3QO ELEMENTARY ZOOLOGY. 



same is true of the vertebrates. One insect agrees with 

 another insect in the mode of its development more 

 nearly up to the complete adult character than an insect 

 will agree with an echinoderm, or either with a vertebrate. 

 In a similar way, all vertebrates have a course of develop- 

 ment parallel for a longer time than they parallel any other 

 group (Fig. 1 60: 6). 



Furthermore, the same principle applies to the sub- 

 groups of the vertebrates, or of any other branch. The 

 vertebrate development begins to differ from the insect 

 development lower down than the divergence of the 

 reptiles from the birds, or the mammals from the fishes 

 (Fig. 1 60: 8} The parallelism between birds and reptiles 

 ends earlier than that between one type of birds and 

 another, or between man and the other mammals. Finally 

 within a species, as of grasshoppers, of reptiles, or of 

 rabbits, the course of development of different individ- 

 uals continues identical through to the mature form. 

 It is just those forms that seem most similar in structure 

 (section 383) in which the embryonic development is 

 most nearly identical. The two forms of similarity, 

 structure and embryology, strengthen the idea of real 

 kinship. Hence we have reached the conclusion that all 

 parallelisms in the fundamentals of individual history 

 marks some degree of relationship. The longer and more 

 profound the parallelism the closer the relationship. 

 A careful study of the diagram (Fig. 160) will serve to 

 make this point more clear. 



387. The Biogenetic Law. Out of such considerations 

 as these has come one of the most important and far- 

 reaching laws that the biologists have ever stated. It is 

 as follows: "Each individual animal, in passing from 



