The Biocliroinc Equation. ()7\ 



this trnnsformaiion from ilic non-fossiliforons in llie 

 fossiliferous pcritxl nia\- 1)C' inia^incd l<» lia\e taken place' 

 Til the heginnin^" life was chietly conl'inecl to the upper 

 levels of the sea; and extended to only those depths to 

 wliich the rays of the sun penetrated, thus supplyini; the 

 source of energ\' for the nutrition ^^\ the smaller Ali^'ae. 

 These latter were almost the only source of nourishment 

 for the animals which there f(^re had not left this region 

 yet; consequentl\' they were nn^stly small and of delicate 

 structure, and without sucli ])arts as could hecome fossil. 

 Afterwards it was the discoxery, as Brooks call^ it. of 

 the possihility of life on the gloomy hottom of the sea, 

 on the dead remains of the swimming organisms sink- 

 ing there, which extended the distrihutic^n (»f life and 

 furnished a new and most variable abode for living be- 

 ings. Thus was started the rai)id and abundant evolution 

 in the numerous directions which now constitute the 

 main lines of organic descent. 



Besides this period of rapid evolution. Brooks, to- 

 «'ether with other writers, assumes that there ha\e been 

 other S]:)ecial periods of great variability; for instance at 

 the time when land-animals and aeain when man originated 

 {loc. cif., p. 217). The distribution of fossils also indi- 

 cates the existence of ])eri(^ds in which the forniatiou 

 of species has been esjxx^ially ra])id.- 



1die cjuestion arises: Were the individual mutations 

 greater in such ]")eriods, (tr did they onl\' follow more 

 rapidly upon one another?"' This f|uesti(^n is one oi 

 com|)arative anatoni}' and of systematic science. Some 



MV. K. Brooks, The Foundations of Zoology, i^y). pp. 215-2.^7. 



'Die Mulotioncn mid }fi(lationspcn'odrn, jv 5^1; also W . K. 

 Brooks, Foundations of Zoohx^y. \\. jiS; (^11 \> A. Wihtk, 'ihc Rc- 

 lation of Biology, p. 296, etc. 



■'' E. KoKEX. Paldontoh^gic und Dcsrcndcn^U-h' <• '"-i' " >o. 



