118 DYNAMICAL GEOLOGY. 



stratum will hereafter be discussed in the light of these 

 facts. 



SECTION IV. GEOGRAPHICAL DISTRIBUTION OF SPECIES. 



No one can go to a foreign country, or even a distant 

 part of our own country, as, for example, from the eastern 

 to the western coast, without being struck with the great 

 difference in the native animals and plants. If such a one 

 has been trained to observe, he will see that nearly all the 

 species are entirely different. As a broad, general fact, 

 every country has its own native species, differing more 

 or less conspicuously from those of other countries. The 

 laws of this distribution and its causes have recently 

 attracted much attention, and are a subject of very great 

 interest. We can only give the briefest outline. 



Faunas and Floras. We shall hereafter frequently 

 use the terms fauna and flora, and must therefore define 

 them. The whole group of animals inhabiting one place 

 is called its fauna, and of plants its flora. Thus, we may 

 speak of the fauna and flora of New York, or Illinois, or 

 Oregon. But science cares nothing for such arbitrary 

 limits it deals only with natural boundaries. A natural 

 fauna or flora is a natural group of animals or plants in 

 one place, differing conspicuously from other groups in 

 other places, and separated from them by natural bound- 

 aries, geographical or climatic. Among the climatic con- 

 ditions limiting faunas and floras, perhaps the most 

 important is temperature, and we shall therefore speak of 

 this first. Again, plants, being fixed to the soil, are 

 more strictly limited than animals, and we shall there- 

 fore illustrate the laws of distribution first "by them. 

 Again, temperature conditions change in elevation above 

 the surface, and in latitude. We take the former first. 



Botanical Temperature Regions in Elevation. 

 For this we take a high mountain, near the seashore in 



