IO 8 ELEMENTARY ZOOLOGY. 



higher animals, in their development, come for a little 

 way along a somewhat common course. It has been 

 found that the nearer alike the animals are in the adult 

 state, the longer their parallel development is. The 

 individual starfish, the grasshopper, the crayfish, the 

 frog, and man begin life quite similarly, as a single cell. 

 The cells divide, and they all pass through a morula, 

 or mass of undifferentiated cells, and later a blastula 

 stage. Soon they begin to be different, but the grass- 

 hopper and the crayfish, being fundamentally more 

 alike when adult, follow a similar course of develop- 

 ment longer than the other types. The frog and man, 

 being vertebrates, have stages in common w^hich they 

 do not share with the grasshopper. This increase of 

 the differences continues until they reach the mature 

 condition. The diagram, Fig. 160, will further illustrate 

 this truth. 



1 1 6. Heredity. Why should a grasshopper and noth- 

 ing else develop from a grasshopper egg? There are 

 only two sets of influences that could act to bring it about : 

 (i) the internal influences which come to the individual 

 from the sperm cell and the ovum, and (2) the external 

 influences. The first we call heredity, the second en- 

 vironment. Any quality or power which the individual 

 gets directly from its parents through the egg or sperm is a 

 hereditary quality; nothing else is. In some way each 

 fertilized egg inherits the power to become a fish or a 

 frog or a man, as the case may be. It may also inherit 

 the power to become a certain kind of frog or man, even 

 down to color of the eyes or personal tendencies. 



The ovum and the sperm then are the connecting link 

 between the individual and its past ancestors and are 



