48 DISTRIBUTION OF ANIMALS. [PART I, 
ground against the rival forms whose numbers were intact. 
Some would probably diminish and rapidly’ die out; others 
which produced favourable varieties, might be so modified by 
natural selection as to maintain their existence under a different 
form; and such changes would take place in varying modes on 
the two sides of the new strait. 
6. But the progress of these changes would necessarily affect 
the other species in contact with them. New places would be 
opened in the economy of nature which many would struggle to 
obtain; and modification would go on in ever-widening circle 
and very long periods of time might be required to bring the 
whole again into a state of equilibrium. 
7. A new set of factors would in the meantime have come 
into play. The sinking of land and the influx of a large body 
of water could hardly take place without producing important 
climatal changes. The temperature, the winds, the rains, might 
all be affected, and more or less changed in duration and amount. 
This would lead to a quite distinct movement in the organic 
world. Vegetation would certainly be considerably affected, and 
through this the insect tribes. We have seen how closely the 
life of the higher animals is often bound up with that of insects ; 
and thus a set of changes might arise that would modify the 
numerical proportions, and even the forms and habits of ‘a great 
number of species, would completely exterminate some, and raise 
others from a subordinate to a dominant position. And all these 
changes would occur differently on opposite sides of the strait, 
since the insular climate could not fail to differ considerably 
from that of the continent. 
8. But the two sets of changes, as above indicated, produced 
by different modes of action of the same primary cause, would 
act and react on each other; and thus lead to such a far-spread- 
ing disturbance of the organic equilibrium as ultimately perhaps 
to affect in one way or another, every form of life upon the 
earth. 
This hypothetical case is useful as enabling us better to realize 
how wide-spreading might be the effects of one of the simplest 
changes of physical geography, upon a compact mass of mutually 
