THE JIE CAPITULATION THEOIiY. 27 



devoid of thern, or of teeth in the embryo of the whalebone 

 whale, are in like manner to be regarded as reminiscences of 

 former ancestral conditions, and as indicating that the ancestors 

 of chicks and rabbits breathed by gills, and that the toothless 

 whalebone whales are descended from toothed progenitors. 



It is on this fact of Recapitulation that the great value of 

 embryology depends. The study of development acquires a new 

 and striking interest when it is realised that through it we are 

 enabled to obtain knowledge, in many cases unattainable by any 

 other means, of the real or blood relationships between animals 

 and groups of animals. 



It is with animals as with men, the only natural classification 

 is a genealogical or phylogenetic one, and the possibility of 

 framing such a classification of animals depends very largely on 

 the success with which we are able to reconstruct their pedigrees 

 from a study of the stages through which they pass in their 

 actual development or ontogeny. 



Recapitulation must apply, not merely to the development of 

 an animal as a whole, but to that of each one of its organs and 

 parts : the formation of the ear, for example, as a pit of the skin, 

 must be interpreted as meaning that the ear, like the other organs 

 of sensation, was in its earliest commencement merely a spe- 

 cialised patch of skin. 



The theory must also apply to the earliest stages of develop- 

 ment equally with the later ones ; and the fact that all Metazoa 

 commence their existence as eggs perhaps the most striking 

 of all embryological facts receives an entirely new significance 

 when we interpret it as a reminiscence of a unicellular ancestry 

 for all Metazoa, and as an indication that all the multicellular 

 animals, or Metazoa, are descended from unicellular Protozoa. 



From this point of view the earliest developmental stages of 

 Metazoa deserve special attention, as possibly indicating the 

 actual lines of descent of Metazoa from Protozoa. Segmentation 

 is simply cell-division ; and the main difference between cell 

 division in Protozoa and segmentation of the egg of a Metazoon 

 is that, in the former case, the products of division separate from 

 each other as independent unicellular animals, while in the latter 

 they remain in close contact and become constituent units of 

 one multicellular animal. The several stages of segmentation, 

 Fig. 2, II to vn, may be compared with colonies of Protozoa ; 



