49° ZOOLOGY 



487. Careful study of the later stages of development of 

 the higher animals gives us further illustrations of these truths 

 and enables us to state even more broadly the principles deduced 

 above. For example, we find that insects have a parallel course 

 of embryonic development in which the great body of insects 

 agree (Fig. 250). The same is true of the vertebrates. One in- 

 sect agrees with another insect in the mode of its developinent 

 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. 250, 1-7). 



Furthermore, the same principle applies to the subgroups 

 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. 250,8). 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 individuals con- 

 tinues identical through to the mature form. It is just those 

 forms that seem most similar in structure (section 484) in which 

 the steps of the embryonic development is most nearly identical. 

 The two forms of similarity, structure and embryology, 

 strengthen the idea of real kinship. If the similar course of 

 embryological development in brothers or cousins marks kin- 

 ship, the shorter parallelism of different types may also mark 

 kinship of a less degree. Hence we have reached the conclusion 

 that all parallelisms in the fundamentals of individual history 

 mark some degree of relationship. The longer and more pro- 

 found the parallelism the closer the relationship. A careful 

 study of the diagram (Fig. 250) will serve to make this point 

 clearer. 



488. 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 



