CHAPTER II 
THE THEORY OF EVOLUTION 
One of the most important considerations in connection 
with the problem of adaptation is that in all animals and 
plants the individuals sooner or later perish and new genera- 
tions take their places. Each new individual is formed, in 
most cases, by the union of two germ-cells derived one from 
each parent. As a result of this process of intermixing, 
carried on from generation to generation, all the individuals 
would tend to become alike, unless something else should 
come in to affect the result. 
So far as our actual experience reaches, we find that the 
succeeding generations of individuals resemble each other. 
It is true that no two individuals are absolutely alike, but if a 
sufficiently large number are examined at a given time, they 
will show about the same variations in about the same pro- 
portionate numbers. Such a group of similar forms, repeat- 
ing itself in each generation, is the unit of the systematists, 
and is called a species. 
It has been said that within each species the individuals 
differ more or less from each other, but our experience 
teaches that in each generation the same kinds of variations 
occur, and, moreover, that from any one individual there may 
arise in the next generation any one of the characteristic 
variations. Certain limitations will have to be made in re- 
gard to this statement, but for the present it will suffice. 
The Law of Biogenesis states that each living thing arises 
from another living thing; that there is no life without ante- 
cedent life, z.e. spontaneous generation does not occur. The 
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