150 ABSENCE OR RARITY 



being sterile and producing sterile offspring, whereas, when 

 varieties are crossed, their fertility is unimpaired ? 



The two first heads will here be discussed ; some miscel- 

 laneous 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 extermi- 

 nate, its own less improved 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 transitional 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 Record ; and I will here 

 only state that I believe the answer mainly lies in the record 

 being incomparably less perfect than is generally supposed. 

 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 inhabit the same territory, we surely ought to find 

 at the present time many transitional forms. Let us take 

 a simple case: in travelling from north to south over a 

 continent, we generally meet at successive intervals with 

 closely allied or representative species, evidently 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 absolutely distinct from each other in every 

 detail of structure as are specimens taken from the metropo- 

 lis inhabited by each. By my theory these allied species 

 are descended from a common parent ; and during the pro- 

 cess of modification, each has become adapted to the con- 



