No. 2.] THE HARD PARTS OF THE MAMMALIA. 139 
the writer discussed the mechanical causes of the structures of 
the elbow and other joints, in the Artiodactyla, and the origin 
of the peculiar intervertebral articulations in that order.! This 
enumeration covers, so far as I am aware, the work done in this 
field. A good deal of it has been tentative, while to other por- 
tions of it considerable precision and conclusiveness may be 
granted. 
At the present time we are in a position to understand what 
the structural changes are, which the Mammalia have gradually 
acquired through the operation of causes long continued through 
geologic time. In other words, we are now, thanks to verte- 
brate paleontology, in possession of the phylogeny of most of 
the lines of Mammalian descent. We are thus able to distin- 
guish between primitive characters and characters of degener- 
acy. We can understand the origin and progress, and also the 
decadence, of structural characters. The more my attention has 
been directed to the facts thus presented, the more convinced I 
have become that, in the language of Lamarck, it is the habit 
that has given rise to the structures of animals, and not the 
structures which have forced animals to adopt their special 
habits. 
In the following pages the salient characters of the skeleton 
and of the dentition of the Mammalia are examined, and the 
attempt is made to discover in the light of the descent traced 
by paleontology, the mechanical causes for their existence. It 
is believed that in a considerable proportion of instances this 
attempt has been successful; while in others a tentative stage 
only has been reached. The intelligent reader will be able 
to determine to which category any given discussion may be 
referred. 
The position of the Post-Darwinians is clearly set forth in an 
abstract of a lecture delivered by Prof. E. Ray Lankester, at 
the London Institution, which appears in Nature of February 
28, 1889. Professor Lankaster declares that the error of La- 
marck (and consequently of the Neolamarckians) consists in the 
assumption that acquired characters can be inherited. He says: 
“Congenital variation is an admitted and demonstrable fact ; 
transmission of congenital variations is also an admitted and 
1 American Naturalist, March, 1889. 
