EVOLUTION IOI 
progress as exhibited in the vertebrate kingdom; 
the very plasticity of these creatures has thwarted 
progressive development, as we understand the 
phrase, and they readily retrograde, or branch off, 
into bypaths. Nevertheless, progressive develop- 
ment is traceable in certain characters, while in 
other conspicuous features the action of environment 
or individual requirement alone seems responsible. 
It will suffice to take certain leading features and 
organs to summarize what is known concerning 
them, and to point out what conclusions, if any, can 
be based upon them. 
The tidal zone was in all probability the cradle 
of the race, although it has been argued by some 
with much show of reason that the earliest forms 
were pelagic. From the shore the various members 
betook themselves, mostly to deeper and deeper 
water on the one hand, but also, though perhaps 
more tentatively and gradually, to fluviatile and 
terrestrial conditions on the other. 
Now, the first requirement of a soft-bodied animal, 
and especially of one considered by its fellows to be 
good eating, is protection. In early days, however, 
enemies were far fewer than now, and it was rather 
from the force of the elements that preservation was 
needed. a 
This first requirement is supplied by the shell, 
and all three types, Univalve, Bivalve, and Multi- 
