1880.] ARRANGEMENT OF THE MAMMALIA. 651 



would have to be placed iii one order of the class Mammalia and 

 their descendants in another. 



It may be suggested that it might be as well to wait until the 

 primordial Hippoid is discovered before discussing the difficulties 

 which will be created by its appearance. But the truth is, that the 

 problem is already pressing in another shajie. Numerous " Lemurs," 

 with marked ungulate characters, are being discovered in the older 

 Tertiaries of the United States and elsewhere ; and no one can study 

 the more ancient mammals with which we are already acquainted, 

 without being constantly struck with the " Insectivorous " characters 

 which they present. In fact, there is nothing in the dentition of 

 either Primates, Carnivores, or Ungulates which is not foreshadowed 

 in the Insectivora ; and I am not aware that there is any means of 

 deciding whether a given fossil skeleton, with skull, teeth, and limbs 

 almost complete, ought to be ranged with the Lemurs, the Insecti- 

 vores, the Carnivores, or the Ungulates. 



In wliatever order of Mammals a sufficiently long series of forrris 

 has come to light, they illustrate the threefold law of evolution as 

 clearly, though perhaps not so strikingly, as the Equine series does. 

 Carnivores, Artiodactyles, and Perissodactyles all tend, as we trace 

 them back through the Tertiary ejioch, towards less modified forms 

 which will fit into none of the recognized orders, but come closer 

 to tlie Insectivora than to any other. It would, however, be most 

 inconvenient and misleading to term these jirimordial forms " Insec- 

 tivora," the mammals so called being themselves more or less spe- 

 cialized modifications of the same common type ; and only, in a 

 partial and limited sense, representatives of that type. 



The root of the matter appears to me to be that the palseontological 

 facts which have come to light in the course of the last ten or fifteen 

 years have completely broken down existing taxonomical conceptions, 

 and that attempts to construct fresh classifications upon the old 

 model are necessarily futile. 



The Cuvierian method, which most modern classifiers up to the 

 time of the appearance of Haeckel's ' Generelle jNIorphologie ' have 

 followed, has been of immense value in leading to the close inves- 

 tigation and the clear statement of the anatomical characters of 

 animals. But its principle, the construction of sharp logical cate- 

 gories defined by such characters, was sapped when Von Baer showed 

 that, in estimating the likenesses and unlikeuesses of animals, de- 

 velopment must be fully taken into account ; and if the importance 

 of individual development is admitted, that of ancestral development 

 necessarily follows. 



If the end of all zoological classification is the clear and concise 

 expression of the morphological resemblances and differences of ani- 

 mals, then all such resemblances must have a taxonomic value. But 

 they fall under three heads:— (1) those of adult individuals; 

 (2) those of successive stages of embryological development or indi- 

 vidual evolution ; (3) those of successive stages of the evolution of 

 the species, or ancestral evolution. 



An arrangement is "natural" (that is, logically justifiable in view 



43* 



