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not to be seen on the heath ; and the heath was frequented by two or three distinct insectivorous birds. Here we see how potent has been the effect of the introduction of a single tree, nothing whatever else having been done, with the exception that the land had been enclosed, so that cattle could not enter. But how important an element enclosure is, I plainly saw near Farnham, in Surrey. Here there are extensive heaths, with a few clumps of old Scotch firs on the distant hilltops : within the last ten years large spaces have been enclosed, 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

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 unenclosed heath, and literally I could not see a single Scotch fir, 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 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

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

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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. We began this series by insectivorous birds, and we have ended with them. Not that in nature the relations can ever be as simple as this. Battle within battle must ever be recurring with varying success; and yet in the long run the forces are so nicely balanced, that the face of nature remains uniform for long periods of time, though assuredly the merest trifle would often give the victory to one organic being over another. Nevertheless, so profound is our ignorance, and so high our presumption that we marvel when we hear of the extinction of an organic being ; and as we do not see the cause, we invoke cataclysms to desolate the world, or invent laws on the duration of the forms of life!

I am tempted to give one more instance showing how plants and animals, most remote in the scale of nature, are bound together by a web of complex relations. I shall hereafter have occasion to show that the exotic Lobelia fulgens, in this part of England, is never visited by insects, and consequently, from its peculiar structure, never can set a seed. Many of our orchidaceous plants absolutely require the visits of moths to remove their pollen-masses and thus to fertilise them. I find from experiments that humble-bees are almost indispensable to the fertilisation of the heartsease (Viola tricolor), for other bees do not visit this flower. I have also found that the visits of bees are necessary for the fertilisation of some kinds of clover: for instance, 20 heads of Dutch clover (Trifolium repens) yielded 2,290 seeds ; but 20 other heads protected from bees produced not

Again, 100 heads of red clover (T. pratense) produced 2,700 eds, but the same number of protected heads produced not a single seed. Humble-bees alone visit red clover, as other bees cannot reach the nectar. It has been suggested that moths may serve to fertilise the clovers; but I doubt this in the case of the red clover, from their weight being apparently not sufficient to depress the wing-petals. Hence we may infer as highly probable that if the whole genus of humble-bees

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became extinct or very rare in England, the heartsease and red clover would become very rare, or wholly disappear. The number of humble-bees in any district depends in a great degree on the number of field-mice, which destroy their combs and nests ; and Mr. H. Newman, who has long attended to the habits of humblebees, believes that more than two-thirds of them are thus destroyed all over England.” Now the number of mice is largely dependent, as every one knows, on the number of cats; and Mr. Newman says, “ Near villages and small towns I have found the nests of humble-bees more numerous than elsewhere, which I attribute to the number of cats that destroy the mice.” Hence it is quite credible that the presence of a feline animal in large numbers in a district might determine, through the intervention first of mice and then of bees, the frequency of certain flowers in that district ! ]

In the case of every species, many different checks, acting at different periods of life, and during different seasons or years, probably come into play ; some one check or some few being generally the most potent, but all concur in determining the average number or even the existence of the species. In some cases it can be shown that widely-different checks act on the same species in different districts. When we look at the plants and bushes clothing an entangled bank, we are tempted to attribute their proportional numbers and kinds to what we call chance. But how false a view is this ! ]Every one has heard that when an American forest is cut down, a very different vegetation springs up; but it has been observed that ancient Indian ruins in the Southern United States, which must formerly have been cleared of trees, now display the beautiful diversity and proportion of kinds as in the surrounding virgin forests. What a struggle between the several kinds of trees must here have gone on during long centuries, each annually scattering its seeds by the thousand; what war between insect and insect-between insects, snails, and other animals with birds and beasts of prey-all striving to increase, and all feeding on each other or on the trees or their seeds and seedlings, or on the other plants which first clothed the ground and thus checked the growth of the trees ! Throw up a handful of feathers, and all must fall to the ground according to definite laws; but how simple is the problem where each

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shall fall compared to that of the action and reaction
of the innumerable plants and animals which have
determined, in the course of centuries, the proportional
numbers and kinds of trees now growing on the old
Indian ruins !
The dependency of one organic being on another, as
of a parasite on its prey, lies generally between beings
remote in the scale of nature. This is often the case
with those which may strictly be said to struggle with
each other for existence, as in the case of locusts and
grass-feeding quadrupeds. But the struggle almost invari-
ably will be most severe between the individuals of
the same species, for they frequent the same districts,
require the same food, and are exposed to the same
dangers. In the case of varieties of the same species,
the struggle will generally be almost equally severe,
and we sometimes see the contest soon decided : for
instance, if several varieties of wheat be sown together,
and the mixed seed be resown, some of the varieties
which best suit the soil or climate, or are naturally the
most fertile, will beat the others and so yield more
seed, and will consequently in a few years quite
supplant the other varieties. To keep up a mixed stock
of even such extremely close varieties as the variously
coloured sweet-peas, they must be each year harvested
separately, and the seed then mixed in due proportion,
otherwise the weaker kinds will steadily decrease in
numbers and disappear. So again with the varieties
of sheep : it has been asserted that certain mountain-
varieties will starve out other mountain-varieties, so
that they cannot be kept together. The same result
has followed from keeping together different varieties
of the medicinal leech. It may even be doubted whether
the varieties of any one of our domestic plants or
animals have so exactly the same strength, habits, and
constitution, that the original proportions of a mixed
stock could be kept up for half-a-dozen generations, if
they were allowed to struggle together, like beings in
a state of nature, and if the seed or young were not
annually sorted.
As species of the same genus have usually, though by

means invariably, some similarity in habits and constitution, and always in structure, the struggle will generally be more severe between species of the same genus, when they come into competition with each other,

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became extinct or very rare in England, the heartsease and red clover would become very rare, or wholly disappear. The number of humble-bees in any district depends in a great degree on the number of field-mice, which destroy their combs and nests ; and Mr. H. Newman, who has long attended to the habits of humblebees, believes that more than two-thirds of them are thus destroyed all over England.” Now the number of mice is largely dependent, as every one knows, on the number of cats; and Mr. Newman says, Near villages and small towns I have found the nests of humble-bees more numerous than elsewhere, which I attribute to the number of cats that destroy the mice.” Hence it is quite credible that the presence of a feline animal in large numbers in a district might determine, through the intervention first of mice and then of bees, the frequency of certain flowers in that district ! ]

In the case of every species, many different checks, acting at different periods of life, and during different seasons or years, probably come into play ; some one check or some few being generally the most potent, but all concur in determining the average number or even the existence of the species. In some cases it can be shown that widely-different checks act

the same species in different districts. When we look at the plants and bushes clothing an entangled bank, we are tempted to attribute their proportional numbers and kinds to what we call chance. But how false a view is this ! ]Every one has heard that when an American forest is cut down, a very different vegetation springs up; but it has been observed that ancient Indian ruins in the Southern United States, which must formerly have been cleared of trees, now display the beautiful diversity and proportion of kinds as in the surrounding virgin forests. What a struggle between the several kinds of trees must here have gone on during long centuries, each annually scattering its seeds by the thousand ; what war between insect and insect-between insects, snails, and other animals with birds and beasts of prey-all striving to increase, and all feeding on each other or on the trees or their seeds and seedlings, or on the other plants which first clothed the ground and thus checked the growth of the trees ! Throw up a handful of feathers, and all must fall to the ground according to definite laws; but how simple is the problem where each

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