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, i.e. spontaneous generation does not occur. The 



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