608 ZOOLOGY SECT. 



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 doc- 

 trine 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 type forms 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 land- 

 marks 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, Reptiles, 

 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 the fore-foot 

 of the Horse of essentially the same bony elements. More difficult 

 still would it be to explain the cases in which what is a 



