CERTAIN LAWS OF VARIATION. 221 



One of the few conditions which have been generally 

 held to lead to increased variability is that of Domesti- 

 cation. Darwin * laid considerable stress on the fact 

 that " with extremely few exceptions all animals and 

 plants which have been long domesticated have varied 

 greatly." This increased variability he attributed 

 partly to the conditions of life being less uniform, and 

 to a lesser degree to the effects of excess of food. He 

 concluded that organic beings must be exposed to the 

 new conditions for several generations before much in- 

 crease of variability is observed, and hence it seemed as 

 if the changed conditions of life acted not only directly 

 on the organism as a whole, but also indirectly through 

 the reproductive system. 



In tie first place it is probable that Darwin consider- 

 ably overestimated the variability of domesticated or- 

 ganisms, as compared with that of undomesticated ones. 

 Thus we have already seen how widely many naturally 

 occurring organisms may vary, and Schwalbe has even 

 denied altogether that domestication produces any in- 

 crease of variation. Thus he states t that Pfitzner has 

 found thit parts of the skeleton of the fore and hind 

 feet of vild animals vary just as much as the corre- 

 sponding parts in man. The size and the indices of the 

 skull of tlie otter, as determined by examining over 200 

 skulls derived from a limited district in Alsace, were 

 found by Schwalbe to vary just as much as the corre- 

 sponding measurements in the most widely separated 

 human races. Again, Kohlbriigge has shown that in all 



* " Anmals and Plants," ii. p. 401. 



f Anatcmischer Anzeiger. Verhandlungen, xii. p. 2, 1898. 



