ORGANIC EVOLUTION 285 



use develops such an organ as a muscle, and that persistent disuse causes 

 it to dwindle and to lose its power of functioning, leading eventually 

 perhaps to abortion or even to suppression. If this law is conceived of 

 as applying to every organ of a plant or an animal, the results might be 

 as deep-seated and general as could be demanded by the origin of new 

 forms. 



According to this theory, the use or disuse of an organ is determined 

 by the environment. A change in the environment might shift the de- 

 mands upon the different organs, and so build up or modify some and 

 allow others to degenerate, resulting in a different kind of plant or animal. 

 This process is sometimes called " adaptation," the idea being that plants 

 and animals can " adapt " themselves to fit their environment. La- 

 marck used the neck of the giraffe as one of the striking illustrations of 

 his theory. He imagined that a grazing animal, thrust into an environ- 

 ment where feeding upon the foliage of trees became more or less neces- 

 sary, would call upon its neck in such a way that it would become some- 

 what elongated; and that the gain in length secured by any individual 

 would be transmitted to its offspring, so that generations of such animals 

 would gradually build up the enormously elongated neck of the modern 

 giraffe. Such a result would mean the transmission of small changes 

 acquired during the lifetime of an individual, and the possibility of such 

 transmission is now generally disbelieved. 



The three factors recognized by this theory are (1) a changing environ- 

 ment, (2) the effect of use and disuse, and (3) the inheritance of acquired 

 characters. The first two factors are evidently important, but they are 

 of no avail in producing new forms, according to Lamarck, unless the 

 third factor operates. 



Natural selection. — The explanation of organic evolution by means 

 of natural selection is more widely known than any other evolutionary 

 theory. Its announcement in 1858 by Charles Darwin and the appear- 

 ance in 1859 of his book entitled Origin of species by weans of natural 

 selection introduced a new epoch in scientific thought and method. 

 Modern biology, in a very real sense, may be said to date from this 

 book, and what, is called Darwinism has dominated it for nearly fifty 

 years. The enormous mass of facts, obtained from world-wide obser- 

 vations and prolonged experiments, was organized in such a convincing 

 way to support the theory that only wider observation and more careful 

 experiment could make it appear unsatisfactory. In fact, the theory of 

 natural selection as presented by Darwin led to a wide acceptance of 



