THE PRINCIPLES OF CLASSIFICATION. 7 



of animal forms in the Artiodactyles ; and we may 

 almost say, as a general rule, that the earliest known 

 mammalia belong to less specialised types than our 

 existing species. They are not well-marked Carnivores, 

 Rodents, Marsupials, &c., but rather constitute a group 

 of generalised forms from which our present well-marked 

 orders appear to have diverged. Among the Inverte- 

 brata, Carpenter and Williamson have proved that it is 

 almost impossible to divide the Foraminifera into well- 

 marked species ; and, lastly, among plants, there are 

 laro-e genera, as, for instance, Rubus and Hieracium. 



O O ' > ' ' 



with reference to the species of which no two botanists; 

 are agreed. 



The principles of classification point also in the 

 same direction, and are based more and more on the 

 theory of descent. Biologists endeavour to arrange 

 animals on what is called the ' natural system.' Xo one 

 now places whales among fish, bats among birds, or 

 shrews with mice, notwithstanding their external simi- 

 larity ; and Darwin maintained that ' community of 

 descent was the hidden bond which naturalists had 

 been unconsciously seeking.' How else, indeed, can we 

 explain the fact that the framework of bones is so 

 similar in the arm of a man, the wing of a bat, the fore- 

 leg of a horse, and the fin of a porpoise that the neck 

 of a giraffe and that of an elephant contain the same 

 number of vertebrce ? 



Strong evidence is, moreover, afforded by embryo- 

 logy ; by the presence of rudimentary organs and tran- 

 sient characters, as, for instance, the existence in the 

 calf of certain teeth which never cut the gums, the 

 shrivelled and useless wings of some beetles, the presence 



