DIFFICULTIES ON THEORY 155 



such wonderful structure, as the eye, of which we hardly 

 as yet fully understand the inimitable perfection ? 



Thirdly, can instincts be acquired and modified 

 through natural selection ? What shall we say to so 

 marvellous an instinct as that which leads the bee to 

 make cells, which has practically anticipated the dis- 

 coveries 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 shall be here discussed — Instinct 

 and Hybridism in separate 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 or other less- 

 favoured forms with which it comes into competition. 

 Thus extinction and natural selection will, as we have 

 seen, go hand in hand. Hence, if we look at each species 

 as descended from some other unknown form, both the 

 parent and all the transitional varieties will generally 

 have been exterminated by the very process of forma- 

 tion 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 much 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 imperfection of the 

 record being chiefly due to organic beings not inhabiting 

 profound depths of" the sea, and to their remains being 

 embedded and preserved to a future age only in masses 

 of sediment sufficiently thick and extensive to withstand 

 an enormous amount of future degradation ; and such 

 fossiliferous masses can be accumulated only where much 



