GREGARINIDA, RHIZOPODA, SrONGIDA, AND INFUSORIA. 3 



As a matter of fact, however, no such mutual independence 

 of animal forms exists in nature. On the contrary, the different 

 members 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 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 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 

 morphological 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 defi- 

 nition of the group. 



Thus, among animals with vertebra?, 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 mammaB 

 and non-nucleated reel 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 Mam- 

 malia : 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- 



13 2 



