4 Proceedings of the lioyal Physical Society. 



theory of Natural Selection. This theory is the expression of the following 

 facts : — 



(1) The fact of the variability of organisms; 



(2) The fact that organisms by their multiplication tend to increase 



beyond the numbers which the conditioning factors allow to exist ; 



(3) The fact that the numbers are kept within the necessary limits by 



death rate ; 



(4) The fact that death rate is necessarily selective — the more fit tending 



to live longer and leave more descendants ; 



(5) The fact that such a selective death rate mu&t necessarily involve, 



when acting for prolonged periods, evolutionary progress of the 

 race in the direction of greater fitness — or better adaptation to 

 environment. 



These are fundamental facts which have to be admitted whatever be one's 

 view as to the potency of Natural Selection. 



The only aspect of Natural Selection open to question has to do with the 

 rate at which it produces evolutionary change : whether it is rapid enough in 

 its action to account for any considerable part of the evolutionary change 

 which has taken place in the organic world. In our present state of 

 ignorance, both as regards the rate of change which may be produced by 

 Natural Selection, 1 and as regards geological time, 2 it is clear that we are 

 not in a position to discuss this question with profit. 



In Natural Selection, then, we have a factor which is necessarily producing 

 evolutionary change. Few will doubt any more than Darwin himself doubted 

 that there are other factors at work. That Natural Selection is a very potent 

 factor is indicated by the wide-spread phenomena of adaptation. A good 

 deal is already known about such phenomena, but I think many, particularly 

 such as have spent some time observing Nature in wild parts of the Tropics, 



1 It is clear that in a large proportion of the animals in which the sexes are separate, 

 the loss of a female individual is of much greater importance to the species than the loss of 

 a male. It follows of necessity that Natural Selection will produce evolutionary change 

 more rapidly in secondary sexual characters of the female than in those of the male : or in 

 a i,dven period of time Natural Selection will produce a greater extent of evolutionary change 

 in such characters of the female than in those of the male. Consequently where such 

 characters are adaptive they will, on the whole, be more perfectly adaptive in the female 

 than in the male. This principle is well brought out in cases of mimicry in Butterflies, 

 where indeed "the female is often alone mimetic" (Poulton). The same principle 

 apparently also holds in regard to the greater variability of the female in certain groups 

 ill' Buttei'lIii'H ( l'oul ton) — mere variability being, as already indicated, a character of the 

 utmost importance I'm the persistence of the species. 



The recent discovery of Radium has afforded a good example of the kind of disturb- 

 ing factors of which there may exist any number still unknown, which must necessarily 

 render absolutely unreliable all calculations regarding geological time. 



