130 
CRITICAL EPISODE IN EVOLUTION 
habits, while the parent stem, competing with both at a disad¬ 
vantage, is exterminated. Among the animals which we know 
best evolution leads to a branching tree-like genealogy with the 
topmost twigs represented by living animals while the rest of 
the tree is buried in the dead past. 
Even at the present day things are somewhat diilerent in the 
open ocean; and they must have been very different in the prim¬ 
itive ocean, for a pelagic animal has no fixed home, one locality 
is like another, and competitors and enemies of each individual 
are determined] in great part by accidents. We accordingly find, 
even now, that the evolution of the pelagic animals is often linear 
instead of divergent. Ancient forms, such as sharks, often live 
on side by side with the later and more evolved forms. 
Many of the oldest fossils, like the Pteropods, are modified 
descendants of ancestors with hard parts, and there is no reason to 
suppose that the first animals which were capable of preservation 
as fossils have been discovered; but it is interesting to note that 
the oldest known fauna is an unmistakable approximation to the 
primitive fauna of the bottom. 
The Cambric fauna is usually regarded as a half-way station in 
a series of animal forms which stretches backwards into the past 
for an immeasurable period; and it is even stated that the history 
of life before Cambric times is larger by many fold than its 
history since. So far as this opinion rests on the diversity of 
types in Cambric times it has no good basis; for if the views 
here advocated are correct, the evolution of the ancestral stems 
took place at the surface and all the conditions necessary for the 
rapid production of types were present when the bottom fauna 
first became established. 
As we pass backwards towards Early Cambric times we find 
closer and closer agreement with the zoological conception of the 
character of primitive life on the bottom. Although we cannot 
regard the oldest fossil fauna which has been discovered as the 
first which existed on the bottom, we may feel confident that the 
first fauna of the bottom resembled that of the Early Cambric 
rocks in its physical conditions and in its most distinctive peculiari¬ 
ties. We must regard it as a decided and unmistakable ap¬ 
proximation to the beginning of the modern fauna of the earth, 
as distinguished from the more ancient and simple fauna of the 
open ocean. 
