200 PEOF. HTJXLEY ON THE 



While entertaining the same general conception of classificatory 

 method, Cuvier saw the importance of an exhaustive analysis of 

 the adult structure of animals. The most complete investigation of 

 the kind ever made under the direction of a single mind, and far 

 surpassing all previous attempts in extent and thoroughness, is 

 contained in the ' Legons d'Anatomie Comparee ' and the ' Eegne 

 Animal.' Cuvier's classification is purely morphological ; it is 

 an attempt to enunciate the facts of structure determined in his 

 time, and largely by his own efforts, in a series of propositions 

 of which the most general are the definitions of the largest groups, 

 and are connected by a series of subordinate, differential proposi- 

 tions with those which constitute the definition of the species. 



In his great work, the 'Entwickelungs-Geschichte der Thiere,' 

 Yon Baer, among other contributions to science of first-rate im- 

 portance, showed that our knowledge of an animal's true struc- 

 ture must be imperfect, unless we are acquainted with those 

 developmental stages (which are successive structural conditions) 

 through which the animal has passed in its way from the ovum 

 to the adult state ; and, since 1828, no philosophical naturalist 

 has neglected embryological data in forming a classification. 



In 1859, Darwin, in the ' Origin of Species,' laid a new and firm 

 foundation for the theory of the evolution of living beings, which 

 had been hypothetically sketched out by Lamarck, and thereby 

 introduced a new element into Taxonomy. If a species, like an 

 individual, is the product of a process of development, the 

 character of that process must be taken into account when we 

 attempt to determine its likeness or unlikeness to other spe- 

 cies ; and Phylogeny, or the history of the evolution of the 

 sj)ecies, becomes no less important an element than Embryo- 

 geny in the determination of the systematic place of an animal. 

 The logical value of phylogeny, therefore, is unquestionable ; but 

 the misfortune is, that we have so little real knowledge of the 

 phylogeny even of small groups, while of that of the larger groups 

 of animals we are absolutely ignorant. To my mind there is full 

 and satisfactory proof of the derivation of Equns from Sipparion, 

 and of this from an Anchitherioid ancestor ; and there is much 

 to be said in favour of the derivation of other genera of existing 

 Mammals from their Tertiary predecessors. There are also pretty 

 clear indications of the series of changes by which the Ornithic 

 arose out of the Eeptilian type, and the Amphibian from the 

 Fish ; but I do not know that as much can be said of other large 



