300 Darwin, and after Darwin. 



to be useful, it will degenerate by the withdrawal of selection 

 alone. Which, of course, is merely a re-statement of the 

 doctrine of panmixia, or cessation of selection, in somewhat 

 varied terminology provided that the birth-mean be taken 

 over a number of generations, or not only over a few follow- 

 ing the selection-mean of the structure while still in its 

 highest state of efficiency. For the sake of brevity I will 

 hereafter speak of these " few following " generations by the 

 term of " first generations." 



It remains to consider the views of Professor Lloyd 

 Morgan upon the subject. In my opinion he is the 

 shrewdest, as well as the most logical critic that we have 

 in the field of Darwinian speculation; therefore, if possible, 

 I should like to arrive at a full agreement with him upon 

 this matter. His latest utterance with regard to it is as 

 follows : 



"To account for the diminution of organs or structures 

 no longer of use, apart from any inherited effects of disuse, 

 Mr. Romanes has invoked the Cessation of Selection ; and 

 Mr. Francis Galton has, in another connexion, summarized the 

 effects of this cessation of selection in the convenient phrase 

 ' Regression to Mediocrity.' This is the Panmixia of Professor 

 Weismann and his followers ; but the phrase regression to 

 mediocrity through the cessation of selection appears to me 

 preferable. It is clear that so long as any organ or structure 

 is subject to natural selection through elimination, it is, if not 

 actually undergoing improvement, kept at a high standard of 

 efficiency through the elimination of all those individuals in 

 which the organ in question falls below the required standard. 

 But if, from change in the environment or any other cause, the 

 character in question ceases to be subject to selection, elimina- 

 tion no longer takes place, and the high standard will no longer 

 be maintained. There v will be reversion to mediocrity. The 

 probable amount of this reversion is at present a matter under 

 discussion '." 



1 Presidential Address to the Bristol Naturalists' Society, 1891. 



