JEvohition 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, Keptile, 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 stnicture. (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 rise — 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 j^roportions. 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 l)y the 

 environment. 



/. 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 tliem. 



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

 actly like their parents, but coinljine individual character- 



