SEC. II PRINCIPLES AND PRACTICE OF CLASSIFICATION 105 



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 definition of the 

 group. 



"Thus, among animals with vertebras, the class Mammalia is 

 definable as those which have two occipital condyles, with a well- 

 ossified basi- occipital ; Avhich 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 mammee 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 generalisation based upon, and constantly 

 verified by, very wide experience ; and, secondly, a belief arising 

 out of that generalisation. The generalisation 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 

 structures, from which the most important conclusions are deducible " 

 (Introd. to Classif. of Animals, 8vo, London, 1869, pp. 2, 3). 



But broad as such laws of correlation of structure are, and 

 important as are the conclusions deducible, we must constantly be 

 on our guard against presuming upon the infallibility either of the 

 data or of the deduction, as the author just quoted goes on to show. 

 Such caution is specially required where there is no obvious reason 

 for the particular combination that may be found to exist. In the 

 case of the ostrich-like birds {Ratike), for example, we can understand 

 how a flat, unkeeled breast-bone, a particular arrangement of the 

 shoulder -bones, and a rudimentary state of the wing -bones, are 

 found in combination, because all these modifications of structure 

 are evidently related to loss of the power of flight ; and, in point 

 of fact, no exception is known to the generalisation, that such 

 conditions of the sternal, coraco-scapular, and humeral bones always 

 coexist. But in all known struthious (ratite) birds, this state of 

 the bones in mention coexists also with a peculiar modification of 

 the bones of the palate, and no necessary connection between these 

 two sets of diverse characters is conceivable. Now, if we only 

 knew struthious birds, and found the combination in mention to 

 hold with them all, we should doubtless declare our belief that any 



