CHAPTER XVIII 



THE PHYLOGENY OF ANIMALS 



In this chapter the writer will endeavor to trace broadly 

 the i)hylogeny of the animal kingdom, in conformity with 

 views expressed so far in the foregoing pages. This phylogeny 

 will illustrate four principles that have already been empha- 

 sized: first, that animal life, like vegetable life, primarily 

 originated in fresh- water areas; second, that the main line, 

 as well as many lateral lines of animal advance, have long 

 remained amid such surroundings, but gradually adopted 

 a terrestrial life; third, that successive migrations from fresh 

 to salt water have taken place along the entire evolving ani- 

 mal scale, and that the marine immigrants usually developed 

 or further strengthened either a hard, often heavy, calcareous 

 shell or a chitinous investment along with creeping or swim- 

 ming habit; or became soft, pelagic, and much modified in 

 structural detail; /o?<r//?, that all the great invertebrate groups 

 had already been fully established and broadly distributed 

 by the close of the archsean epoch. 



Distributional and structural evidence has already been 

 advanced to prove that all of the primitive protozoan groups 

 are now mainly fresh-water in habit, wliile we have also ac- 

 cepted it that an easy transition from colonial flagellate in- 

 fusors like Proterospongia to the multicellular or metazoan 

 sponges is clearly indicated. For in sponges a colonial group 

 of cell individuals, formed by repeated division from a single 

 cell individual as in young stages of Proterospongia, has merged 

 into a multicellular organism often of complicated structure. 

 Now Proterospongia and some of the simplest as well as world- 

 wide s})onges are fresh- water, and give no indication of ever 

 having had a marine existence. But, while Spongilla — of 

 world-wide distribution — and allies of it are still fresh-water, 



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