The Biochronic Equation. 671 
this transformation from the non-fossiliferous to the 
fossiliferous period may be imagined to have taken place. 1 
In the beginning life was chiefly confined to the upper 
levels of the sea ; and extended to only those depths to 
which the rays of the sun penetrated, thus supplying the 
source of energy for the nutrition of the smaller Algae. 
These latter were almost the only source of nourishment 
for the animals which therefore had not left this region 
yet ; consequently they were mostly small and of delicate 
structure, and without such parts as could become fossil. 
Afterwards it was the discovery, as BROOKS calls it, of 
the possibility of life on the gloomy bottom of the sea, 
on the dead remains of the swimming organisms sink- 
ing there, which extended the distribution of life and 
furnished a new and most variable abode for living be- 
ings. Thus was started the rapid and abundant evolution 
in the numerous directions which now constitute the 
main lines of organic descent. 
Besides this period of rapid evolution, BROOKS, to- 
gether with other writers, assumes that there have been 
other special periods of great variability; for instance at 
the time when land-animals and again when man originated 
(he. cit., p. 217). The distribution of fossils also indi- 
cates the existence of periods in which the formation 
of species has been especially rapid. 2 
The question arises : Were the individual mutations 
greater in such periods, or did they only follow more 
rapidly upon one another?" This question is one of 
comparative anatomy and of systematic science. Some 
1 W. K. BROOKS, The Foundations of Zoology, 1899, pp. 215-237. 
~Dic Mutationen nnd Mutationsperioden, p. 56; also W. K. 
BROOKS, Foundations of Zoology, p. 218; CHAS. A. WHITE. The Re- 
lation of Biology, p. 296, etc. 
3 E. KOKEN, Paldontologie und Descendenslehre, TOOT. p. 30. 
