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 embryos begin 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 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 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 



