66 HUXLEY, ON CLASSIFICATION. 
bers of the animal kingdom, from the highest to the lowest, are 
marvellously interconnected. Every animal has a something in 
common with all its fellows; much, with many of them; more, with 
afew; and, usually, so much with several, that it differs but little 
from them. 
Now, a morphological classification is a statement of these grada- 
tions of likeness which are observable in animal structures, and its 
objects and uses are manifold. In the first place, it strives to throw 
our knowledge of the facts which underlie, and are the cause of, the 
similarities discerned into the fewest possible general propositions— 
subordinated to one another, according to their greater or less degree 
of generality ; and in this way it answers the purpose of a memoria 
technica, without which the mind would be incompetent to grasp and 
retain the multifarious details of anatomical science. 
But there is a second and even more important aspect of morpho- 
logical classification. Every group in that classification is such in 
virtue of certain structural characters, which are not only common 
to the members of that group, but distinguish it from all others ; and 
the statement of these constitutes the definition of the group. 
Thus, among animals with vertebre, the class Mammalia is de- 
finable as those which have two occipital condyles, with a well-ossified 
basi-occipital; which have each ramus of the mandible composed of 
a single piece of bone and articulated with the squamosal element of 
the skull; and which possess mamme and non-nucleated red blood- 
corpuscles. 
But this statement of the characters of the class Mammalia is 
something more than an arbitrary definition. It does not merely 
mean that naturalists agree to call such and such animals Mammalia ; 
but it expresses, firstly, a generalization based upon, and constantly 
verified by, very wide experience; and, secondly, a belief arising out 
of that generalization. The generalization is that, in nature, the 
structures mentioned are always found associated together: the 
belief is, that they always have been, and always will be, found so 
associated. In other words, the definition of the class Mammalia is 
a statement of a law of correlation, or coexistence, of animal struc- 
tures, from which the most important conclusions are deducible. 
For example: if a fragmentary fossil be discovered, consisting of 
no more than a ramus of a mandible and that part of the skull with 
which it articulated, a knowledge of this law may enable the palzon- 
tologist to affirm, with great confidence, that the animal of which it 
formed a part suckled its young and had non-nucleated red blood- 
corpuscles; and to predict that should the back part of that skull be 
discovered, it will exhibit two occipital condyles and a well-ossified 
basi-occipital bone. 
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