362 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



VILA HYPOTHESIS OF THE ORIGIN OF THE DIVISIONS OF 



THE VERTEBRATA. 



In order to estimate the part which has been 

 played by the movements of the Vertebrata in chang- 

 ing their environment in past geologic ages, we have to 

 rely principally on inferences derived from the present 

 physical characteristics of the earth. Formerly, as 

 now, conditions of temperature, humidity, soil, shel- 

 ter, food, etc., were avoided or appropriated by ani- 

 mals, through their capacity for moving from place to 

 place. What concerns us chiefly here, is the effects 

 on their structure produced by the movements of Ver- 

 tebrata. In examining this question I will take it up 

 in systematic order, so as to observe whether kineto- 

 genesis has been the principal or only a subordinate 

 agency in the evolution of this branch of the animal 

 kingdom. 



The most conspicuous index of the serial succes- 

 sion of the vertebrate classes, is, as has been already 

 remarked, the circulatory system. The modifications 

 of this system have been immediately connected with 

 those of the method of respiration, which the exigen- 

 cies of the environment induced in vertebrates. The 

 existence of branchial arteries and veins dates from 

 the earliest vertebrate, if not from prevertebrate life. 

 They are already established in the Tunicata, and con- 

 tinued throughout the rising scale in diminished num- 

 bers, so long as Vertebrata were exclusively aquatic 

 in their modes of life. When at the close of the De- 

 vonian system the land masses assumed great propor- 

 tions in both the Eastern and Western Hemispheres, 

 it is probable that many fishes were entangled in shal- 



