NATURAL SELECTION 237 



Fourthly, how can we account for species, when crossed, being 

 sterile and producing sterile offspring, whereas, when varieties are 

 crossed, their fertiUty is unimpaired ? 



ANSWER TO THE FIRST DIFFICXJLTY 



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 hes 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 hfe, then the difficulty of believing 



