Classification of Mammalia^ Birds^ Reptiles, <^c. 395 



Classification of Mammals, Birds, Beptiles, and Fishes, from 

 Embryonic and Palceozoic data. 



The principle, says Agassiz, which has regulated our classifications 

 in zoology for the last half century, is that which Cuvier worked out 

 by his anatomical investigations ; I mean the arrangement of the 

 whole animal kingdom according to the natural affinities of animals 

 as ascertained by the investigation of their internal structure. This 

 fruitful principle, applied in various ways, has produced a series of 

 classifications, agreeing or differing more or less in their outlines, 

 but all resting upon the idea, that a certain amount of anatomical 

 characters may be easily ascertained, expressing the main relations 

 which exist naturally among animals, and affording a natural basis 

 for classification. Structure, therefore, internal as well as external, 

 is, according to the principles of Cuvier, the foundation of all natural 

 classifications ; and, undoubtedly, his researches, and those of his fol- 

 lowers, have done more in the way of improving our natural methods, 

 than all the efforts of former naturalists put together, and this prin- 

 ciple will doubtless regulate, in the main, our further efforts. 



Nevertheless, so much is left in this method to the arbitrary de- 

 cision of the observer, that it would be in the highest degree desir- 

 able to have some principle by which to regulate the internal details 

 of the edifice. 



We may indeed form natural divisions simply from structural 

 evidence, bring together all fishes as they agree in the most impor- 

 tant details of their structure, and combine all reptiles into one class, 

 notwithstanding the extreme differences in their external form. 

 We may also recognise the true affinity of whales, and bring them 

 together with other mammalia, notwithstanding their aquatic habits 

 and their fish-like form ; we may even subdivide those classes into 

 inferior groups upon structural evidence, and thus introduce orders, 

 like the Quadrumana, Carnivora, Rodentia, lluminantia, &c., &c., 

 among Mammalia. But we are at once at a loss how to determine 

 the relative value of those groups, and to find a scale for the natural 

 arrangement of further subdivisions. After having, for example, 

 circumscribed the Carnivorous Mammalia into one natural family, 

 how are we to group the minor divisions like that of the swimming 

 Carnivora, the Plantigrada, and the Digitigrada ; or, after circum- 

 scribing the reptiles into natural groups like those of Chelonians, 

 Saurians, Ophidians, and Batrachians, how shall wo, for instance, 

 arrange the various types of Batrachians ] To those who have been 

 familiar with our proceedings in all these attempts, it must be evi- 

 dent that the grouping of our subdivisions have been almost arbitrary 

 and entirely left to our decision without a regular guide. We have, 

 it is true, subdivided the Batrachians into the more fish-liko forms 

 which preserve their gills and tails, or at least their tails ; and into 

 another group containing those which undergo a complete metamor- 



