TRANSITIONAL VARIETIES 179 



leads the bee to make cells, and which has practically antici- 

 pated 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 Ranty 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-favoured 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 embedded in countless 

 numbers in the crust of the earth? It will be more con- 

 venient to discuss this question in the chapter on the Imper- 

 fection of the Geological Record; and I will here only state / 

 that I believe the answer mainly lies in the record being in- ■^ 

 parably 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 conti- 

 nent, 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. Those represen- 

 tative species often meet and interlock; and as the one be- 

 comes 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 ab- 



