NATURAL SELECTION 237 
Fourthly, how can we account for species, when crossed, being 
sterile and producing sterile offspring, whereas, when varieties are 
crossed, their fertility is unimpaired ? 
ANSWER TO THE FIRST DIFFICULTY 
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-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 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 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 crust of the earth is a vast museum; but the natural collections 
have been imperfectly made, and only at long intervals of time. 
ANSWER TO THE SECOND DIFFICULTY: ORGANS OF EXTREME 
PERFECTION AND COMPLICATION 
To suppose that the eye with all its inimitable contrivances for 
adjusting the focus to different distances, for admitting different 
amounts of light, and for the correction of spherical and chromatic 
aberration, could have been formed by natural selection, seems, I 
freely confess, absurd in the highest degree. When it was first said 
that the sun stood still and the world turned round, the common sense 
of mankind declared the doctrine false; but the old saying of Vox 
populi, vox Dei, as every philosopher knows, cannot be trusted in 
science. Reason tells me, that if numerous gradations from a simple 
and imperfect eye to one complex and perfect can be shown to exist, 
each grade being useful to its possessor, as is certainly the case; if 
further, the eye varies and the variations be inherited, as is likewise 
certainly the case; and if such variations should be useful to any ani- 
mal under changing conditions of life, then the difficulty of believing 
