3G ORGANIC EVOLUTION 



method so well adapted to this end, it has not yet 

 been conceived by the mind of man. 



But the critic may reply, with the late Lord 

 Salisbury, that, in point of fact, " no one has seen 

 natural selection at work." What positive experi- 

 mental evidence is there that this thing exists out- 

 side the pages of modern treatises ? It is not 

 sufficient to show that, if it existed, it could do 

 many things ; it is necessary to show that it does 

 exist. This challenge had to be met. We could 

 not be content even with showing, from a priori 

 considerations, that natural selection must be and 

 must have been at work. If others could be 

 content with a priori argument — that is, argument 

 that, since so-and-so is so-and-so, then something 

 else must follow — men of science could not, 

 for they have been fighting, since the dawn of 

 science, against the countless lies which the a priori 

 method has asserted as true. If possible, we must 

 certainly establish the fact of natural selection by 

 the a posteriori or specifically scientific method, 

 which begins by recording facts and facts and more 

 facts, and then proceeds to base general conclusions 

 upon them. And already, even within the few 

 years since Lord Salisbury issued his challenge, 

 we have been able to adduce actual observations of 

 what no one can doubt to be " natural selection at 

 work." 



In 1893, 1895, and 1898 careful measurements 

 were made of the shells of a certain kind of crab 

 (Carcinus mosnas) which lives in Plymouth Sound. It 

 was found — one need not give all the details here 1 — 



1 See Report of British Association, 1898, p. 887. 



