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Show 70 MU'l'U.AL CHECKS TO INCRl~ASE. [CnAP. lll could not enter. But ho'v important an element enclosure is, I plainly sa-w near Farnha1n, in Surrey. I-Im·e there are extensive heaths, with a fe-vv clun1ps of old Scotch firs on the distant hill-tops : within the last ten years large spaces have been enclosed, and self-sown firs are now springing up in n1ultitudes, so close together that all cannot live. When I ascertained that these young trees had 11ot been sown or planted, I was so inuch surprised at their ntunbers 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 thirtytwo little trees ; and one of then1, judging from the rings of growth, had during twenty-six years tried to raise its head above the stems of the heath, and had failed. No 'vonder 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 1nost 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 nun1ber in Paragl1ay 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 birds. Hence, if certain insectivorous birds (whose numbers are probably regulated by hawks .or beasts of prey) were to increase in Paraguay, the fhes would decrease-then cattle and horses would become feral, and this would certainly greatly alter (as indeed I CHAP. III.] MUTUAL CHECKS TO INCREASE. 71 ha.ve ob~erved in parts of South America) th . ~his again would largely affect the insects . a~;~~~tation : JUst have seen in Staffordshire, the inse~tivorou~s, a~ we and so onwards in ever-increasing circles of com 1 br~·ds, We beg~n this series by insectivorous birds d P ehity. ended With them. Not that in nature h ' an .we ave ~ver be a~ sim~le as this. Battle within tb!tfi~~~~ts ca~ e recurnng With varying success. and . evel run the for?es ar~ so nicely bala~ced tl.:: ~~:17 l?n~ nature remmns uniform for long periods of t. ace o assuredly the merest trifle would oft . hmez though one organic being over another Ne~v~~ le VIctory to found is our ignorance and so hi . h our ei e ess . so prow~ marvel when we h~ar of the ~xtinct:presufmption, th~1 bmng; and as we do not see the cau IOn o . an organic t c!ysmfs to desolate the world, or inventsela;: ~~~~~edcataIon o the forms of life ! ura-l am tempt.ed to give one Inore instance showin h plants and animals, most remote in the scale of n~tu~·w hre b~~nd ~ogether b~ a web of complex relations. I shaTI f 1rea e~ ahv~ occasion to show that the exotic Lobelia u gens, In t IS part of England, is never visited b . sects, ~nd condsequently, from its peculiar structure Je;~; can se a ~ee . Many of our orc11idaceous lant~ b lutely require the visits of moths to remove fheir ~I SO· masses and thus to fertilise them I h 1 po en· b 1· th t h · ave, a so, reason to ti~ Ievf tha humble-bees ~re indispensable to the fertilisa-no~ o. "t :h. extsease (VIola tricolor), for other bees do . VISI IS ower. From experiments which I have tried, I have found that the visits of bees if not ind. able, tre at least highly beneficial to tl;e fertilisat~s!:~} our c ove~s; .but humble.-bees alone visit the common red clover (Tnfohum pratense ), as other bees cannot reach the nectar. Hence I have very little doubt, that if the whole genus of humble-bees became extinct or ver rare in England, the heartsease and red clover wouia become b:~f :are, or tyhoiiy disappear. The number of humble· b m any d1strwt depends in a great degree 011 the nu~ er of :field-mice, which destroy their combs and nes s ; and Mr. H. Newman, who has long attended to |