AMERICAN ZOOLOGISTS AND EVOLUTION. 7 



tions surrounding the first form in the Mohawk River, he had reason 

 to believe that the rapid currents which pass over it bear along sub- 

 stances that, coming in contact with the exposed edges of the shell, 

 break them down, thus retarding the growth of the shell at this point, 

 and the animal concentrates its growth -powers to the repairs of the 

 broken portion. The same gentleman also shows that the so-called 

 species Lymnana elodes, catascopium, and marffinata, " are modifica- 

 tions of one type or species, influenced by locality and temperature 

 varying the method of development." ' 



A. G. Wetherby 2 calls attention to the variation in form of a group 

 of fresh - water snails, found in the greatest abundance in certain 

 streams of Tennessee and North Alabama. In showing the varied 

 influences they are subjected to he cites the rapid currents of the 

 channels, and the greater liability of the snails being torn from the 

 rocks. He shows that they are exposed in various ways to the effects 

 of these currents, with all their changing impetus of high and low 

 water exposed also to privation of food from the scouring sand re- 

 moving the conferva?, upon which they subsist, from the rocks. He 

 takes into account temperature, chemical action, and the like, and 

 says, " No greater vicissitude can be imagined than this growth in an 

 unstable element." Coincident with these diverse conditions he finds 

 an enormous variety of forms, and frankly acknowledges that many 

 of those described as distinct species must be reduced to synonyms. 



George W. Ti-yon, in his large work on the American Melanians, 

 published by the Smithsonian Institution, having finished his manu- 

 script in 1865, says, under date of 1873, when the work was finally 

 published, "A more enlarged acquaintance with fresh-water shells con- 

 vinces me that a much greater reduction of the number of species 

 than I have attempted must eventually be made." 



If we now look upon the definition of a species, as given by a gen- 

 tleman foremost in the ranks as a describer of species, we find it formu- 

 lated as follows : A species represents " a primary established law, 

 stamped with a persistent form (a type) pertaining solely to itself, 

 with the power of successively reproducing the same form, and none 

 other ;" and this gentleman has not hesitated to base these " primary 

 organic laws" upon the evidence of a single specimen, and in some 

 cases even the fragments of one have offered him a sufficient induce- 

 ment ! 



But it has been argued by some that a wide variation may be the 

 case "with many species. Prof. Agassiz, 3 at a meeting of the American 

 Academy, reiterated his opinion that what are called varieties by 

 naturalists do not in reality exist as such. He found a great abun- 

 dance of diverging forms in Echinoderms, which, without acquaintance 



1 " Proceedings of the Boston Society of Natural History," vol. v., pp. 121-128. 



2 Proceedings of Cincinnati Society of Natural Science, No. 1, June, 1876. 

 8 " Proceedings of the American Academy," vol. v., p. 72. 



