58 General Part. 



function and acquired a new one. Organs or parts of the body, which, 

 have had a common origin, may perform the same, or different,, 

 functions ; such are called homologous : the arm of a man is. 

 homologous with the fore-leg of a dog and the wing of a bird, 

 although the function in each case is different. On the other hand,. 

 it frequently happens that the same function is performed by 

 different organs in different animals. Those which, differing in 

 origin, are functionally equivalent, are said to be analogous: 

 the eye of a Vertebrate is, for instance, analogous with that of a. 

 Gastropod. That branch of science, whose special task is to 

 trace homologies and to notice changes in organs, is called 

 Comparative Anatomy: together with Embryology, it 

 forms the Morphology of Animals. In contrast to this, it is. 

 the task of Physiology to study the functions of organs. 



VI. Biology. 



Biology, in the narrow sense,* treats of the habits of animals,, 

 their relations to the environment, and so forth. Various biological 

 questions have already been touched upon in the sections on Organs 

 and on Embryology : others of more general interest will now be 

 considered. 



1. Dispersal of Animals. 



Animals are adapted to their environment in various ways ; some 

 forms flourish in one set of conditions, others in another ; members, 

 of the same group, occurring in diverse situations, are differentiated 

 in correspondence with the environment. On the other hand, the 

 same environment often induces, in many respects, a similar type 

 in members of different groups (Pish, Whales). 



Mammalia, Aves, Eeptilia, Insecta, Arachnida, 

 and Myripoda, are emphatically terrestrial; the Amphibia, 

 which are partly aquatic, are less distinctive in type. There are 

 also many forms which, although belonging to essentially marine 

 groups, have become adapted to a terrestial life, such as the great 

 division, Pulmonata, not a few Crustacea, some Annelids. 

 (Earthworm, Leech) , a few Platyhelminths. The land animals 

 have one characteristic, in common; they are provided with a, 

 special respiratory apparatus, and almost always breathe by means 

 of lungs, or lung-like organs. The fauna of a place varies in 

 correspondence with the character of the district, of the soil and of 



* Biology in the wider sense comprises the whole science, of organisms, the whole, 

 of Zoology and Botany. 



