No. 549] A CASE OF POLYMORPHISM 



537 



Heterogenesis is confined to special periods or caused by 

 special conditions as above set forth. 



However, not only is the question of species confused 

 by the presence of heterogenesis in the genus, but another 

 difficulty which I had not anticipated manifests itself; 

 viz., the discovery that the descriptive work which has 

 already been done upon the genus has not, even the best 

 of it, been sufficiently accurate to be trustworthy. I 

 make this statement with the utmost reluctance, and only 

 after I have spent weeks of effort to bring my observa- 

 tions upon single points into accord with the statements 

 of Rousselet, who is not only the highest authority on the 

 group in question, but who has, as already stated, made 

 the last and most detailed pronouncement upon the 

 species of the genus. I have failed, however, in my 

 efforts. Nor are the discrepancies such as may, with 

 probability, be explained by the assumption of differ- 

 ences in the material which we have examined. Thus, to 

 take an example from Rousselet 's description of the jaws. 

 He says: 



Hudson, in his supplement, has also spoken to similar 

 effect, viz. : 



