2 PRIMITIVE ANIMALS 



objective manner we are forced to accept a great 

 deal of it on trust. The greatest triumphs in this 

 attempt to piece together the actual history of 

 animal change have been won within the last fifty 

 years from the study of vertebrate animals, especially 

 of the most modern and dominant group of them, the 

 Mammalia, and it is no exaggeration to say that 

 we can now trace the history of all the principal 

 kinds of living placental Mammalia through a series 

 of almost continuous gradations back to their 

 common origin in Eocene times. But the Mammalia, 

 geologically speaking, are modern animals, their 

 evolutionary history has been accomplished with com- 

 parative rapidity, and their durable fossilized remains 

 have been deposited in strata which in many cases 

 have not been excessively disturbed by subsequent^ 

 events. We might dwell upon other achievements" 

 won by the application of the evolutionary theory 

 to the study of living and extinct animals, but our 

 present object is rather to pass the animal kingdom 

 hurriedly in review 1 and indicate the serious gaps in 

 the evolutionary scheme which there seems little 

 prospect of ever filling in, except by unverifiable 

 conjectures. 



The simplest animals known are those whose 



1 In Appendices A and B (pp. 150, 152) a summarized classifica- 

 tion of the animal kingdom and a table of the chief geological 

 periods may be found useful for reference. 



