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Show CHAP. YI. DIFFICULTIES ON THEORY. 174 t t . t meet with numerous transitional vari-presen Ime o . · · ·h region though they must have existed etws In eac ' . 1! ·1 d' · h d ay be embedded there In a 10SSI con Ition. t ere, an m . . · eli t B t · the intermediate regwn, having Intermo a e coUn diItilol ns of life, why do we not now find cl ose l y-lI' n k.,..'I ng intermediate varieties? This cli~cult! for a lo~g time quite confounded me. But I think It can be In large part explained. . In the first place we should be extrer:nely cauho~s in inferring, because an area is now cont:nuous, that It has been continuous during a long penod. Geology would lead us to believe that almost every continent has been broken up into islands even during the later tertiary periods ; and in such islands distinct speci~s might have been separately formed without the possibility of intermediate varieties existing in tho intermediate zones. By changes in the form of the land and of climate ' marine areas now continuous must . often have existed within recent times in a far less contmuous and uniform condition than at present. But I will pass over this way of escaping from the difficulty; for I believe that many perfectly defined species have been formed on strictly continuous areas ; though I do not doubt that the formerly broken condition of areas now continuous has played an important part in tho for~ation of new species, more especially with freely-crossmg and wandering animals. . . In looking at species as they are now d1stnbutecl over a wide area, we generally find them tolerably numerous over a large territory, then becoming somewhat abruptly rarer and rarer on the confi?-os, and finally disappearing. Hence the neutral terntory b~tween two representative species is generally narrow Ill comparison with the territory proper to eac~ w ~ see the same fact in ascending mountains, and sometimes CIIA.P. VI. TRANSITIONAL VARIETIES. 175 it is quite remarkable how abruptly, as Alph. De Candolle has observed, a common alpine species disappears. The same fact has been noticed by Forbes in sounding the depths of the sea with the dredge. To those who look at climate and the physical conditions of 1ife as the all-important elements of distribution, these facts ought to cause surprise, as climate and height or depth graduate away insensibly. But when we bear in mind t~at almos.t every species, even in its metropolis, would Increase rmmensely in numbers, were it not for other competing species; that nearly all either prey on or. ser;e ~s pre~ for others ; in short, that each organic ?e~ng Is either directly or indirectly related in the most Important manner to other organic beings, we must see that the range ~f the inhabitants of any country by no n:eans ex~l~Isively depends on insensibly changing physwal co.nditions, b~t in large part on the presence of other spemes, on which it depends, or by which it is destroyed, or ':ith which it comes into competition; and as these spemes are already defined objects (however the! may.have become so), not blending one into another by Inse~sible ?radations, the range of any one species, depending as It does on the range of others, will tend to be sharp~y defined. Moreover, each species on the confi.~ es of Its range, where it exists in lessened numbers ·will, d:uing fluctuations in the number of its enemie~ or of Its prey,. or .in the seasons, be extremely liable to. utter extermination; and thus its geographical range Will come to be still more sharply defined. If. I am right in believing that allied or representative species, when inhabiting a continuous area are gene-ranY so dI' stri'b uted that each has a wide r' ange with a. cornp arat 'I ve 1y narrow neutral territory between ' them ~~ whwh th~y ~ecome rather suddenly rarer and rarer ; en, as varieties do not essentially differ from species, |