624 ZOOLOGY SECT. 



the science. It is observed that an animal possesses a certain 

 structure, develops in a certain way, has certain affinities with 

 other animals, has a certain geographical and geological range ; 

 and the attempt is made to find a satisfactory explanation of these 



facts. 



Evolution. Of these facts there is, to all intents and purposes, 

 but one explanation requiring consideration here. The animal- and 

 plant-life of the globe has come to be as it now is by a process 

 of evolution which has been'going on continuously from an early 

 period in the history of the earth to the present time. The plant- 

 and animal-worlds, in other words, have been evolved by a gradual 

 process of development, in the course of which the higher forms 

 have originated from the lower. Evidence bearing on this doctrine 

 has already been encountered in abundance in fact the theory of 

 evolution has to be looked upon as in many respects a guiding 

 principle in the study of our science ; and it has, accordingly, 

 been necessary in many parts of previous sections to take its truth 

 for granted. In discussing the relations of the various phyla to one 

 another, the relations of the various classes of each phylum, and 

 the position of the described examples within the classes ; in 

 referring to the homologies borne by the organs of the members 

 of one class to those of the members of another, it has been necessary 

 to assume the truth of a theory of evolution. 



For the evidence, then, in favour of a doctrine of evolution the 

 reader is referred to the substance of previous sections, where it 

 will be found on almost every page. For his guidance some 

 landmarks may, however, be here pointed out. 



Anatomical and Embryological Evidence. A consider- 

 able body of the evidence in favour of the view that the higher 

 animals have been derived from lower forms is obtained from the 

 provinces of comparative anatomy and embryology. The mere 

 fact that we are able conveniently to express the resem- 

 blances and differences in structure between different groups by 

 the construction of such genealogical trees as have been given 

 in some of the previous sections tells strongly in favour of a theory 

 of descent ; for, though it is by assuming evolution that such 

 diagrams are constructed, the resemblances which they represent 

 point strongly to common ancestry. A theory of evolution 

 explains also the fact that there is running through a whole series 

 of forms let us say Fishes, Amphibians, Keptiles, Birds, and 

 Mammals a common type of structure, in which the same 

 essential parts, though perhaps differently modified in accordance 

 with differences in function, are to be found in the same mutual 

 relations. It would be difficult, on any other view of the facts, 

 to explain, for example, the occurrence in the wing of the Bird 

 and of the Bat, the flipper of the Whale, and in the fore-leg of the 

 Horse, of essentially the same bony elements. More difficult 



