Corresponding changes take place in the arteries, and we shall
find that this change in structure is of very great importance in
the evolution of the higher types of land-life. The heart of the
higher land-animals, we may add, passes through these stages in
its embryonic development.
Externally the chief change in the Amphibian is the appearance of
definite legs. The broad paddle of the fin is now useless, and
its main stem is converted into a jointed, bony limb, with a
five-toed foot, spreading into a paddle, at the end. But the legs
are still feeble, sprawling supports, letting the heavy body down
almost to the ground. The Amphibian is an imperfect, but
necessary, stage in evolution. It is an improvement on the
Dipneust fish, which now begins to dwindle very considerably in
the geological record, but it is itself doomed to give way
speedily before one of its more advanced descendants, the
Reptile. Probably the giant salamander of modern Japan affords
the best suggestion of the large and primitive salamanders of the
Coal-forest, while the Caecilia--snake-like Amphibia with scaly
skins, which live underground in South America--may not
impossibly be degenerate survivors of the curious Aistopods.
Pages:
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203