Chap. XIV. RECAPITULATIOiN. 4I5 



difBculty of the wide diffusion of tlic species of the same genus 

 is in some def^ree lessened. 



As uccording' to the theory of natural selection an intermi- 

 nable number of intermediate forms must have existed, linking* 

 together all the species in each group by gradations as fine as 

 are our [jrescnt varieties, it may be asked, Why do v,'o not 

 see these linking forms all around us ? Wliy are not all organ- 

 ic beinfTS blended tos:ether in an inextricable chaos ? AVith 

 respect to existing forms, we should remember that we have 

 no right to expect (excepting in rare cases) to discover directhj 

 connecting links between them, but only between each and 

 some extinct and supplanted form. Even on a Avide area, 

 which has during a long period remained continuous, and of 

 which the climate and other conchtions of life change insensibly 

 in proceeiling fi'om a district occupied by one species into 

 another district occupied by a closely-allied species, we have 

 HO just right to expect often to find intermediate varieties in 

 the intermediate zone. For we have reason to believe that 

 only a few species of a genus ever imdergo change ; the other 

 species becoming utterly extinct and leaving no modified pro- 

 geny. Of the species which do change, only a few Avithin the 

 same country change at the same time ; and all modifications 

 are slowly effected. I have also shoAvn that the intermediate 

 varieties which prcjbably at first existed in the intermediate 

 zones, would be liable to be supplanted by the allied forms on 

 either hand; for the latter, from existing in greater numbers, 

 would generally be modified and improved at a quicker rate 

 than the intermediate varieties, which existed in lesser num- 

 bers ; so that the intermediate varieties Avould, in the long- 

 run, be supplanted and exterminated. 



On this doctrine of the extermination of an infinitude of 

 connecting links, between the living and extinct inhabitants 

 of the Avorld, and at each succcssIa'C period betAA'cen the ex- 

 tinct and still older species, Avhy is not every geological forma- 

 tion charged Avith such links ? Why does not every collection 

 of fcjssil remains afford plain evidence of the gradation and 

 mutation of the forms of life? Although geological research 

 has undoubtedly rcA'caled the former existence of many links, 

 bringing numerous forms of life much closer together, it does 

 not yield the infinitely many fine gradations between i)ast and 

 present species required on the theory ; and this is the most 

 obvious of the many objections Avhich may be urged against it. 

 Why, again, do Avhole groups of allied sjiecies apj)car, though 



