CHAPTER XV 



THE EVOLUTION OF ANIMALS (Continued) 



In a study of the palseontological record of animals, and of 

 the facts it reveals as to the periods when different species, 

 genera, and families appeared, one is often impressed by the 

 apparent suddenness with which many of the leading groups 

 are ushered in. This apphes not merely to invertebrates, 

 but even to vertebrates, as in the case of fishes, which appear 

 apparently suddenly in such biologically ancient rocks as the 

 Silurian. But it is permissible to estimate the comparative 

 evolution of the species, and the length of time needed for 

 such evolution, by the thickness of the strata in which their 

 remains occur. Then the appearance of arthropods and of 

 gasteropodous molluscs in the cambrian, as well as of fishes 

 in the lower silurian, rocks is appropriate. For the frequent 

 thickness of these rocks, in various parts of the earth, indicates 

 long ages of biochemical activity. 



A variety of genealogical tables and of classifications has 

 been proposed, each one of w^hich sets forth the supposed main 

 fine of evolutionary advance from a unicellular type to man, 

 as well as the Important side branches that have diverged 

 from it. Most of these in passing upward from the Protozoa 

 early reach some of the annelidan sea types, that conduct 

 thence by rather sudden and discontinuous morphological 

 variations to the Hemichordata or series that includes Balano- 

 glossus, thence to a primitive free-swimming member of the 

 Ascidia or Urochordata and again to the Cephalochordata 

 that has as sole representative Amphioxus {Branchiostoma), 

 while from it a saltatory advance is made to the scaled fishes. 

 Therefore, from the early "vermes" or worm ancestors, on 

 to the fishes, a marine environment is claimed for the verte- 

 brate phylum. 



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