CHAPTER XVI 



THE EVOLUTION OF ANIIVIALS, Part III 



A consideration of the evolution of craniate animals has 

 engaged many naturalists since Darwin's "Origin of Species" 

 and some of Haeckel's earlier works appeared. It would be 

 superfluous here to attempt a review of the various classi- 

 fications proposed, though in connection with some often 

 valuable treatises vague and even random statements have 

 been made, that might prejudice a fair consideration of the 

 facts before one. Thus when within a few pages of each other 

 in one volume it is said, "if degeneracy consists in the loss of 

 parts without complimentary addition of other parts, then 

 the batrachian line is a degenerate line," and "the Reptilia 

 of the permian present us with types wdth fish-like vertebrae, 

 from which the class Mammalia may be distinctly traced," 

 and again "the later reptiles diverged farther and farther from 

 the mammalian type with the advance of geological time," 

 we have generalized assertions made that — unless backed by 

 detailed facts — may produce impressions wide of phylogenetic 

 value. 



In the present chapter we shall endeavor to compare struc- 

 tural details of the Craniata — alike living and fossil — and 

 see how far these may contribute to some helpful positions. 

 In doing this we shall dwell rarely on the Urochordata or 

 Tunicata, since the consensus of opinion now is that they 

 are very primitive chord ate forms that have become much 

 degraded and modified even from the larval state owing to 

 their assuming a sessile life. Aviphioxus, though greatly 

 higher in structure, we shall view as a much modified and in 

 not a few respects degraded type from the main vertebrate 

 line (p. 543). The burrowing habit has conferred both modi- 

 fied and degraded conditions of structure. Wliile greatly 



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