416 THE PRINCIPLES OF SCIENCE. 



fication. Naturalists have never pointed out any separate 

 method of deciding what are the results of distinct crea- 

 tions, and what are not. As Darwin says^, * the definition 

 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. 



The eminent naturalist Agassis seems to consider that 

 he has discovered an important principle, to the effect that 

 general plan or structure is the true ground for the dis- 

 crimination of the great classes of animals, which may be 

 called branches of the animal kingdom 1 ". He also thinks 

 that genera are definite and natural groups. ' Genera/ 

 he says 8 , 'are most closely allied groups of animals, differ- 

 ing neither in form, nor in complication of structure, but 

 simply in the ultimate structural peculiarities of some 

 of their parts ; and this is, I believe, the best definition 

 which can be given of genera.' But it is surely apparent 

 that there are endless degrees both of structural peculi- 

 arity and of complication of structure. It is impossible to 

 define the amount of structural peculiarity which consti- 

 tutes 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 limited number of such classes ; thus 

 Linna3us adopted Class, Order, Genus, Species, and 

 Variety, and even seemed to think that there was some- 

 thing essentially natural in a five-fold arrangement of 

 groups *. 



( i 'Descent of Man,' vol. i. p. 228. 



r Agassiz, 'Essay on Classification,' p. 219. s Ibid. p. 249. 



* ' Philosopbia Botanica/ 155, p. 98. 



