TWO ee ee ee TERI 
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1894.] The Origin of Pelagic Life. 587 
introduced many and new opportunities for divergent modifi- 
cations. 
Another result was the escape of varieties from competition 
with their allies by flight from the crowded bottom to the 
open water above. The influence of these emigrants upon 
strictly pelagic forms is seen in the evolution at the surface of 
complicated forms like the siphonophores. But, on the whole, 
ocean space is so great and conditions of life in open water so 
easy that many of the pelagic organisms retain their primitive 
simplicity, existing simultaneously with the large and highly 
organized invaders from the shore and bottom. 
The colonization of the bottom formed an important era in 
the evolution of marine life and the author devotes a section 
to a consideration of the characteristics of this primitive fauna 
of which the following is a summary: 
“1. It was entirely animal, and it at first depended directly 
upon the pelagic food supply. 
“2. It was established around elevated areas and in water 
deep enough to be beyond the influence of the shore. 
“3. The great groups of metazoa were rapidly established 
from pelagic ancestors. 
“4. There was a rapid increase in the size of the bottom 
animals and hard parts were quickly acquired. 
* 5. The bottom fauna soon produced development among 
pelagic animals. 
“6, After the establishment of the bottom fauna, elabora- 
tion and differentiation among the representatives of each 
primitive type soon set in and led to the extinction of the 
connecting forms." 
In comparing these characteristics with those of the earliest. 
known fauna as sketched by Walcott, Mr. Brooks finds that in 
going backward toward the lower Cambrian he finds a closer 
and closer agreement with the biological conception of the 
primitive life at the bottom. And while he does not regard 
the Olenellan fauna as the first bottom fauna, since it contains 
forms secondarily adapted to pelagic life, such as pteropods, 
still, ^a biologist must regard it as an unmistakable approx- 
imation to the primitive fauna of the bottom, beyond which 
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