86 ANIMAL LIFE 
bone, or, more accurately speaking, of a notochord, a back. 
bone-like structure. Now, an insect and a vertebrate di- 
verge very soon in their development from each other; but 
two insects, such as a beetle and a honey-bee, or any two 
vertebrates, such as a frog and a pigeon, do not diverge 
from each other so soon. That is, all vertebrate animals 
diverge in one direction from the other great groups, but 
all the members of the great group keep together for some 
time longer. Then the subordinate groups of the Verte- 
brata, such as the fishes, the birds, and the others diverge, 
and still later the different kinds of animals in each of 
these groups diverge from each other. In the illustration 
(Fig. 41) on the opposite page will be seen pictures of the 
embryos of various vertebrate animals shown as they appear 
at different stages or times in the course of development. 
The embryos of a fish, a salamander, a tortoise, a bird, and 
a mammal, representing the five principal groups of the 
Vertebrata, are shown. In the upper row the embryos are 
in the earliest of all the stages figured, and they are very 
much alike. They show no obvious characteristics of 
fish or bird. Yet there are distinctive characteristics of 
the great class Vertebrata. Any of these embryos could 
readily be distinguished from an embryonic insect or worm 
or sea-urchin. In the second row there is beginning to be 
manifest a divergence among the different embryos, al- 
though it would still be a difficult matter to distinguish 
certainly which was the young fish and which the young 
salamander, or which the young tortoise and which the 
young bird. In the bottom row, showing the animals in a 
later stage of development, the divergence has proceeded 
so far that it is now plain which is a fish, which batrachian, 
which reptile, which bird, and which mammal. 
54. The laws or general facts of development.—That the 
course of development of any animal from its beginning to 
fully developed adult form is fixed and certain is readily 
seen. Every rabbit develops in the same way ; every grass- 
