Page:Popular Science Monthly Volume 10.djvu/17

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7
AMERICAN ZOÖLOGISTS AND EVOLUTION.

tions surrounding the first form in the Mohawk River, he had reason to believe that the rapid currents which pass over it bear along substances 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 Lymnaæ elodes, catascopium, and marginata, "are modifications of one type or species, influenced by locality and temperature varying the method of development."[1]

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 removing the confervæ, 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. Tryon, in his large work on the American Melanians, published by the Smithsonian Institution, having finished his manuscript in 1865, says, under date of 1873, when the work was finally published, "A more enlarged acquaintance with fresh-water shells convinces 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 gentleman foremost in the ranks as a describer of species, we find it formulated 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 inducement!

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