274 : COPE. (Vo. III. 
< 
Insectivora, and Rodentia. The reduction of the number of the 
digits in the posterior limb only when this is extensively used 
for rapid progression, as in leaping: this is seen in the kan- 
garoo and jerboas, in the orders Marsupialia and Rodentia.! 
There are a good many structures in the skeleton of the 
Mammalia which have not yet received a satisfactory expla- 
nation on the ground of mechanical necessity. Such, for in- 
stance, appears to me to be the history of the origin of the 
canine tooth; that is, its use in preference to an incisor for 
raptorial purposes. Such may be also the history of the origin 
of the complex vertebral articulations of the American Edentata 
as compared with the simple articulations of those of the Old 
World. In these as in similar cases, however, an element enters 
which must be taken into account in seeking for explanations ; 
that is, that every evolution is determined at its inception by 
the material or type from which it originates. Thus is explained 
the fact that identical uses have not produced identical struc- 
tures in the limbs of all aquatic vertebrates. The fin of the fish 
is essentially different from the paddle of the Ichthyosaurus or 
the whale. The beak of the raptorial bird is different from the 
canine tooth of the rapacious mammal. When this principle is 
duly considered, many mechanical explanations will become clear 
which now seem to be involved in difficulty or mystery. 
V.. LITERATURE: 
1809. Lamarck, J. B. P. A. Philosophique Zoologique, Chap. VII. ; transla- 
tion, American Naturalist, 1888, p. g60. 
1865. Spencer, Herbert. Principles of Biology, II., pp. 167 and 195. 
1871. Cope, E. D. Proceedings of the American Philosophical Society, p. 
259; on the Method of Creation of Organic Types, reprinted in 
Origin of the Fittest, 1887, p. 210. 
1872. Cope, E. D. Penn Monthly Magazine, on Evolution and its Conse- 
quences ; reprint, Origin of the Fittest, 1887, p. 30. 
1877. Ryder, J. A. American Naturalist, p. 607, on the Laws of Digital 
Reduction. 
1877. Ryder, J. A. Proceedings of the Academy of Natural Sciences of 
Philadelphia, p. 314; on the Significance of the Diameter of the 
Incisors in the Rodents. 
1 An able presentation of the facts embraced in this section has been made by 
Professor Ryder in the American Naturalist, October, 1887, p. 606. 
