144 EVOLUTION ۳٦ 
mice depends on the number of cats; hence, if the cats 
were destroyed the field-mice would increase and destroy 
the bees, in which case the clover would produce no seed, 
and the cattle would soon be deprived of a most important 
article of food. The same author calls attention to the 
fact of cattle determining the existence of trees: “Here 
there are extensive heaths, with a few clumps of old Scotch 
firs, on the distant hill-tops: within the last ten years large 
spaces have been inclosed, and self-sown firs are now 
springing up in multitudes, so close together that all 
cannot live. When I ascertained that these young trees 
had not been sown or planted, I was so much surprised at 
their numbers that I went to several points of view whence 
I could examine hundreds of acres of the uninclosed 
heath, and, literally, I could not see a single Scotch frr, 
except the old planted clumps. But on looking closely 
between the stems of the heath, I found a multitude of 
seedlings and little trees, which had been perpetually 
browsed down by the cattle. In one square yard, at a 
point some hundred yards distant from one of the old 
clumps, I counted thirty-two little trees; and one of them, 
with twenty-six rings of growth, had, during many years, 
tried to raise its head above the stems of the heath, and 
had failed. No wonder that, as soon as the land was 
inclosed, it became thickly clothed with vigorously grow- 
ing young firs. Yet the heath was so extremely barren, 
and so extensive, that no one would ever have imagined 
that cattle would have so closely and effectually searched 
it for food. Here we see that cattle absolutely determine 
the existence of Scotch fir. But in several parts of the 
world insects determine the existence of cattle. Perhaps 
Paraguay offers the most curious instance of this; for here 
neither cattle nor horses nor dogs have ever run wild, 
though they swarm southward and northward in a feral 
State; and Azara and Rengger have shown that this is 
