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Page:Popular Science Monthly Volume 32.djvu/113

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103
AMERICAN ZOÖLOGISTS AND EVOLUTION.

ated, and so stunted in size as to be mistaken for distinct species." Dr. W. H. Dall,[1] in some general considerations regarding the environment of the deep-sea mollusks, as compared with the shallow water and littoral forms, shows how much the littoral forms have to contend with in the struggle for existence as compared with the deep sea forms, and the delicate sculpture and extreme fragility of many of the shells occurring in the deeper abysses of the sea are to be explained on the ground of their habitat. Dr. Carl F. Gissler[2] has presented some interesting evidences of the effect of chemico-physical influences in the evolution of the branchiopod crustaceans.

The effect of mechanical strains as producing like morphological effects has been treated in a masterly way by Dr. John A. Ryder.[3] He cites the vertebral axes of turtles and extinct armadillos, also the sacra of birds and mammals, and says: "These observed coincidences, it is believed, are neither accidental, nor designed by an active cause external to these organisms or their cosmic environment. I would rather believe that the structures, so far as they have been evolved in parallel or similar ways, are the results of like forces conditioning growth and nutrition in definite modes and determinate directions. The manner of incidence of the modifying forces being in all cases determined by the voluntary actions of the organisms, the actions in turn are determined by the degree of intelligence of the animal manifesting them."

In considering the "Laws of Digital Reduction,"[4] Dr. Ryder gives a concise presentation of the various groups of animals, showing in each the line of mechanical strain in the extremities and its correlation with the increased development of those digits bearing this strain, and the consequent reduction or atrophy of those digits out of this line. These considerations led him to the following conclusions:

"1. That the mechanical force used in locomotion during the struggle for existence has determined the digits which are now performing the pedal function in such groups as have undergone digital reduction.

"2. That where the distribution of mechanical strains has been alike upon all the digits of the manus or pes, or both, they have remained in a state of approximate uniformity of development.

"3. It is held that these views are Lamarckian and not Darwinian—that is, that they more especially take cognizance of mechanical force as a mutating factor in evolution, in accordance with the doctrine of the correlation of forces."

Dr. Ryder further says, "It seems a most convincing proof of the doctrine of descent to find man an instance of the same kind of

  1. "Bulletin of the Museum of Comparative Zoölogy," vol. xii, No. 6, p. 183.
  2. "Proceedings of the American Associated Antiquarian Society," vol. xxix, p. 657.
  3. "American Naturalist," vol. xii, p. 157.
  4. Ibid., vol. xi, p. 603.