34 IOWA STUDIES IN NATURAL HISTORY 



*ASTBOCNIDA OpHIOMITRA 



*ASTROPHYTUM *OpHIAC: NTHELLA 



*ASTROGORDIUS *MlCROFHIURA 



*ASTROCYCLUS *0PHI0BLENNA 



*ASTROCYNODUS *OpHIONEMA 



[ ?Ophiochondrus] *Ophiothyreus 



*Ophiochondrella *Amphipholizona 



*Ophiop^pale 



While a number of these genera will undoubtedly eventually 

 be found in other parts of the world, still the fact that they 

 represent 24% of the total number of genera occurring in the 

 West Indies is significant, and the fact that all but three of them, 

 or 83%, are monotypic is still more so. 



The existence in the West Indian region of these endemic 

 genera indicates that this region constitutes a very marked 

 faunal entity, while the occurrence of so many monotypic genera 

 suggests that faunal stability was attained a long while ago; in 

 other words that, as compared to the other portions of the 

 oceans, faunal evolution has been retarded so that the West 

 Indian region may be described as faunally the most ancient 

 portion of the recent seas. 



So far as we are able to judge from the geological record, 

 taken in connection with the recent fauna, there are two quite 

 distinct types of distribution, always, however, more or less 

 superimposed, in every homogeneous animal group. 



A newly arisen animal type immediately spreads to the limits 

 of its possible distribution, and within the area overrun by it all 

 sorts of variants appear which may be considered as of varietal, 

 subspecific or specific value. 



Thus a genus or any other homogeneous group represented in 

 a given faunal district by a number of closely related types is a 

 group of recent origin, or of recent introduction. 



As time passes a number of factors begin to assert themselves, 

 internal and external parasitism, economic pressure of great 

 numbers of individuals, and economic pressure of competing 

 types, which tend to restrict the possible scope of variation and 

 to confine the variants within well marked limits which con- 

 stantly become narrower and narrower until only a few well 

 marked and distinct types remain each of which has now ac- 

 quired the status of a monotypic genus. 



