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Show 17'Q EFFECTS OF CHANGES OF CLIMAT~ [Ch.X. great change of climate must be fata~ ~o many. wh.ich can find no place of retreat, when their ongmal hab1tat1ons become unfit for them. For if the general temperature be on the rise, then is there no cooler region whither the pol~r species can take refug~; if it be on the decline, then the. ammals and plants previously established between the tropics have no resource. Suppose the general beat of the atmosphere to in· crease, so that even the arctic region became too warm. for the musk-ox and rein-deer, it is clear that they must per1sh ; so, if the torrid zone should lose so much of its heat.<by the progressive refrigeration of the earth's surface, as to be a~ unfit habitation for apes, boas, bamboos, and palms, these tnbes of animals and plants, or at least most of the species now belonging to them, would become extinct, for there would be no warmer latitudes for their reception. It will follow, therefore, that as often as the climates of th~ globe are passing from the extreme of heat to that of coldfrom the summer to the winter of the great year before alluded to by us ill<-the migratory movement will be directed constantly from the poles towards the equator; and for this reason the $pecies inhabiting parallel latitudes, i~ the ~orthern and southern hemispheres, must become widely different. For we assume, on grounds before stated t, that the original stock of each species is introduced into one spot o: t~e earth .only, and, consequently, no species can be at once md1genous m the arctic and antarctic circles. Dut when, on the contrary, a series of changes in the phy-sical geography of the globe, or any other supposed cause, occasions an elevation of the general temperature,-when there is a passage from the winter to one of the vernal or summer seasons of the great cycle of climates, then the order ~f the migratory movement is inverted. The different spec1es of animals and plants direct their course from the equator towards the poles ; and the northern and southern hemisp~eres may become peopled, to a great degree, by identical spec1es. Such • yol. i, p. 116. t Chap. VIII. Oh.X.J ON TilE DISTRIBUTION OF SPECIES. 171 is not tl~e actual state of the inhabited earth, as we have already shown m our sketch of the gcoO'raphical distribution of its living productions; and this fac~ adds one more additional proof to a great body of evidence, derived from independent sou~·ces, that the general temperatme has been cooling down durmg the epochs which immcdiateJy preceded our own. We do not mean to speculate on the entire transposition of a group of animals and plants from tropical to polar latitudes, or the reverse, as a probable, or even possible, event; for although we believe the mean annual temperature of one zone to he transferrible to another, we know that the same climate cannot be so transferred. Whatever be the general temperature of the earth's surface, comparative equability of heat will charactedze the tropical regions, while great periodical variations will belong to the temperate, and still more to the polar lati. tudes. These, and many other peculiarities connected with heat and light, depend on fixed astronomical causes such as the motion of the earth and its position in relation t~ the sun, and not on those fluctuations of its surface, which may influence the general temperature. Among many obstacles to such extensive transferences of habitations, we must not forget the immense lapse of time required, according to any hypothesis yet suggested especiall that which has appeared to us most feasible to brii:g about y cons1' d erable change in climate. During a p' eriod so vast, thea other causes of extirpation, before enumerated by us, would ex~rt so po.werful an infl~ence as to prevent all, save a very few hatdy speCies, from passmg from equatorial to polar regions or from the tropics to the pole. ' But the power of accommodation to new circumstances is great in certain species, and might enable many to pass from one zone to another, if the mean annual heat of the atmosphere and ~he ocean were greatly altered. To the marine tribes, ;:eCJally: such. a p.assa~e would be possible, for they are Jess pede~ m their migratiOns, by barriers of land, than are the terrestrial by the ocean. Add to this, that the temperature o{ |