AMERICAN ZOOLOGISTS AND EVOLUTION. 107 



subjected to these influences, and hence has resulted a variety of forms 

 which have gone on continually widening to the present day. 



Professor A. G. Wetherby,* in a paper on the geographical distri- 

 bution of certain fresh-water mollusca and the possible cause of their 

 variation, shows the paucity of forms of Unionidce on the Pacific and 

 Atlantic coasts as compared to the richness and profusion of those 

 forms in the central portion of the continents. He remarks also on 

 the absence of the family Strepototnidce^ east of the Alleghanies. He 

 assumes that the first fresh-water forms were lacustrine. lie points 

 out the well-known geological fact of large inland inclosures and their 

 subsequent drainage, and shows the vicissitudes which must have been 

 encountered by species in the variety of physical conditions implied 

 by these changes. In this connection I may be permitted to call at- 

 tention to the fact that at a meeting of this Association, at Hartford, 

 in 1874, I made a communication on the origin of the !N^orth Ameri- 

 can Unionidm, in which I urged some of the points made by Dr. 

 White and Professor Wetherby.f 



Dr. Thomas H. Streets,^ in studying the immature plumage of the 

 North American shrikes, was much struck with the close resemblance 

 between the plumage of the young of Sida cyanops and the adult 

 plumage of another species. Recalling a generalization made by 

 Darwin, that " when the young differs in color from the adult, and the 

 colors of the former are not, as far as we can see, of any special serv- 

 ice, they may generally be attributed, like various embryological 

 structures, to the retention by the young of the characters of an early 

 progenitor." He then shows the gradation between the several spe- 

 cies of shrikes from this standpoint, and traces their descent from a 

 common ancestor. 



* "Journal of the Cincinnati Society of Natural Uistory," vol. iii, p. o'^")^ and vol. iv, 

 p. 156. 



f The following is a brief abstract which was published in the Bartford " Courant," 

 August, 18Y4 : "Mr. Morse, in explaining the origin of the North American UnionidcE, 

 did not pretend to point out the absolute line of descent in these forms, but wished to 

 call attention to some curious features in the possible derivation of the fresh-water fami- 

 lies of mollusks from cognate genera living in salt water. It is observed, first, that the 

 few families of fresh-water mollusks are intimately related to those forms which live in 

 the sea between high and low water mark, and those which can withstand the influence 

 of brackish water. lie cited certain families of fresh-water mollusks which are so close- 

 ly related to tidal forms as hardly to be distinguished from them. ... In explaining the 

 immense number of species of fresh-water mussels in America compared to the very few 

 forms in Europe, we might look to an explanation of this feature in the past geological 

 history of the two continents. 



" In Europe there have been no great inland seas, while in America its past history 

 shows the inclosing of large tracts of water in which freshening from brackish water 

 went on, and, while many forms succumbed to these changed conditions, only those forms 

 survived which resemble certain littoral species. And with the curious modifications 

 that must have taken place in these changed conditions, one gets a possible explanation 

 of the great variety of mollusks in our Western rivers." 



X " American Naturalist," vol. xvii, p. 389. 



