480 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.46. 



of wide geographic range and of notable strength numerically, are 

 without known antecedents. As an instance of this sort, we have 

 Ha'plocrinus, a form curiously primitive in many respects despite its 

 Middle Devonian occurrence. This genus has a laiown range from 

 Germany to New York State. As elsewhere suggested, this genus 

 may well have been derived from a form not greatly dissimilar to 

 Homocrinus. One or two such cases of apparently isolated types 

 might be explained on the assumption of sudden introduction into a 

 given area of hitherto excluded faunas. To attempt to explain all 

 such cases and the related phenomena on such a basis would involve 

 an unnecessary assumption of unstable seas and barriers. 



The importance of such a simple group in determining or influenc- 

 ing the evolution of the Crinoidea is largely dependent upon the 

 ability of its constituent members under the impetus of changed con- 

 ditions or for other cause, to vary and give rise to sturdy lines in 

 which the tendency toward mutation is perpetuated. One must 

 predicate such power as latent in these minute forms, else their inter- 

 est and importance hes solelj^ in their existence. As is well known, a 

 type that persists for a long time apparently loses its power to vary, 

 at least fundamentally. So it is in the case of many long-Uved 

 brachiopods. Such instances are those preeminently of genera and 

 species. It probably is true that in larger groups much the same con- 

 dition of affairs obtains, though in a less marked degree. With them 

 the tendency toward variation is arrested rather than destroyed, 

 however, and though somewhat impaired in vigor may be revived by 

 the appUcation of competent stimuli. Subsequent to such stimula- 

 tion it may well be that the resultant Unes do not have the inherent 

 strength of those evolved earUer in the history of the stock, but such 

 differences tend to be quantitative rather than qualitative. Such 

 limitations necessarily apply only to the minute primitive fomas of 

 the later Paleozoic. The status of such forms in the early history of 

 the Pelmatozoa is probably quite different. Here there existed any- 

 thing but a condition of stagnation. In their small way mutations 

 doubtless were of frequent occurrence and of appreciable weight. 



How very acceptable such an hypothesis will prove may readily be 

 seen. Given a persistent stock of primitive character and one may 

 predicate offshoots in the evolution of which convergence and paral- 

 lelism wUl generate types strikingly similar in many respects and yet 

 incapable of derivation one from the other. That conditions exist 

 among the Crinoidea explicable only on the assumption of the exis- 

 tence of numerous polyphyletic strains seem capable of demon- 

 stration. Indeed it is probable that few of the groups into which 

 the Crinoidea have been subdivided are monophyletic — unless such 

 groups be comparatively smaU and closely circumscribed. 



