218 COPE. (Vox. Tr. 
present forms, the Megatheriide. Their long pubis has been 
generally pressed downwards and backwards by the weight of 
the viscera, while the short pubis of the Quadrumana has not 
experienced any such change of direction. The relation of this 
effect to the cause in question is demonstrated by the history 
of the Dinosauria and of the birds. In the former, the direc- 
tion of the pubis has become downwards and then backwards, 
directly as the animals have more or less entirely adopted the 
erect attitude, by walking on their hind limbs. The viscera have 
been gradually swung backwards from in front of to below the 
point of suspension, the acetabulum, by the force of gravity ; 
and with the advent of complete bipedism, in the birds, a part 
of them have moved even posterior to this point, thus more 
perfectly maintaining the balance in progression. 
A similar posterior direction of the pubis exists in the Tal- 
pide. We may with some probability ascribe this to the con- 
stant pressure on the abdomen upwards and backwards, which 
their mode of life entails on them. The viscera and pelvis are 
depressed by superincumbent weight, and the forward movement 
of the body in progression furnishes the pressure in the poste- 
rior direction. 
PL THE DENTITION 
The distinction of teeth into incisors, canines, and molars 
appears independently at various points in the line of Verte- 
brata. Incisors and molars are distinguished in Sparoid fishes, 
and in Placodont and Diadectid reptiles. Canine-like teeth, or 
pseudo-canines, appear in Clepsydropid and Crocodilian reptiles, 
and in Saurodont fishes. Canine-like incisors appear in the 
Clepsydropide. The variety of character in these structures 
presented by the Mammalia to be considered here is great, and 
the principles deduced from observation of them are applicable 
to the Vertebrata in general. 
As mechanical causes of the origin of dental modifications, I 
enumerate the following : — 
I. Increase of size of a tooth, or a part of a tooth, is due to 
increased use, within a certain maximum of capacity for in- 
creased nutrition. 
II. The use and change of direction of a tooth take place 
