THE PRESIDENT'S ADDRESS. 77 



characters, and this is perhaps the reason why, in the higher 

 organisms, non -adaptive characters are less frequently met with 

 than in lower forms. Moreover, the effect of natural selection 

 will tend to become cumulative and the rate of evolution corres- 

 pondingly increased. 



It may be objected that even in the highest organisms 

 characters often vary independently of one another, but who 

 knows how many characters are really involved in each such 

 variation ? Moreover, it by no means follows from what has 

 been said above that new characters, whether valuable or 

 otherwise, may not arise singly and remain quite independent 

 of others. 



In any case the principle of correlation could hardly help us 

 to explain the specific forms assumed by sponge microscleres, or 

 indeed the exact nature of any non-adaptive character ; it could 

 only help to explain why such characters should exist at all and 

 why they should undergo progressive evolution. 



If it be asked, what are the adaptive characters with which, 

 in our own particular case, the non-adaptive characters of 

 the microscleres are supposed to be correlated ? it must be 

 admitted that this question cannot at any rate at present be 

 answered, but it would be sufficient for the general argument if 

 it were granted that a modification in the constitution of the 

 germ -plasm which gives rise to a useful character may at the 

 same time give rise also to a useless one, or perhaps even to 

 many useless ones. 



The question, why are the specific forms of sponge microscleres 

 what they are 1 ? is probably one that will have to be answered, 

 if it ever is answered, by the chemist and physicist rather than 

 by the mere biologist ; or perhaps by that happy combination 

 of chemist, physicist and biologist whose advent is so much 

 to be desired. I have suggested that the form is probably 

 determined by the hereditary constitution of the mother-cell, 

 including its power to select silica as the raw material to be 

 worked up, but this is no more than to say that the nature of 

 the product turned out by a factory depends upon the character 

 of the work-people employed and of the machinery and raw 

 material with which they have to deal. In the case of our 

 microscleres we want to know a great deal more about the 

 nature of the machinery and the manner in which it is controlled 



