g8 Darwin, and after Darwin. 



that a rigid selection of individuals with well-developed wings 

 at once ceases among its descendants. Hence, in the course 

 of generations, a deterioration of the organs of flight must 

 necessarily ensue ^" 



Or, to state the case in another way: if any 

 structure which was originally built up by natural 

 selection on account of its use, ceases any longer 

 to be of so much use, in whatever degree it ceases 

 to be of use, in that degree will the premium before 

 set upon it by natural selection be withdrawn. And 

 the consequence of this withdrawal of selection as 

 regards that particular part will be to allow the 

 part to degenerate in successive generations. Such 

 is the principle which Weismann calls Panmixia, 

 because, by the withdrawal of selection from any 

 particular part, promiscuous breeding ensues with 

 regard to that part. And it is easy to see that 

 this principle must be one of very great importance 

 in nature ; because it must necessarily come into 

 operation in all cases where any structure or any 

 instinct has, through any change in the environment 

 or in the habits of a species, ceased to be useful. 

 It is likewise easy to see that its effect must be 

 the same as that which was attributed by Darwin 

 to the inherited effect of disuse ; and, therefore, that 

 the evidence on which he relied in proof of the 

 inherited effects both of use and of disuse is vitiated 

 by the fact that the idea of Panmixia did not occur to 

 him. 



Here, however, it may be said that the idea first 

 occurred to me'' just after the publication of the 



' Essays, i. p. 90. 



' Nature, vol. ix. pp. 361-2, 440-1 ; and vol. x. p. 164. 



