ia flic 



DARWINISM AND HEREDITY. 43 



is the product of these two sets of factors, and can only 

 be reproduced when both are present. Only those char, 

 acters reappear regularly in successive generations which 

 depend for their development on stimuli always present 

 in the normal environment. Others, depending on a new 

 or occasional stimulus, do not reappear in the next 

 generation unless the stimulus is present. In popular 

 language the former are said to be inherited, and the 

 latter are said not to be inherited. But both are equally 

 due to factors of inheritance and to factors of environ- 

 ment ; in this respect the popular distinction between 

 acquired and not acquired characters is illusory. In 

 every case it is the capacity to acquire, to become modified 

 or to respond, which is really transmitted ; the direction 

 and extent of the modification depends on the stimulus 

 encountered. The presence of a given hereditary factor 

 cannot be determined by mere inspection of the characters 

 of an organism ; the factor may be present, but the cor- 

 responding character fail to show itself owing to the 

 absence of the necessary stimulus. On the other hand, 

 dissimilar stimuli acting on different factors may give 

 apparently similar results. Heredity must be defined 

 afresh as the transmission of the factors of inheritance, 

 and not as the reappearance of characters in successive 

 generations. 



Modifications, then, are not transmitted as such. They 

 could only be "transmitted" if the new environment 

 produced such a change in the factors of inheritance 

 themselves, that when replaced in the old environment 

 they continued to respond as if the new stimuli were still 

 present. "We will not say that such a thing is impossible ; 

 but it is in the highest degree improbable, and it is very 

 difficult to conceive how such a result could be reached. 

 At all events no case of the supposed transmission of 

 modifications has yet been brought forward which could 

 not be explained in other ways. 



