1 72 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



3. THE LAW OF THE UNSPECIALIZED. 



The facts cited in the preceding parts of this chap- 

 ter show that the phylogenetic lines have not been 

 continuous, but that they may be represented by a 

 system of dichotomy. Jn other words, the point of 

 departure of the progressive lines of one period of time 

 has not been from the terminal types of the lines of 

 preceding ages, but from points farther back in the 

 series. Thus it is not the highly developed or spe- 

 cialized plants which have given origin to the animal 

 kingdom, but the lowest forms or Protophyta, which 

 are not distinguishable from the Protozoa. Among 

 animals it is not the specialized Arthropoda or Mol- 

 lusca which present the closest affiliations with the 

 Vertebrata, but the simple Vermes or Tunicata, from 

 which the origin of the latter may be traced. In the 

 Vertebrata it is not the higher fishes (Actinopterygia) 

 which offer the closest points of affinity to the succeed- 

 ing batrachian class, but that more generalized type 

 of the Devonic period, the Rhipidopterygia, which 

 probably occupies that position. The modern types of 

 Batrachia (Urodela, Salientia) have plainly not fur- 

 nished the starting-point for the reptiles, but the an- 

 cient order of the Stegocephali, which are also fish- 

 like, is evidently their source. The Reptilia of the 

 Permian present us with types with fish-like vertebrae 

 (Cotylosauria, Pelycosauria), from which the class 

 Mammalia may be distinctly traced. The later reptiles 

 diverged farther and farther from the mammalian type 

 with the advance of geologic time. The same prin- 

 ciple has been found to be true in tracing the history 



