484 ZOOLOGY 



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. 2 50, 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 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 mark 

 some degree of relationship. The longer and more profound the 

 parallelism the closer the relationship. A careful study of the 

 diagram (Fig. 250) will serve to make this point more clear. 



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 

 individual animal, in passing from the egg to the adult, repeats 

 in an abbreviated way, in a few days or years, many of the steps 

 taken by the race to which it belongs, in its evolution from its 

 single-celled ancestors to its present condition." Put briefly, it 

 reads : Individual history is a brief recapitulation of race history. 



Why can the individual egg cell of a frog, in a few months, 

 pass through a morula, a blastula, a water-breathing legless fish- 

 like stage, and into an air-breathing, four-legged adult? Be- 

 cause its ancestors were first single-celled animals, and through 

 millions of years and countless generations gradually developed 

 first into a mass of cells, later into a gastrula-like stage, and later 

 still for long ages breathed by gills, like the fishes, and last 

 of all became what they now are water-breathing tadpoles 

 with the power to breathe air in the adult life. 



