66 BASIS OF THE PKOBLEM 



2. The nucleus must not be thought of as isolated from the 

 rest of the cell ; there is a constant and active interchange between 

 the nucleus and the cytoplasm. It has actually been demonstrated 

 that at times particles stream out from the nucleus into the 

 cytoplasm. The nucleus is the centre of activity ; without the 

 nucleus the cell cannot live. This being so, how are we to 

 view the process of development from the fertilized egg to 

 the adult ? The process can be studied in detail. The egg divides 

 into two cells, then into four, eight, sixteen, thirty-two cells, and 

 so on. The gradual differentiation of the organs can be watched 

 and their lineage to certain cells in earlier stages traced. At 

 every stage the developing organism comes under the influence 

 of certain stimuli provided by the surrounding conditions. Very 

 many elements in the environment act as stimuli ; among them 

 are light, temperature, gravitation, food, and so on. Provided 

 that the environment is normal, provided, that is to say, that the 

 variations in the stimuli do not exceed certain limits, an adult 

 member of the species will be formed. What is implied in the 

 term ' normal environment ' will be more fully discussed in 

 Chapter XIV, where the results of subjection to an abnormal 

 environment will also be studied. That a large number of stimuli 

 varying within only narrow limits are necessary, will there also 

 be shown to have been proved. 



The process of development, therefore, takes the shape of the 

 growth of a particular organic form through the play of certain 

 stimuli upon the germinal constitution. The germinal constitu- 

 tion has, as we have seen, apparently to be identified with the 

 constitution of the chromosomes of the fertilized egg, of which 

 half are derived from the father and half from the mother. It is 

 known that the characters, which the organism exhibits, in some 

 manner have a basis in the germinal constitution. The number 

 of these separate characters is very large. What view then are 

 we to take of the nature of the germinal constitution ? In the 

 first place every character as seen in the organism is the result 

 of the play of certain stimuli upon something in the germinal 

 constitution. This something can only be thought of as pre- 

 dispositions to the development of certain characters under 

 certain stimuli and to the development of characters differing in 



cannot be considered as established beyond doubt. The element of doubt, how- 

 ever, has no bearing upon the argument. 



