lo Plant Genetics 



the theory was being constructed the theory of natural 

 selection was widely accepted. It should be understood 

 that Weismann did not criticize natural selection, but 

 he did not believe that the individual variations upon 

 which natural selection was based were to be explained as 

 the inheritance of acquired characters. The theory of 

 germinal selection, therefore, was intended to explain the 

 origin of individual variations in some other way. The 

 explanation of the theory begins as follows : 



I believe that it is possible to suggest that the origin of heredi- 

 tary individual characters takes place in a manner quite different 



from any which has as yet been brought forward In the 



first place, it may be argued that external influences may not 

 only act on the mature individual, or during its development, 

 but that they may also act at a stUl earlier period upon the germ 

 cell from which it arises. It may be imagined that such influ- 

 ence of different kinds might produce corresponding minute 

 alterations in the molecular structure of the germ plasm ; and as 

 the latter is, according to our supposition, transmitted from one 

 generation to another, it follows that such changes would be 

 hereditary Without altogether denying that such influ- 

 ences may directly modify the germ cells, I nevertheless believe 

 that they have no share in the production of hereditary individual 

 characters Hereditary individual differences must there- 

 fore be derived from some other source. I believe that such a 

 source is to be looked for in the form of reproduction by which the 

 great majority of existing organisms are propagated, sexual 



reproduction The object of sexual reproduction is to 



create those individual differences which form the material out of 

 which natural selection produces new species. 



Weismann forestalls a criticism which was sure to 

 come by saying that the opinion has already been 

 expressed that deviations from the specific type are 

 rapidly destroyed by the operation of sexual reproduc- 



