412 ORIGINATIVE FACTORS IN EVOLUTION: 



fluctuations or minute variations in the Darwinian sense, 

 but the recent work of Castle (1916), for instance, shows 

 that it is in some cases demonstrable. 



It is a curious fact that one of the reasons why Darwin 

 attached little importance to sports or mutations was his 

 belief that they would be swamped in the inter-crossing. 

 In reality they are highly transmissible. When they come 

 they often come to stay unless they are pathological on the 

 one hand, or too superlative, like geniuses, on the other. 

 What is desirable at present is more evidence of the trans- 

 missibility of the small fluctuations of germinal origin — a 

 transmissibility which Darwin assumed without ques- 

 tion. 



To emphasise the contrast between fluctuating or con- 

 tinuous variations, and saltatory or discontinuous mutations, 

 we may quote a couple of vivid sentences from one of Samuel 

 Eutler's Essays. 



When circumstances are changing, an ^^ organism must 

 act in one or other of these two ways : It must either change 

 slowly and continuously with the surroundings, paying cash 

 for everything, meeting the smallest change with a cor- 

 responding modification so far as is found convenient ; or 

 it must put off change as long as possible, and then make 

 larger and more sweeping changes ". 



" It may be questioned whether what is called a sport is 

 not the organic expression of discontent which has been long 

 felt, but which has not been attended to, nor been met step 

 by step by as much small remedial modification as was found 

 practicable: so that when a change does come it comes 

 by way of revolution. Or, again (only that it comes to much 

 the same thing), a sport may be compared to one of those 

 happy thoughts which sometimes come to us unbidden after 



