THE LIFE CYCLE 85 
the young animals get further and further along in the 
course of their development, they become more and more 
different until each finally reaches its fully developed ma- 
ture form, showing all the great structural differences be- 
tween the star-fish and the dove, the beetle and the horse. 
That is, all animals begin development alike, but gradually 
diverge from each other during the course of development. 
There are some extremely interesting and significant 
things about this divergence to which attention should be 
given. While all animals are alike structurally * at the 
beginning of development, so far as we can see, they do not 
all differ at the time of the first divergence in development. 
This first divergence is only to be noted between two kinds 
of animals which belong to different great groups or classes. 
But two animals of different kinds, both belonging to some 
one great group, do. not show differences until later in their 
development. This can best be understood by an example. 
All the butterflies and beetles and grasshoppers and flies 
belong to the great group of animals called Insecta, or in- 
sects. There are many different kinds of insects, and these 
kinds can be arranged in subordinate groups, such as the 
Diptera, or flies, the Lepidoptera, or butterflies and moths, 
and soon. But all have certain structural characteristics 
in common, so that they are comprised in one great group 
or class—the Insecta. Another great group of animals is 
known as the Vertebrata, or back-boned animals. The class 
Vertebrata includes the fishes, the batrachians, the reptiles, 
the birds, and the mammals, each composing a subordinate 
group, but all characterized by the possession of a back- 
* They are alike structurally, when we consider the cell as the unit 
of animal structure. That the egg cells of different animals may dif- 
fer in their fine or ultimate structure, seems certain. For each one of 
these egg cells is destined to become some one kind of animal, and no 
other; each is, indeed, an individual in simplest, least developed con- 
dition of some one kind of animal, and we must believe that difference 
in kind of animals depends upon difference in structure in the egg itself, 
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