The old artificial systems were
based too much on merely external characteristics, the results of
adaptation. When the internal anatomy had been thoroughly studied
their groups had to be rearranged.
Reptiles and amphibia were at first united in one class because of
their resemblance in external form. Our common salamanders look so
much like lizards that they generally pass by this name. But the
young salamander, like all amphibia, breathes by gills, its skeleton
differs greatly from, and is far weaker than, that of the lizard,
and there are important differences in the circulatory and other
systems. Moreover, practically all amphibia differ from all reptiles
in these respects. Evidently the fact that the alligator and many
snakes and turtles (of which neither the young nor the embryos ever
breathe by gills) live almost entirely in the water, is no better
reason for classifying these with amphibia than to call a whale a
fish, and not a mammal, because of its form and aquatic life.
When the comparative anatomy of fish, amphibia, and reptiles had
been carefully studied it was evident that the amphibia stood far
nearer the fish in general structure, while the higher reptiles
closely approached birds. Then it was noticed that our common fish
formed a fairly well-defined group, but that the ganoids, including
the sturgeons, gar-pikes, and some others, had at least traces of
amphibian characteristics.
Pages:
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51