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Show 74 MUTUAL CHECKS TO INCREASE. CHAP. III. extinct or very rare in England, the heartsease. and red clover would become very rare, or wholly d1sappear. 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 humble-bees, beheves 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 nun1erous 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 concurring 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 the trees now growing on the ancient Indian mounds, in the Southern U :p.ited States, display the same beautiful diversity and proportion of kinds as in the surrounding virgin forests. vVhat a struggle between the several kinds of trees CHAP. III. MUTUAL CHECKS TO INCREASE. 75 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 gTowth of the trees! Throw up a handful of feathers, and all must fall to the ground according to definite laws ; but how simple is this problem compared to the action andre~ action 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 invariably 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 propor- E2 |
