Evolution of Animal Life. 147 



isted before the appearance of either group ; the second, in 

 the recognized advance of life on the whole, as for instance, 

 in the series Mollusk, Fish, Eeptile, Mammal, Man; the 

 third, in the successive culmination of each of the groups 

 of the series just named, in the Silurian, Devonian, Meso- 

 zoic, Tertiary and Quaternary, and Present geological ages 

 respectively. 



D. The inclusion of all past and present animal forms 

 within a few great, persistent types of structure. (Proto- 

 zoans, Radiates, Mollusks, Articulates and Vertebrates.) We 

 think mostly of the Vertebrates, when we speak of ani- 

 mals. In the series just named as an illustration, all the 

 members except the first are vertebrates. Yet of more than 

 500,000 species determined, the vertebrates number only 

 25,000. It is a noteworthy fact that of the myriad other 

 forms not one has ever been found that could not be recog- 

 nized as belonging to one of the few great types mentioned. 



E. The facts revealed by comparative anatomy concern- 

 ing the adaptations to special uses, within each type, of the 

 structural elements common to the type, or their retention 

 without use — the facts of homology, morphology, rudi- 

 mentary organs, etc. 



F. The facts of embryology — particularly the wonder- 

 ful passage of the embryo through successive stages of re- 

 semblance to features characteristic of species of earlier or- 

 igin, in the order in which those species appeared in nature. 

 This phenomenon is not everywhere discernible ; but it has 

 been proved in certain instances — notably with regard to 

 the brain of the human embryo, which resembles succes- 

 sively that of a fish, a reptile, and a mammal, before assum- 

 ing human shape and proportions. There are other facts of 

 embryology, of which time will not permit the mention here. 



G. The geographical distribution of species. 



H. The fact that, within the life of a single individual, 

 organs are affected in size and structure by change of func- 

 tions, use or disuse, and, to some extent, directly by the 

 environment. 



I. The fact of heredity : that offspring always resemble 

 their parents and ancestors, and that inherited peculiarities 

 are likely to be intensified when both parents or many an- 

 cestors have possessed them. 



K. The fact of variation : that offspring are never ex- 

 actly like their parents, but combine individual character- 



