208 ABSENCE OR RARITY [Chap. YL 



believe that natural selection could produce, on the one 

 hand, an organ of trifling importance, such as the tail of 

 a giraffe, which serves as a fly-flapper, and, on the other 

 hand, an organ so wonderful as the eye ? 



Thirdly, can instincts be acquired and modified 

 through natural selection ? What shall we say to the 

 instinct which 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 

 miscellaneous 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 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 convenient to discuss this question in the 

 chapter on the Imperfection of the Geological Eecord ; 



