726 THE PRINCIPLES OF SCIENCE. [CHAP. 



in another work ; lie says, 1 " Novas species immo et genera 

 ex copula diversarum specierum in regno vegetabilium oriri 

 primo intuitu paradoxum videtur ; interim observationes sic 

 tieri non ita dissuadent." Even supposing in the present 

 day that we could assent to the notion of a certain number 

 of distinct creational acts, this notion would not help us in 

 the theory of classification. Naturalists have never pointed 

 out any method of deciding what are the results of distinct 

 creations, and what are not. As Darwin says, 2 " the de- 

 finition must not include an element which cannot possibly 

 be ascertained, such as an act of creation." It is, in fact, 

 by investigation of forms and classification that we should 

 ascertain what were distinct creations and what were not ; 

 this information would be a result and not a means of 

 classification. 



Agassiz seemed to consider that he had discovered an im- 

 portant principle, to the effect that general plan or structure 

 is the true ground for the discrimination of the great classes 

 of animals, which may be called branches of the animal 

 kingdom. 3 He also thought that genera are definite and 

 natural groups. " Genera," he says, 4 " are most closely 

 allied groups of animals, differing neither in form, nor in 

 complication of structure, but simply in the ultimate struc- 

 tural peculiarities of some of their parts ; and this is, I be- 

 lieve, the best definition which can be given of genera." 

 But it is surely apparent that there are endless degrees both 

 of structural peculiarity and of complication of structure. 

 It is impossible to define the amount of structural pecu- 

 liarity which constitutes the genus as distinguished from 

 the species. 



The form which any classification of plants or animals 

 tends to take is that of an unlimited series of subaltern 

 classes. Originally botanists confined themselves for the 

 most part to a small number of such classes. Linnseus 

 adopted Class, Order, Genus, Species, and Variety, and even 

 seemed to think that there was something essentially natu- 

 ral in a five-fold arrangement of groups. 5 



1 Amcenitates Academicce (1744), voL i. p. 70. Quoted in Edin- 

 burgh Review, October 1868, vol. cxxviii. pp. 416, 417. 



2 Descent of Man, vol. i. p. 228. 



3 Agassiz, Essay on Classification, p. 219. 4 Ibid. p. 249. 

 3 Philosophic Botanica, 155, p. 98. 



