156 EVOLUTION OF LIFE, 
as every one cannot work at the same trade, and the de- 
pendence of one upon another becomes very great. This is 
immediately seen if we consider the confusion that would 
arise in a city if the butchers and bakers were suddenly to 
die. But the community in general is not only differentiated 
by the Struggle for Existence into divers interests, but, sooner 
or later, the individuals are affected in the same way. For 
the individual whose organization is most specialized and 
whose functions are many is better fitted to maintain him- 
self against the changing conditions of life than one whose 
organization is more simple. But we have seen that one 
variation entails another, and that the peculiarities of the 
parents are transmitted to their offspring: hence in the 
course of generations the organization becomes extremely 
complex. Thus the Division of Labor is carried out to 
such an extent in the organization of the human body that 
it requires volumes to describe the anatomy of man. The 
Division of Labor, or a complex organization, does not 
necessarily follow from the Struggle for Existence; for there 
are animals who, when young, lead a free active life, and 
have quite a complex organization, but, growing older, 
they adopt a parasitic mode of life, and then lose many of 
their organs through disuse. Prof. Haeckel aptly observes, 
“The traveler lightens his journey who throws away his 
pack." So of many parasites: the one who first gets rid 
of any useless muscles or nerves will have the best chance 
of surviving; complex conditions of existence bring about, 
sooner or later, complex organization, while simple struc- 
tures are the result of simple conditions of existence. Sup- 
posing this view of Nature to be correct, the plants and 
animals that first appeared on the earth ought to have 
been simply and lowly organized, the later ones highly 
complex. In our chapter on Geology we have shown that 
such is the case,—that there has been a progress from 
the lower to the higher forms of life, accompanied at the 
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