CHAPTER VIII 



OTHER THEORIES OF EVOLUTION 



In the preceding chapter we have reviewed the evidence for 

 Natural Selection as the best documented and most elaborated 

 theory of the cause of evolution. We held that this theory- 

 is essentially one which seeks to explain (a) how a new 

 variant spreads through a population, and (b) how certain 

 types are eliminated so that group divergence results. We 

 have questioned the assumption that the whole process 

 of evolution is to be regarded as a summation of the changes 

 currently assumed to have been produced by selection — 

 whether adaptation and the major trends of evolution are the 

 product of continuous ' speciation.' This question is discussed 

 in Chapter X. We have now to ascertain what the other 

 theories of evolution are competent to explain. 



I. Lamarckism and ' the Inheritance of Induced 

 Modifications.' — The evidence on the origin of variation is 

 dealt with in Chapter II. It remains to discuss these theories 

 in their wider evolutionary bearing. It has been contended 

 that they are essentially theories which explain the origin of 

 new characters. In so far as the changes of habit and environ- 

 ment which affect individuals may also affect populations, 

 they may also be held to explain how variants multiply. How- 

 ever, those who believe that the effects of use and disuse and the 

 modification of the parental soma or of the germ cells by the 

 environment are inherited have rarely considered the question 

 whether mass transformation of this kind actually takes place. 

 As far as we know, Rensch (1929) is the only author who has 

 of recent years attempted to ascertain whether there is any 

 correlation between environmental factors and structural 

 divergence of such a nature as to satisfy the requirements of 

 this aspect of the problem. Furthermore, it has not been con- 

 sidered by what means such modification by the environment 



