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Page:Popular Science Monthly Volume 32.djvu/117

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AMERICAN ZOÖLOGISTS AND EVOLUTION.

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,[1] in a paper on the geographical distribution of certain fresh-water mollusca and the possible cause of their variation, shows the paucity of forms of Unionidæ 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 Strepotomidæ, east of the Alleghanies. He assumes that the first fresh-water forms were lacustrine. He 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 attention to the fact that at a meeting of this Association, at Hartford, in 1874, I made a communication on the origin of the North American Unionidæ, in which I urged some of the points made by Dr. White and Professor Wetherby.[2]

Dr. Thomas H. Streets,[3] in studying the immature plumage of the North American shrikes, was much struck with the close resemblance between the plumage of the young of Sula 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 service, 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 species of shrikes from this standpoint, and traces their descent from a common ancestor.

  1. "Journal of the Cincinnati Society of Natural History," vol. iii, p. 357, and vol. iv, p. 156.
  2. The following is a brief abstract which was published in the Hartford "Courant," August, 1874: "Mr. Morse, in explaining the origin of the North American Unionidæ, 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 families 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. He cited certain families of fresh-water mollusks which are so closely 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."

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