80 THE RELATIONS OF MAN 



essential structure as that of the Dog : — the yelk of that 

 egg always undergoes division, or ' segmentation ' as it is 

 often called : the ultimate products of that segmentation 

 constitute the building materials for the body of the young 

 animal ; and this is built up round a primitive groove, in 

 the floor of which a notochord is developed. Further- 

 more, there is a period in which the young of all these 

 animals resemble one another, not merely in outward 

 form, but in all essentials of structure, so closely, that the 

 differences between them are inconsiderable, while, in 

 their subsequent course, they diverge more and more 

 widely from one another. And it is a general law, that, 

 the more closely any animals resemble one another in 

 adult structure, the longer and the more intimately do their 

 embryos resemble one another: so that, for example, the 

 embryos of a Snake and of a Lizard remain like one an- 

 other longer than do those of a Snake and of a Bird ; and 

 the embryo of a Dog and of a Cat remain like one another 

 for a far longer period than do those of a Dog and a Bird ; 

 or of a Dog and an Opossum ; or even than those of a 

 Dog and a Monkey. 



Thus the study of development affords a clear test of 

 closeness of structural affinity, and one turns with impa- 

 tience to inquire what results are yielded by the study of 

 the development of Man. Is he something apart ? Does 

 he originate in a totally different way from Dog, Bird, 

 Frog, and Fish, thus justifying those who assert him to 

 have no place in nature and no real affinity with the 

 lower world of animal life ? Or does he originate in a 

 similar germ, pass through the same slow and gradually 

 progressive modifications, — dej)end on the same contri- 

 vances for protection and nutrition, and finally enter the 

 world by the help of the same mechanism ? The reply is 

 not doubtful for a moment, and has not been doubtful any 



