302 GENERAL ZOOLOGY 



Evidence from Embryology and Morphology. Frequently 

 the only way the zoologist may know of the kinship of cer- 

 tain groups is by studying their early stages of development. 

 Since all the animals composed of more than one cell repro- 

 duce at one time or another by means of eggs and sperms, 

 we see that all animals, however great the differences be- 

 tween the adults may be, are alike in being composed of one 

 cell at the beginning of development. Furthermore, the 

 degree to which the young of two different groups of ani- 

 mals resemble each other in the details of their development 

 is believed to be a measure of the relationship existing be- 

 tween the groups. This theory is based upon the very 

 generally accepted belief that each individual in the course 

 of its development repeats in an extremely short time the 

 history of its race. The Recapitulation Theory is the name 

 by which this generalization is commonly known. 



All those animals which have few organs show relatively 

 few changes in development, in comparison with those ani- 

 mals which have numerous complicated organs. A hydra, 

 for example, is a very simple form compared with an earth- 

 worm. Not only has a hydra fewer organs than an earth- 

 worm, but reference to the latter's development will show 

 that the two layers of cells which constitute an entire hy- 

 dra represent a very early stage in the development of an 

 earthworm. 



The third germ layer, the mesoderm, offers a beginning 

 place for organs not developed in the ectoderm and endo- 

 derm, and groups of animals possessing it develop more 

 kinds of organs than those which do not have it ; that is to 

 say, they show greater differentiation. All animals except 

 the Protozoa and the ccelenterates have a mesoderm.' 



If we were to begin at once to arrange all animals in a 

 regular graded series, on the basis of what we learn from 

 morphology and from embryology, we should have a diffi- 



