220 HUMAN DISEASES 



retrogressive variations tend to predominate over progressive 

 variations; and (4) that these three fundamental characters of 

 living beings are themselves adaptations which have resulted from 

 Natural Selection. 



361. Nearly all biologists at the present day accept the theory 

 of Natural Selection; but some accept it with considerable 

 reservations, and none, as far as I am aware, accept it in the form 

 I have suggested. For example, the elimination of useless 

 characters (variations and old-established parts) is still generally 

 attributed to reversed selection. Sometimes progression is 

 thought to result from an inherent growth-force that is only kept 

 in check by Natural Selection, which therefore moulds the race 

 by planing away superfluities. In other words, progression is 

 thought to result, not from selection, but from a natural tendency 

 for progressive variations to predominate over retrogressive varia- 

 tions. According to this theory, therefore, all species would 

 progress were it not for Natural Selection. There is massive 

 evidence, however, that this view, which appears to be especi- 

 ally popular amongst the supporters of the mutation theory, 

 is mistaken. All parts, for example vestigial parts and the 

 special features of prize domestic breeds (e.g. race-horses), which 

 have no survival value, tend to disappear on cessation of selection, 

 and we have only to observe Natural Selection in actual opera- 

 tion to perceive plainly that its special role is the causation of 

 progression. 1 



362. My excuse for the positive attitude I assume must be 

 that I offer, in the chapters that now follow, easily verified and, 

 I think, very conclusive evidence of Natural Selection in actual 

 operation ; whereas the opinions I controvert are founded on a 

 consideration of lower animals and plants, amongst which, 



1 As a rule the idea that one or the other kind of variations tends to predominate 

 is implied rather than expressly stated in the theories of biologists. The problem 

 has not often been especially formulated. I think most students of evolution 

 believe, more or less vaguely, that progressive and retrogressive variations about 

 balance one another. This view, however, leaves unexplained the retrogression 

 of parts that have no selection value. Weismann's theory of Panmixia implies 

 that retrogressive variations predominate, and he has attempted to account for 

 the fact by his hypothesis of Germinal Selection. The view I have ventured to 

 set before the reader differs from his merely in that I attribute the predominance 

 to a tendency evolved by Natural Selection, and do not speculate on the 

 mechanism of processes in the germ-plasm concerning which I have no data. 

 Mutationists and Mendelians ignore retrogression. At any rate, the subject is 

 not to my knowledge mentioned in their works. Indeed the fact that retro- 

 gression constantly occurs on cessation of selection is not only quite ignored, 

 but also is by implication controverted by many of them. See 285. 



