MORPHOLOGICAL CLASSIFICATION. 



more, with a few ; and, usually, so much with several, that it 

 differs but little from them. 



Now, a morphological classification is a statement of these 

 gradations 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 pos- 

 sible 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 

 morphological classification. Every group in that classification 

 is such in virtue of certain structural characters, which are not 

 only common to the members of the group, but distinguish it 

 from all others ; and the statement of these constitutes the defi- 

 nition of the group. 



Thus, among animals with vertebrae, the class Mammalia is 

 definable as those which have two occipital condyles, with a 

 well-ossified basi-occipital ; which have each ramus of the man- 

 dible composed of a single piece of bone and articulated with 

 the squamosal element of the skull ; and which possess mammae 

 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 asso- 

 ciated together : the belief is, that they always have been, and 

 always will be, found so associated. In other words, the defini- 

 tion of the class Mammalia is a statement of a law of correlation, 

 or coexistence, of animal structures, from which the most im- 

 portant 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 



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