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Show 72 MUrl'UAL CITECKB TO INCREASE. {CHAP. III. the habits of humble-bees, 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 sntall 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 creditable that the presence of a feline animal in large nu1nbers 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 co1ne 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. vVhen 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 au American forest is cut down, a very different vegetation springs up; but it has been observed that the trees no'v growing on the ancient Indian mounds, in the Southern United States, display the san1e beautiful diversity and proportion of kinds as in the surrounding virgin forests. What a struggle between the several kinds of trees 1nust here have gone on during long centuries, each annually scattering its seeds by the thousand ; ·what war between insect and insect-between inse~ ts, 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 plants which first clothed the ground and thus checked the growth of the trees ! Thro'v 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 and reaction of the innumerable plants and animals which have determined, in the course of centuries, the propor- MU~UAL CIIECKS TO INUREASE . . 73 tlonal numoers and k' d f 0DAP. III.] Indian ruins! In so trees now growing on the old The dependency of one or . b . of a parasite on its re 1' ganic mng on another as remote in the scale ofp nal~r~es ~~~e~allyf between bei~gs those which may stricti b . . lS IS o ten the case with other for existence as fn :h said to struggle with each fe~ding quadrupeds: But th: case of locusts ~nd grassWill. be most severe between thtr~gff:Ie. almost Invariably species, for they fi·equent th e In l~Id~als of the same same food, and are exposed : ~hme distncts, require the case of varieties of the sa o e s~me dangers. In the generally be almost equall me species, the struggle will the contest soon decided . f se.ve~e, an~ we sometimes see of wheat be sown togethe~r ::d ahce, I~ several varieties so:wn, some of the varietie; whi t e mixe~ seeds be rechmate, or are naturall th ch bes~ suit. the soil or others and so yield mole see~ most f~rtile, Will beat the few years quite supplant the dthn~ Wil~ c.onsequently in a a mixed stock of even h el varieties. To keep up the variously coloured s~~~t- :~!r~~ely close varieties as har~ested separately, and thepseed hey m:'-st b~ each year portion, otherwise the we 1 . t en mixed In due proIn numbers and disa a rer ~nnds 'Yill s.teadily decrease of sheep: it has b ppear. o a~aln With the varieties varieties will starve~~~ ~:herted t at. certa.in mountainthey cannot be kept together mThtaln-varieties, so that lowed from kee in er. ~same result has folthe medicinal lee~h g Ittogether drfferent varieties of the varieties of an ~ne f may even be doubted whether have so exactl tl o our domestic plants or animals tion, that the. o~igi:als;~~pst~~ngth,f habi~s, and constitu~ be kept up for half a d or Ions_ o a miXed stock could allowed to stru le t ozen ~enerations, if they were nature and I'f thgg dogether, hke beings in a state of As ' species ofe tsheee sao r you.n g were not annually sorted. no means in . bl me g~n~s ~ave usually, though b t t' varia y, some S1m1lanty in hab't d Y u Ion, and always in structur h 1 san consh~ be more seyere between specf~st of st~uggle will generally they co4m e Into competition with each ~t~emr, e thgaenn ubes ,twweheenn |