CHAPTER VI. 



GEOGRAPHICAL DISTRIBUTION OF ORGANISMS. 



This subject is not directly connected with physi- 

 ology, although it is with morphology, but its extreme 

 interest and importance in connection with evolution 

 justifies its treatment with some fullness. 



It is well known that the observant traveler from 

 one continent to another — as, for example, to take an 

 extreme case, from Europe or the United States to 

 Australia — finds all the animals and plants entirely dif- 

 ferent from those he has been accustomed to see at 

 home. The same \» true in less degree in going from 

 America to Europe, or even from the Atlantic to the 

 Pacific side of our own continent. Until comparatively 

 recently the facts of this distribution of species were a 

 mere chaotic mass without a guiding principle. The 

 theory of evolution has brought law and order into this 

 chaos. All we can do here is to give a bare outline of the 

 general laws of this distribution and their explanation. 



Fauna and Flora. — In popular language fauna 

 and flora means the group of animals and plants inhabit- 

 ing any place; but in scientific language it is a natural 

 group of organisms differing from other natural groups 

 and separated from them by geographical boundaries, or 

 by temperature or climate, or physical conditions of 

 some sort, and in harmonic relations with one another 

 and with the environment. 



47i 



