CLARK: CORRELATION OF PHYLOGENETIC SPECIALIZATION 317 



including those under the heading ''General," and below 250 

 fathoms the line representing those without. 



If we do this, we find the highest degree of specialization in 

 the recent crinoids above 250 fathoms, and especially between 

 50 and 150 fathoms, which represents a zone above the optimum 

 temperature for the group. 



The least degree of specialization is reached between 550 and 

 750 fathoms, and this is possibly the maximum depth to which 

 a crinoid can ordinarily descend without undergoing semi- 

 pathological degeneration. 



Below 750 fathoms the excess of primitive characters beco^ies 

 slowly less and less pronounced through semi-pathological changes 

 simulating true phylogenetic advance, so that at 2500-3000 

 fathoms it is only slightly less than the excess of specialized 

 characters at the surface. 



If the' preceding deductions are justified, 



(1) The most marked phylogenetical advance, which is always 

 evidenced bj^ a greater or lesser suppression of some structural 

 feature, occurs not under optimum conditions for the type 

 under consideration, but under the more or less unfavorable 

 conditions of the warm littoral. 



(2) Progressively more and more unfavorable conditions induce 

 a correlated phylogenetical conservatism, and finally a phylo- 

 genetical stagnation. 



(3) Very unfavorable conditions induce a progressively in- 

 creasing semi-pathological degeneration which, though usually 

 very different in the details of its manifestation, is biologicall}^ 

 the equivalent of phylogenetical advance under the optimum 

 conditions. 



(4) Many deep sea types, or types living under similarly 

 unfavorable conditions, which exhibit an extraordinary mixture 

 of very primitive and very highly speciaHzed characters, are to 

 be interpreted as primitive types upon which is superposed a 

 pseudo-specialization induced by the pathological effect of their 

 environment. 



