20 
The Australasian Scientific Magazine . [August i, 1885. 
distribution of plants depends very largely on the distribution of heat. 
And, indeed, what better illustration of the influence 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 isothermal zones. 
Throughout the old continent, with the exception of the torrid zone, 
heath abounds; 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 Australasia, the forests present a melancholy— 
a shadeless character, from their casnarinas, 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 temperate 
zone, eastward beyond the Caspian there are men whose complexion 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 leopard and 
tiger, Brazil has had, in times long past, representatives of its existing 
sloths and armadillos. Australia neither has apes nor monkeys, no cats, 
tigers, wolves, bears, hyenas, horses, squirrels, rabbits ; no woodpeckers or 
pheasants. In place of them it has the kangaroo, wombat, ornithoryncus, 
cockatoo, and lories. 
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 condi- 
tions, such as the rainfall, the character of the soil, etc. It has been found 
convenient to group all these together, and to speak of them as, under a 
single designation, “The environment. 1 ” 
Change in the environment and change in organisths go hand-in-hand. 
Were the warmth of the tropics diffused into the polar circle, a tropical 
vegetation would replace the vanishing snows. Were the ices of the poles 
to spread over the temperate region, the reindeer would accompany their 
invading edge. While the environment thus influences the organism, the 
organism reacting, influences the environment. 
The most striking instance of this, perhaps, will be found on comparing 
the constitution of the atmosphere before and since the carboniferous 
epoch. Prior to that epoch all the myriads of tons of coal, a substance now 
inclosed in the strata of the earth, existed as carbonic acid in the air. By 
the agency of the sunlight, acting on the leaves of the luxuriant vegetation 
of those times, this noxious gas was gradually removed, and replaced by an 
equivalent volume of oxygen. 
Anterior to the coal deposit, the fauna was cold blooded and slow 
respiring. The flora thus changed the aerial environment, and this in its 
turn reacting, changed the fauna. It is on all sides admitted that plants tend, 
by their removal of carbonic acid from the atmosphere, and replacing it by 
oxygen, to compensate for the disturbance occasioned by animals. In this 
way, through very many centuries, the same percentage constitution of the 
atmosphere is maintained, the sum total of vegetable being automatically 
adjusted to the sum total of animal life— automatically, and not by any 
interference of Providence— a fact of great value in its connection with the 
