672 Geological Periods of Mutation 
investigators hold one of these views, others the other. 
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." 1 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 Pangencsis 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 
a K. C. SCHNEIDER, Lchrbuch dcr vcrglcichendcn Anatomic, Jena 
1902, pp. 244. 248. 
