KINETOGENESIS. 247 



2. Degeneracy from disuse: examples; the rudimental legs 



and digits of numerous lizards. 



3. Degeneracy from disuse and complementary excess else- 



where : examples ; reduction in number of molars and in- 

 cisors in man ; reduced mammae in male Mammalia ; re- 

 duction of lateral digits in the true horses (Equus). 



4. Physico-chemical causes : here must be probably included 



various color-patches and color tints, for which no other 

 explanation is accessible. 



Darwin considers several of the above conditions, 

 and endeavors to explain some of them as consequences 

 of natural selection. Equivalent to the Section I. 

 above, he enumerates extraordinary developments of 

 particular parts. 1 He says, "A part developed in any 

 species in an extraordinary degree or manner in com- 

 parison with the same part in an allied species, tends 

 to be highly variable.' 3 He does not attempt any ex- 

 planation of the origin of such characters, except 

 through natural selection. Of the characters coming 

 under Section II. above, he says, " Multiple, rudimen- 

 tary and lowly organized structures are variable." Of 

 these he remarks, " I presume that lowness here means 

 that the several parts of the organism have been but 

 little specialized for particular functions ; and as long 

 as the same part has to perform diversified work, we 

 can perhaps see why it should remain variable, that 

 is, why natural selection should not have preserved or 

 rejected each little deviation of form so carefully as 

 when the part has to serve for some one special pur- 

 pose." Here Mr. Darwin well illustrates his unwilling- 

 ness to look to disuse as the cause of the conditions he 

 describes. Under "Compensation and Economy of 

 Growth" he quotes from Goethe that "In order to 



IT/ie Origin of Species, Ed. 1872, p. 119. 



