102 



THE ZOOLOGIST. 



The opponents of the Theory, on the other hand, while 

 giving credit to Mr. Darwin for his great candour, logical skill, 

 and his extensive knowledge of Natural History, say that he has 

 not proved his case, viz. that species are mutable. One of them 

 concludes with the statement which he says has never been im- 

 pugned : " Classification is the work of science, but species the 

 work of Nature." 



[Such, then, is a brief outline of the hypothesis of Evolution 

 as expounded by Mr. Darwin, and which has shed a new light on 

 biological researches, and, on the other hand, of some of the 

 antagonistic views. Evolution, as Prof. Allman tersely puts it, 

 depends on two admitted faculties of living beings — heredity, 

 or transmission of character from parent to offspring, and 

 adaptivity, or the capacity of having these characters more or 

 less modified. 



This theory has met with wide acceptance, and is held by many 

 to suggest a more satisfactory explanation of the main facts in 

 zoology, botany, and geology than any other. Moreover, Darwin 

 has enabled observers to extend the effect of known causes to 

 cases in which they have not been suspected, and has given a fresh 

 impulse to studies of the structure, development, and relation- 

 ship of animals. The meaning of this will be more evident by 

 reference to one or two examples. Thus in the Lower Eocene of 

 North America is a small five-toed animal (Phenacodus), from 

 which the ancestry of the Horse can be traced. In the same 

 formation is another — Eohippus — of the size of a Fox, with four 

 well-developed toes and a rudimentary fifth in front, and three 

 toes behind. In the next higher division of the Eocene another 

 ■ — Orohippus (Hyracotherium) — of similar size appears, with four 

 toes in front and three behind. Then a third (Mesohippus), the 

 size of a Sheep, presents itself in the subsequent formation 

 (Miocene), with three functional toes and the splint of another 

 in front, and three behind. In a somewhat higher horizon 

 Miohippus (Onchitherium) occurs with a similar structure, except 

 that the splint-bone is reduced in size. Protohippus (Hipparion) , 

 of the size of a Donkey, again appears in the Pliocene above, 

 and exhibits three toes in front and three behind. Further up- 

 ward comes Pliohippus, a near ally of the Horse, with only a 

 single functional toe to each foot, but differing in the structure 



