Ill 



THE BASIS OF THE POPULATION PROBLEM: 

 (2) THE QUALITATIVE ASPECT 



1. WE have now to consider the qualitative aspect of the 

 population problem. There are two questions to which an answer 

 is required. First we ask : What is the nature of the changes 

 which occur ? In other words, if we may speak of history among 

 animals and plants, we want to know what kind of changes 

 underlie the facts which go to make up that history. The second 

 question is : How have these changes come about ? There is no 

 doubt about the answer to the first question, and it can be given 

 very shortly. Though there is a certain consensus of agreement 

 regarding the answer to the second question, there are consider- 

 able differences of opinion over points which are not unimportant, 

 and in order to deal adequately with these points a long dis- 

 cussion would be required for which there is no space. All that 

 can be done here is to set out the most important facts ; they 

 will provide a satisfactory answer to the first question and enable 

 some indication to be given of the lines which the answer to the 

 second must follow. 



However slight the treatment of the problem, it must begin 

 with a reference to the physical basis of inheritance. Some 

 description has been given of the process of fertilization. It was 

 said that only the head and middle-piece of the spermatozoon 

 penetrate the egg, and that no cytoplasm or ordinary granular 

 protoplasm can be demonstrated in these parts of the sperma- 

 tozoon. The head is the nucleus of the cell, while the middle- 

 piece contains the centrosome, a body which is attached to the 

 nucleus and plays an important part when the nucleus divides. 

 From these facts a very important conclusion follows. It is 

 known that both parents contribute equally to the offspring ; 

 therefore as the male parent contributes only a nucleus and its 

 appanage the centrosome, the basis of the inherited qualities 

 must be sought in the nucleus. 



Attention is thus directed to the nucleus, which can sometimes 



