AMERICAN ZOOLOGISTS AND EVOLUTION. 103 



ated, and so stunted in size as to be mistaken for distinct species." 

 Dr. W. II. Dall,* in some general considerations regarding the en- 

 vironment 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 ex- 

 plained on the ground of their habitat. Di'. Carl F. Gisslerf has pre- 

 sented 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.J 

 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 Avays, 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 mani- 

 festing them." 



In considering the "Laws of Digital Reduction,"* 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 coiTela- 

 tiou 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 pei*- 

 forming 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 re- 

 mained in a state of approximate uniformity of development. 



" 3. It is held that these views are Lamarckian and not Darwin- 

 ian — that is, that they more especially take cognizance of mechanical 

 force as a mutating factor in evolution, in accordance with the doc- 

 trine 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 



* "Bulletin of the Uluseum of Comparative Zoology," vol. xii, Xo. 6, p. 183. 



\ " Proceedings of the American Associated Antiquarian Society," vol. xxix, p. 557. 

 \ "American Naturalist," vol. xii, p. 15*7. 



* Ibid., vol. xi, p. 603. 



