i8 4 THE POPULAR SCIENCE MONTHLY. 



into one of deciduous trees, and this again into one of conifers, the 

 vegetation declining through mosses and lichens as we reach the 

 region of perpetual snow. 



In these cases of horizontal and vertical distribution which thus 

 present such a striking botanical resemblance, there is likewise so 

 clear a meteorological analogy that it is impossible to avoid the con- 

 clusion that the distribution of plants depends very largely on the 

 distribution of heat. And, indeed, what better illustration of the in- 

 fluence of heat could we have than this, that by artificially adjusting 

 the temperature of hot-houses we can cause any plant to grow in any 

 latitude ? 



But temperature alone does not determine the distribution of 

 plants. If it did, we should find the same species in the same isother- 

 mal zones. Throughout the old continent, with the exception of the 

 torrid zone, heaths abound ; but in America not a single heath occurs. 

 In the New World, through forty degrees on each side of the equator, 

 the cactus tribe flourishes ; in the Old not a single cactus is to be seen 

 the spurges there replace them. So, again, in Australia, the forests 

 present a melancholy, a shadeless character, from their casuarinas, 

 acacias, eucalypti, whereas, if temperature alone were concerned, they 

 should offer the same aspect as the forests of North America and 

 Europe. 



As regards animals the same remark may be made. In the tem- 

 perate zone, eastward beyond the Caspian, there are men whose com- 

 plexion is yellow ; in Europe the complexion is white ; the American 

 Indian is red. Asia has its Tibet bear, Europe its brown bear, North 

 America its black bear. The European stag finds in America its 

 analogue in the wapiti, its Asiatic in the musk-deer. The wild-ox of 

 Lithuania differs from the North American buffalo, and this, again, 

 from the Mongolian yak. The llama in America replaces the camel 

 of Asia, the puma replaces the lion. Brazil has had in times long 

 past representatives of its existing sloths and armadillos. Aus- 

 tralia, which has isothermal zones like those of other continents, has 

 no apes or monkeys, no cats or tigers, no wolves or bears, hyenas, 

 horses, squirrels, rabbits ; no woodpeckers or pheasants. Instead of 

 them it has the kangaroo, wombat, ornithorhynchus, cockatoos, and 

 lories, nowhere else found. 



Then, though heat is a dominating influence in the distribution of 

 plants and animals, it is by no means the only one. There are also 

 other conditions, such as the supply of water, the composition of the 

 soil, the access of light, etc. It has been found convenient to group 

 all these together, and to speak of them, as I have already stated, 

 under a single designation, " The Environment." 



Change in the environment, and change in its organisms, go hand- 

 in-hand. Should the warmth of the tropics be diffused into the polar 

 circle, a tropical vegetation would replace the vanishing snows. 



