OCR Text |
Show 146 CHANGES CAUSED BY Tl:IE PROGRESS [Ch. IX. may occur a t once, or at periods separated by differen.t inter- va1 s of t1. me, ough t t o happen before it would be poss1ble for us to declare what ultimate alteration the presence of any new comer, such as the bear before mentioned, might occasion in the animal population of the isle. . . Every new condition in the state of the orgamc or mor- gam.e creati·O n, a new am·m a1 or p1 a n t , an addition.a l sn.o w-clad mountain, any permanent change, however shght . m com-parison to the whole, gives rise to a new order of thmgs, and may make a material change in regard to some one or m~re species. yet a swarm of locusts, or a frost of extreme mtensity, may pass away without any great apparent derang:ment; no species may be lost, and all may soon recover their former relative numbers, because the same scourges may have visited the region, again and again, at some former periods. Every plant that was incapable of resisting su~h a degree of cold, every animal which was exposed to be entirely ~ut off by famine, in consequence of the consumption of vegetatiOn by the locusts, may have perished already, so that the subsequent recurrence of similar catastrophes is attended only by a tem-porary change. We are best acquainted with the mutations brought about by the progress of human population, and the growth of plants and animals favoured by man. To these, therefore, we should, in the first instance, turn our attention. If we conclude, from the concurrent testimony of history and of the evidence yielded by geological data, that man is, comparatively speaking, of very modern origin, we must at once perceive how great a revolution in the state of the animate world the increase of the human race, considered merely as consumers of a certain quantity of organic matter, must necessarily cause. It may, perhaps, be said, that man l1 as, m· some deg ree' compensated for the appropriation to himself of so much fo~d, by artificially improving the natural productiveness of. smls, by irrigation, manure, and a J·U dl'C ·i ous m· t erm1·x t ure of mmer.a l ingredients conveyed from different localities, But it admits Cb.IX.] OF liUMAN POPULATION, 14'"/ of reasonable doubt' wh et h er, upon the whole we fertilize or impoverish .t he land s wh I'c h we occupy. Thi.s ' asserti.o n may seem. startlmg to many' because th ey are so muc h m. t h e habi~ of regarding the sterility or productiveness of land in relatiOn to the wants of man ' and not as regard s the orgam·c world g. enerally. I. t is difficult ' at first , to concei· ve, 1' f a morass Is converted mto arable lane[, and made to yield a crop of grain, even of moderate abundance, that we have not improved the capabilities of the habitable surface-that we have not empowered it to support a larger quantity of organic life. In such cases, a tract, before of no utility to man may b 1 . d ' e rec a1me and become of high agricultural importance but 't • ' 1 may yteld, at the same time, a scantier vegetation. If a lake be drained and turned into a meadow, the space will provide s~stenance to man and many terrestrial animals serviceable to him, but . not perhaps so much food as it previously yielded to the aquatic races. If the pestiferous Pontine Marshes were drained and covered with corn, like the plains of the Po, they might, perhaps, feed a smaller number of animals than they do now; for these morasses are filled with of herds of buffaloes and swine and they swarm with birds, reptiles, and insects. ' . T~e feiling of dense and lofty forests which covered, even Withm the records of history, a considerable space on the globe, now tenanted by civilized man, must usually have lessened the amount of vegetable food throughout the space where these woods grew. "\V e must also take into our account the area covered by towns, and a still larger surface occupied by mads. If we force the soil to bear extraordinary crops one year, we ;re, .perhaps, compelled to let it lie fallow the next. But othmg so much counterbalances the fertilizing effects of human art. as the extensive cultivation of foreign herbs and shrubs, wh~ch, a~though they are often more nutritious to man, seldom t~nv~ With the same rank luxuriance as the native plants of a distnct • M an 1· s, · h . In trut , contmually striving to diminish L 2 |