CiiAr. VI. OF TRANSITIONAL VAKIETIES. 169 



IcriniiKited its ori^i'inal parent-form and all the transitional va- 

 rieties between its past and present states. Hence we ought 

 not to expect at the present time to mccft with numerous tran- 

 sitional varieties in each rei^ion, though they must have ex- 

 isted there, and may be embedded there in a fossil condition. 

 But in the intermediate region, having intermediate conditions 

 of life, why do we not now find closely-linking intermediatt^ 

 varieties? This dilliculty for a long time quite confounded 

 nie. But I think it can be in large part explained. 



In the lii-st jjlace, we should be extremely cautious in infer- 

 ring, be(\iuse an area is now continuous, that it has been con- 

 tinuous during a long period. Geology would lead us to be- 

 lieve that most continents have been broken up into islands 

 even during the later tertiary periods ; and in such islands 

 distinct species might have been separately formed without 

 the possibilit}' of intermediate varieties existing in the inter- 

 mediate 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 continuous and uniform condi- 

 tion than at present. But I will pass over this way of escaj> 

 ing from the difficulty ; for I believe that many perfcctlj^Ie- 

 fmed sjiecies 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 the for- 

 mation of new species, more especially ^vith frcelj'-crossing and 

 wandering animals. 



In looking at species as they are now distributed over a 

 ^vide area, we generally find them tolerably numerous over a 

 large territory, then becoming somewhat abruptly rarer and 

 rarer on the confines, and finally disappearing. Hence the 

 neutral territory between two representative species is gener- 

 ally narrow in comparison with the territory projier to each. 

 ^^'e see the same fact in ascending mountains, and sometimes 

 it is quite remarkable how abruptly, as Alph. de Candollc has 

 (il)served, a common alpine species disappears. The same fact 

 has been noticed by E. Forbes in sounding the depths of the 

 s(>a with the dredge. To those who look at climate and the 

 ])]iysical conditions of life as the all-important elements of dis- 

 tribution, these facts ought to cause surprise, as climate and 

 height or depth graduate away insensibly. But when we bear 

 in mind that almost every species, even in its metropolis, would 

 increase immensely in numbeis, were it not for other com- 

 peting species; that nearly aH either prey on or serve as prey 

 s 



