No. 2.] THE HARD PARTS OF THE MAMMALIA. 163 
external and shorter digits, of the sloths; so that there remain 
but two and three (Choloepus and Bradypus), and in the climbing 
anteater (Cycloturus) but one principal toe, and two rudiments. 
Figure 12.— Manus of Artiodactyla, much reduced; from Kowalevsky. 1, //7zp- 
popotamus ; 2, Hyopotamus ; 3, Sus; 4, Gelocus,; 5, Cervus. 
The excessive strain and impact experienced by certain digits in 
leaping, accounts for the digital reduction in the hinder foot 
of the kangaroos and jerboas, precisely as in the Perissodactyle 
ungulates. 
3. THE Fixep ARTICULATIONS. 
On entering the subject of the articulations, a preliminary 
observation on their origin must be made. No evolutionist can 
doubt that the discontinuity of matter, expressed by an articula- 
tion, is due to flexure of the body exhibiting it. Primitively, the 
softer tissues of which all animals were composed, as the inferior 
forms are to-day, were susceptible of flexure in various direc- 
tions. The deposit of hard mineral or organic substances 
within the tissues, producing calcareous layers, chitin, horn, and 
bone, has not in a majority of cases suppressed motion, but has 
restricted it to definite directions. It cannot have been other- 
wise than that, since the motions of animals continued during 
the evolution of their hard parts, these hard parts grew in exact 
adaptation to these movements. Thus at the points of greatest 
flexure joints would be formed, and between these joints the 
deposit would be continuous. 
