168 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
blastoderm consisting of a single layer of similar undifferentiated cells. 
But soon in the course of development the embryo begins to differ, and 
as 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 mature form, showing all the great 
structural differences between the starfish and the dove, the beetle and 
the horse. That is, all animals begin development apparently alike, 
but gradually diverge from each other during the course of develop- 
ment. 
There are some extremely interesting and significant things about 
this divergence to which attention should be given. While all animals 
are apparently alike structurally at the beginning of development, so 
far as we can see, they do not all differ noticeably at the time of the first 
divergence in development. The first divergence in development is 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 or class of animals called Insecta, or insects. There are many 
different kinds of insects, and these kinds can be arranged in subor- 
dinate groups (orders), such as the Diptera, or flies, the Lepidoptera, 
or butterflies and moths, and so on. But all have certain structural 
characteristics in common, so that they are comprised in one great 
class—the Insecta. Another great group of animals is known as the 
Vertebrata, or backboned 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 backbone or, more accurately speaking, of a notochord, a back- 
bonelike structure. Now, an insect and a vertebrate diverge very 
soon in their development from each other; but two insects, such as a 
beetle and a honeybee, or any two vertebrates, such as a frog and a 
pigeon, do not diverge from each other sosoon. Thatis, 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 Vertebrata, 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. 
That the course of development of any animal from its beginning 
to fully developed adult form is—in all its essentials—fixed and certain 
