DIFFICULTIES OF THE THEORY. I59 



leads the bee to make cells, and which has practically 

 anticipated the discoveries of profound mathematicians? 



Fourthly, how can we account for species, when crossed, 

 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 RAEITT 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 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 onlv 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 traveling from north to south over a con- 

 tinent, 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 



