2 88 CAUSES OF VARIATION 



Disappearing organs. The disappearance of legs from snakes 

 and from whales, the lessening of the fore arms of the kangaroo, 

 and of the wing of certain birds, the loss of toes from many 

 mammals, and rudimentary conditions generally, argue for the 

 gradual disappearance of a part that is no longer useful and no 

 longer used. 



The first stages of this disappearance can be understood as 

 arising through the cessation of selection and the resulting pan- 

 mixia, 1 by which inheritance is from the general average of the 

 whole race, instead of from a selected lot, as heretofore, result- 

 ing necessarily in degeneration as compared with a standard 

 sustained by rigid selection. Later stages may be explained by 

 " reversal of selection," when the hitherto useful organ has not 

 only become useless but in some way detrimental. This would 

 account for still further degeneracy, and is as far as the princi- 

 ple of use and disuse applies. 2 



Space cannot be taken here for the enumeration of instances 

 showing the effects of use and disuse. They may be found on 

 every hand, and they abound in books on general evolution. 3 



Hypertrophy. Unusual enlargement of a part is technically 

 known as hypertrophy. Two kinds are recognized, functional 

 hypertrophy, when a part is enlarged through use ; and com- 

 pensating hypertrophy, which takes place when, one organ 

 being removed or becoming functionless, another enlarges. 4 



The voluntary muscles of the hand and arm grow large 

 through heavy use, but the muscles of the fingers of a musician 

 do not undergo hypertrophy, though the total amount of work 

 may be very large. It is only when muscular work is done against 

 great resistance that enlargement of the muscles takes place. 6 



1 A term coined by Weismann and denoting " universal crossing," literally 

 " all mixed." See Weismann, Essay on Heredity, I, 91, 141 ; also Romanes, Dar- 

 win and After Darwin, Part II, pp. 291-306. 



2 For fuller discussion of disappearing organs, see next chapter. 



8 Darwin, Origin of Species (sixth edition), pp. 108-112 [D. Appleton & 

 Company] ; also Animals and Plants under Domestication (second edition) 

 [D. Appleton & Company] : in general, II, 285-293, 345, 346; in rabbits, I, 129- 

 134; in ducks, I, 299-301. 



4 Morgan, Regeneration, pp. 115-118. 



6 Of course the effects of use are not limited to increase of size ; they are fully 

 as noticeable in nicety of adjustment. 



