DARWINISM AND HEEEDITY. 37 



the external environment and of the characters of the 

 parents on those of their offspring. It was supposed that 

 the changes directly induced in the body of the parent by 

 such stimuli as temperature, moisture, nutrition, repeated 

 use, or exercise and disuse, are inherited as such, and 

 would reappear in the progeny. The theories of evolu- 

 tion propounded by Erasmus Darwin and by Lamarck 

 were founded on this supposition ; and even in the time 

 of Darwin it was not yet questioned whether characters 

 thus acquired by the parent in the course of its lifetime 

 are directly inherited. It was not till Weismann criti- 

 cally examined the evidence for " the inheritance of ac- 

 quired characters" that the theory was definitely over- 

 thrown. He showed convincingly that mutilations (such 

 as the repeated cutting off ojF the tail in dogs), the effects 

 of use and disuse (such as callosities produced by friction, 

 the enlargement of muscles or other organs, the fruits of 

 education, &c.), or any direct modification due to the 

 action of any particular stimulus, have never in any 

 single instance been proved to be transmitted as such 

 from one generation to another, while the evidence that 

 they are not is overwhelming. These conclusions of 

 Weismann, which had been to some extent foreshadowed 

 by Pritchard and Galton, are the most important con- 

 tribution to the science of evolution since the publication 

 of Darwin's Origin of Species. Let us now examine the 

 question more closely, and further analyse the "varia- 

 tions " dealt with above. 



Owing to that universal property of irritability or the 

 power of response to stimuli already described, all organ- 

 isms are the result of the interaction of two sets of 

 factors : the factors of inheritance, and the factors of the 

 environment. By factors of the environment we mean 

 all those conditions or stimuli which are capable of in- 

 fluencing the differentiation, growth, behaviour, or in 

 other words the metabolism of the organism. By factors 

 of the inheritance, on the contrary, we mean that com- 



