UROCOPTID^:. xix 



the result of fortuitous convergence due to similar mechanical 

 conditions need not here be discussed. The latter view seems 

 now the more probable. 



The wide range of conditions of life, no less than the in- 

 stability of the regions in which the evolution of Urocoptida 

 has taken place, has favored the formation of a great number 

 of phyla. Thus in Mexico, Holospira, Spartocentrum and 

 Berendtia live under conditions as totally diverse from the 

 environment of Eucalodium, Coelocentrum, etc., as though 

 they were on another continent. There has been local adap- 

 tive radiation, whereby the various genera of Eucalodiince 

 have diverged to occupy stations where they no longer com- 

 pete with one another, and are exposed to the incidence of 

 different groups of external forces. 



In the unstable Antilles, adaptive radiation has played a 

 great role. With each period of depression, there was evolved 

 on each isolated area series of forms to fill the various sta- 

 tions or sets of conditions available; and upon re-elevation, 

 with consequent union of some formerly separated areas, the 

 more or less parallel specialized series of snails were thrown 

 together, in competition. Thus, western Cuba was appar- 

 ently cut off from eastern Cuba, Haiti and Jamaica, before 

 the evolution of the genus Brachypodella, and remained iso- 

 lated until comparatively recently. It was inhabited by a 

 species, or a homogeneous group of species, of a stock of 

 Urocoptis having the comparatively generalized character of 

 wide central teeth, by Microceramus, and perhaps by other 

 Urocoptid groups now extinct. The former genus, by local 

 adaptive radiation, produced (1) a group of rather large 

 terrestrial species, with many functional radular teeth, Pyc- 

 noptychia, etc. ; a group of smaller forms of the same general 

 type, Gongylostoma elegans, etc. (2) A group of partially 

 arboreal forms, with incomplete or very shortly free peri- 

 stome, Tomelasmus torquata, etc. (3) Elongate, slender, 

 rock-living forms, Callonia; and (4) slender, small, long- 

 necked species, with the inner four teeth of the radula en- 

 larged, Tetrentodon. Now these several groups, from an 

 ecological point of view, are more or less exactly equivalent 



