92 AN INTRODUCTION TO ZOOLOGY. 



distribution of land and sea^ but also of the distribu- 

 tion of land and sea in past ages, so far as it can be 

 ascertained. Two islands may exist at a long dis- 

 tance from another, the animals inhabiting which are 

 the same; yet two other islands comparatively near to 

 one another may differ remarkably from one another 

 in their inhabitants. Such cases may respectively 

 have arisen where islands are the last relics of a sunken 

 continent, which formerly connected them, or where 

 the separating sea-line has been long unchanged. 



The various kinds of animals which inhabit any 

 region are referred to collectively as its fauna, in 

 the same way as the vegetable forms of life are 

 referred to collectively as its flora. The term is a 

 convenient one, because its use serves to remind us 

 that the different types of animal included under it 

 have been moulded by similar causes, and have 

 interacted on each other in a way that makes a 

 knowledge of the whole set necessary for a thorough 

 understanding of the habits and circumstances of any 

 one. 



Still more interesting than the geographical dis- 

 tribution of animals is their distribution in time ; but 

 this is so complicated a subject that it constitutes a 

 special science, to which is given the name of 

 Palaeontology. 



Probably every one now possesses a moderately 

 clear idea of the conditions under which the fossil 

 remains of animals have been preserved. The rocks 

 which contain fossils are those which are called 



