection, based upon a rigorous struggle for existence and survival of the fittest.
If an animal species, or a group of indi viduals of the species, begins to change its habits of life so as to render less important, or even comparatively useless, certain organs of the body, as by the adoption of a cave life which renders seeing a useless function, there will be no advantage to those individuals born with better eyes and no disadvantage to those born with poorer ones. Hence, there will be no selection in the struggle for existence as be tween these two kinds of individuals, and both kinds can persist and produce young like them selves; or they can mate together and produce young with poorer eyes than the better-eyed parent with no disadavantage to the species or part of the species in the caves. Thus a de geheration of eyes and eyesight may be initiated by this miscellaneous mating or panmixia, and may be continued even to a nearly or quite complete loss of eyes by continued panmixia.