THE FACTORS OF ORGANIC EVOLUTION. 775 



are all reduced relatively to the same parts in the wild pigeon." [After detail- 

 ing kindred diminutions in fowls and ducks, Mr, Darwin adds] "The decreased 

 weight and size of the bones, in the foregoing cases, is probably the indirect 

 result of the reaction of the weakened muscles on the bones" (pp. 2'J7-8). 

 "Nathusius has shown that, with the improved races of the pig, the shortened 

 legs and snout, the form of the articular condyles of the occiput, and the posi- 

 tion of the jaws with the upper canine teeth projecting in a most anomalous 

 manner in front of the lower canines, may be attributed to these parts not hav- 

 ing been fully exercised. . . . These modifications of structure, which are all 

 strictly inherited, characterize several improved breeds, so that they cannot 

 have been derived from any single domestic or wild stock. With respect to cat- 

 tle, Professor Tanner has remarked that the lungs and liver in the improved 

 breeds 'are found to be considerably reduced in size when compared with those 

 possessed by animals having perfect liberty ; ' . . . The cause of the reduced 

 lungs in highly-bred animals which take little exercise is obvious" (pp. 299-300). 

 [And on pp. 301, 302, and 303, he gives facts showing the effects of use and 

 disuse in changing, among domestic animals, the characters of the ears, the 

 lengths of the intestines, and, in various ways, the natures of the instincts.] 



But Mr, Darwin's admission, or rather his assertion, that the in- 

 heritance of functionally-produced modifications has been a factor in 

 organic evolution, is made clear not by these passages alone and by 

 kindred ones. It is made clearer still by a passage in the preface to 

 the second edition of his Descent of Man. He there protests against 

 that current version of his views in which this factor makes no appear- 

 ance. The passage is as follows : 



"I may take this opportunity of remarking that my critics frequently assume 

 that I attribute all changes of corporeal structure and mental power exclusively 

 to the natural selection of such variations as are often called spontaneous ; 

 whereas, even in the first edition of the 'Origin of Species,' I distinctly stated 

 that great weight must be attributed to the inherited eftects of use and disuse, 

 with respect both to the body and mind." 



Nor is this all. There is evidence that IVTr. Darwin's belief in the effi- 

 ciency of this factor, became stronger as he grew older and accumu- 

 lated more evidence. The first of the extracts above given, taken 

 from the sixth edition of the Origin of Species, runs thus : 



" I think there can be no doubt that use in our domestic animals has strength- 

 ened and enlarged certain parts, and disuse diminished them ; and that such 

 modifications are inherited." 



Now on turning to the first edition, p. 134, it will be found that in- 

 stead of the words — " I think there can be no doubt," the words origi- 

 nally used were — "I think there can be little doubt." That this delib- 

 erate erasure of a qualifying word and substitution of a word implying 

 unqualified belief, was due to a more decided recognition of a factor 

 originally under-estimated, is clearly implied by the wording of the 

 above-quoted passage from the preface to the Descent of Man ; where 

 he says that " even in the first edition of the * Origin of Species,' " etc.: 

 the implication being that much more in subsequent editions, and sub- 



