MAMMALIA. 
9 
The number of known species of Mammals is, according to 
Van der Hoeven (Philos. Zool. p. 330), 1800 ; but this computa- 
tion was made in accordance with the state of the knowledge on 
the subject in the year 1855, without reference to the discoveries 
made since the publication of ^ Wagner^s Saugethiere.^ 
"^pROF. J. D. Dana has commenced to publish a series of papers, in 
which he exhibits his ideas of a natural system of the animal 
kingdom. The paper on the Classification of Herbivores falls 
within the limits of this Record ; but, in order that the position 
of this subdivision in the class of Mammals, and the principle of 
classification generally employed may be understood, we must 
refer to a preceding paper, On the higher subdivisions in the 
Classification of Mammals,^^ in Amer. Journ. 1863, p. 65. 
The principle of classification used by the author is the 
cephalization of the body ” — that is, the subordination of its 
members and structure to head-uses. This principle and all its 
applications rest on the following facts : 
1. An animal is embodied or concentred force, which force 
manifests polarity in the results of its action in development — 
that is, in the oppositeness of the anterior and posterior ex- 
tremities of the structures evolved, and also in the dorso-ventral 
relations of these structures. 
2. The primarjr potential ’ centre is in the head, or, more 
precisely, in the cephalic nervous mass — an animal being funda- 
mentally a cephalized organism. But, besides this, there may 
be one or more secondary centres. 
3. Species difier (a) in the amount of force concentred ; (b) 
in the degree of control of the systemic force over vegetative 
growth and development; (c) in the distribution of the force 
along the principal (or fore and aft) axis — that is, in its being 
concentrated mainly anteriorly, or diffused, to a greater or less 
degree, from the cephalic extremity posteriorly toward the 
caudal extremity or pole. 
4. The differences just mentioned are expressed in the struc- 
ture of the organism ; and all such expressions are necessarily 
expressions of grade. 
5. Each of these kinds of differences must have expression, or 
be apparent, (a) through the various circumstanccs^atLending 
development or growth, and (b) through all the steps in the 
progress of growth, as well (c) in the resulting structures. 
Although the characteristics afforded by the principle of 
cephalization, like all others appealed to in classification, cannot 
overrule affinities based on obvious resemblances in type of 
structure, yet this subject throws new light on the limits and 
gradal distinctions of groups. The concentration of the anterior 
