But even in molluscan history there was a tragic chapter. The squids
and cuttle-fishes regained the swimming life, and in their latest
forms gave up the protective shell. But its former presence had so
modified their structure that any great advance was impossible. It
was too late. The sins of the fathers were visited upon the children
in the thousandth generation.
The vertebrate developed an internal skeleton. This was necessarily
a slow growth, and the type came late to supremacy. The longitudinal
muscles are arranged in heavy bands on each side of the back, and
the animal swims rapidly. The sense-organs are keen. The brain
contains the ganglia of several or many segments and is highly
differentiated. It has a special centre of perception, thought, and
will; it is an organ of mind. The vertebrate has the physical and
mental advantages of large size.
First the definite form and mode of developing a vertebra is
attained. Then the vertebral column is perfected. The fins are
modified into legs. The lungs increase in size and the heart becomes
double. The animal emerges on land; and, with a better supply of
oxygen and less loss of heat, all the functions are performed with
the highest possible efficiency. First, apparently, amphibia, then
reptiles, and finally mammals of enormous size and strength
appeared.
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