42 DARWINISM AND THE PROBLEMS OF LIFE 



the whole of our living organisms from the simplest 

 forms ? 



We may at once say that we answer in the affirmative, 

 and that we will justify our answer in the following 

 chapters. We shall even go further, and endeavour 

 to prove that natural selection is the sole principle 

 to be used in explaining the evolution of living things. 

 There are other theories in regard to the transforming 

 agencies in the organic world, and one of the chief of 

 these theories was expounded by Lamarck before 

 Darwin's time, and accepted by Darwin himself. This 

 theory affirms that external influences, such as cold or 

 heat, have a modifying action on a species of animals, 

 and that the modification they produce may be 

 transmitted to offspring. It further affirms that an 

 organ may be strengthened by exercise, and that this 

 improvement in the organ is handed on to offspring, 

 and thus by continued use a change of the organ may 

 be brought about. According to Lamarck, these factors 

 may go so far as to produce new species. 



For the moment we will refrain from testing the 

 correctness of this hypothesis, and will return to the 

 general starting-point that led us to consider natural 

 selection and the theory of evolution. But first of all let 

 us put more clearly and succinctly what we have seen 

 as to the changes of species by natural selection. 



Natural selection demands two general conditions for 

 exerting its influence. In the first place, the offspring 

 of a parental couple must differ in some degree from 

 each other, or there must be variations ; in the second 

 place, the characteristics of the parents must be trans- 



