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 enclosed, it became
thickly clothed with vigorously growing 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 the 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 caused by the greater number in Paraguay of a certain
fly, which lays its eggs in the navels of these animals when first born.
The increase of these flies, numerous as they are, must be habitually
checked by some means, probably by other parasitic insects. Hence, if
certain insectivorous birds were to decrease in Paraguay, the parasitic
insects would probably increase; and this would lessen the number of the
navel-frequenting flies--then cattle and horses would become feral, and
this would certainly greatly alter (as indeed I have observed in parts of
South America) the vegetation: this again would largely affect the
insects; and this, as we have just seen in Staffordshire, the insectivorous
birds, and so onwards in ever-increasing circles of complexity.
Pages:
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138