PHYLOGENY. 75 



of life-forms has been from the simple to the complex, 

 and from the generalized to the specialized. These 

 two forms of expression are not identical. In the 

 phrase, "from the simple to the complex," is implied 

 an ascending scale of evolution. In the phrase, "from 

 the generalized to the specialized," we may include 

 both progressive and retrogressive evolution. Retro- 

 gressive or degenerative evolution has been a frequent 

 phenomenon in the past, and scarcely an organism ex- 

 ists which does not display degeneracy in some detail of 

 its structure. Progressive evolution has, how r ever, not 

 been prevented by the frequent occurrence of an op-, 

 posite process ; and, indeed, degeneracy of parts, or of 

 types of life, have been necessary to the advance of 

 other and better organs or forms. 



It is necessary to an understanding of the laws of 

 evolution to get beforehand some idea of what that 

 evolution has actually been. I will, therefore, give a 

 general outline of the phylogeny of plants and animals, 

 and will thus illustrate the subject in full detail in the 

 case of the Vertebrata, where our facilities are espe- 

 cially good. 



It is well known that the Protophyta and the Pro- 

 tozoa are not distinguishable by any sharp line of de- 

 marcation. Chlorophyll, which is so characteristic of 

 plants, is absent from many of the lowest forms, in- 

 cluding the entire class of Fungi, while it is present 

 in a few of the lowest animals. The capacity for mo- 

 tion from place to place, so general in animals, at 

 least in their earlier stages, is present in the earlier 

 stages of some of the Algae, and is universal, except at 

 the period of reproduction, in the Myxomycetes. If it 

 be denied that the latter are plants, then they are ani- 

 mals which do not reproduce by the ordinary process 



