CRITICAL EPISODE IN EVOLUTION 
121 
MOST CRITICAL, EPISODE IN EVOLUTION'^ 
By W. K. Brooks 
Darwin says in his Origin of Species '‘To the question why we 
do not find such fossiliferous deposits belonging to these assumed 
earliest periods prior to the Cambrian System I can give no satis¬ 
factory answer/' On its geological side this difficulty is even 
greater than it was in Darwin’s day, for we now know that the 
fauna of the Early Cambric Period was rich and varied; that 
most of the modern types of animal life were represented in the 
oldest fauna which has been discovered; and that all its types have 
modern representatives. 
Fossiliferous beds of Early Cambric date rest upon beds which 
are miles in vertical thickness, and are identical in all their physical 
features with those which contain this fauna. They prove beyond 
question that the waters in which they were laid down were as 
fit for supporting life at the beginning as at the end of the 
enormous lapse of time which they represent, and that all the 
conditions have since been equally favorable for the preservation 
and discovery of fossils. 
Modern discovery has brought the difficulty which Darwin 
points out into clearer view; but geologists are no more prepared 
to give satisfactory solution, although I shall now try to show 
that the study of living animals in their relations to the world 
around them does help us, and that comparative anatomy and 
comparative embryology and the study of the habits and 
affinities of organisms tell us of times more ancient than the oldest 
1 This article is the second installment of a symposium on the geological aspects 
of the evolution of life. It is a succinct statement of the contents of a paper which 
under different title originally appeared in the Journal of Geology for 1894, but 
which at that time attracted little or* no attention from paleontologists. It is really 
the most important contribution ever made to paleontology in this country and perhaps 
in the world. Partly for this reason the article in condensed form is here republished. 
It spans a wide gap in Darwinian theory to which the author of the Origin of Species 
himself directed especial attention. Not its least important feature is its basic bear¬ 
ing upon the length of geologic time.— Editor. 
