110 Canadinn Record of Science. 
moisture ; but these in turn are variously modified by ele- 
vation, pressure and latitude, as well as those influences 
which originate in the movements of air, proximity of 
water, ocean currents and diversified character of the great 
land areas. Add to all these the influence of ocean currents, 
winds, animals and man, in effecting a wider distribution ; 
‘while we also keep in mind that those very conditions of 
environment, which serve to induce wider distribution in 
some species, are the limiting conditions for other species, 
and some conception may be formed of the peculiarly com- 
plicated nature of the problem before us. 
But if climate directly influences vegetation, it is also 
true, though in a much more restricted sense, that vegeta- 
tion exerts a counter influence upon climate, with a ten- 
dency to modify it in more than an important respect. 
This will be found to hold true, chiefly, in plants of arbor- 
escent form, and instead of affecting wide areas, the in- 
fluence is usually of a mere local nature. While, therefore, 
less direct and certainly far less potent, the effect of vege- 
tation on climate is felt in the purity of the air ; its relative 
humidity and consequently its temperature, local rainfall, 
and even upon the air, as a medium for the distribution of 
septic organism. At the same time, many of these effects, 
either positive or negative, are to a large extent susceptible 
of control at the hands of man. The changes which he 
effects in the vegetation of a given district, either through 
ignorant waste or to meet actual requirements, find their 
final expression in their climatic influence. This fact is so 
well attested, not only by our present experience, but by 
the history of the world for centuries, that it needs no 
special argument at this time to enforce it upon our atten- 
tion. 
As the influences already referred to are by no means 
uniformly distributed over the surface of the earth, which 
is also variously modified as to surface and geological char- 
acter, there are found large areas between which extreme 
variations occur, in consequence of which there is a corres- 
ponding inequality in the distribution of vegetation. From 
