DIFFICULTIES OF THE THEORY 137 



objections in the following chapter; Instinct and Hybridism in the 

 two succeeding chapters. 



ON THE ABSENCE OR RARITY OF TRANSITIONAL VARIETIES 



As natural selection acts solely by the preservation of profitable 

 modifications, each new form will tend in a fully stocked country 

 to take the place of, and finally to exterminate, its own less im- 

 proved parent-form and other less-favored forms with which it 

 comes into competition. Thus extinction and natural selection go 

 hand in hand. Hence, if we look at each species as descended 

 from some unknown form, both the parent and all the transi- 

 tional varieties will generally have been exterminated by the 

 very process of the formation and perfection of the new form. 



But, as by this theory innumerable transitional forms must have 

 existed, why do we not find them imbedded in countless numbers 

 in the crust of the earth? It will be more convenient to discuss this 

 question in the chapter on the Imperfection of the Geological Rec- 

 ord ; and I will here only state that I believe the answer mainly lies 

 in the record being incomparably less perfect than is generally sup- 

 posed. The crust of the earth is a vast museum; but the natural 

 collections have been imperfectly made, and only at long intervals 

 of time. 



But it may be urged that when several closely allied species in- 

 habit the same territory, we surely ought to find at the present 

 time many transitional forms. Let us take a simple case: in travel- 

 ling from north to south ever a continent, we generally meet at suc- 

 cessive intervals with closely allied or representative species, evi- 

 dently filling nearly the same place in the natural economy of the 

 land. These representative species often meet and interlock; and 

 as the one becomes rarer and rarer, the other becomes more and 

 more frequent, till the one replaces the other. But if we compare 

 these species where they intermingle, they are generally as abso- 

 lutely distinct from each other in every detail of structure as are 

 specimens taken from the metropolis inhabited by each. By my 

 theory these allied species are descended from a common parent; 

 and during the process of modification, each has become adapted 

 to the conditions of life of its own region, and has supplanted and 

 exterminated its original 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 meet with numerous transitional va- 

 rieties in each region, though they must have existed there, and 

 may be imbedded there in a fossil condition. But in the intermedi- 

 ate region, having intermediate conditions of life, why do we not 



