672 Geological Periods of Mutation 



investigators hold one of these views, others the otlier. 

 If we assume that the individual mutations in such pe- 

 riods were changes of a greater amplitude, they might 

 be designated by a special name, for instance by the one 

 suggested by Schneider, the "descenses."^ There is no 

 fundamental difference between these and mutations, and 

 the same changes may, according to Schneider, in some 

 lines attain to the magnitude of descenses, whilst in 

 others they may remain of merely subordinate impor- 

 tance. 



At present, however, I am concerned merely with an 

 approximate and average estimate, and the knowledge at 

 our disposal suggests that an estimate of a few thousand 

 years fairly closely represents the truth. 



A third question relates to the number of elementary 

 characters of which one of the higher animals or plants 

 is composed. According to the theory of selection an 

 almost unlimited number of complications would be pos- 

 sible. In my Intracellular Pangenesis I have shown that, 

 quite on the contrary, the number in question cannot be 

 so inordinately great; for we repeatedly see the same 

 characters recurring in different organisms, many of 

 them in systematic groups widely remote from one an- 

 other, as for instance in the higher plants and the higher 

 animals. I need only mention the close similarity be- 

 tween the chemical processes involved in digestion in the 

 stomach and in the leaves of insectivorous plants. Ten- 

 drils and climbing plants, submerged or swimming water- 

 plants, heterostylic and cleistogamous flowers, parasitism 

 and saprophytism and numerous other instances could 

 be adduced. Everywhere nature has built up the whole 



*K. C. ScHNETPER. Lchrbuch dcr vcrgleichenden Anatomie, Jena 

 1902, pp. 2-14. 24«. 



