172 
DIFFICULTIES ON THEOKY, 
Chap. VI, 
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 
natm’al selection ? What shall we say to so marvellous 
an instinct as that which leads the bee to make cells, 
which has practically anticipated the discoveries of pro¬ 
found 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 inliabiting 
