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 (i) 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 means 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 
