Seite - 179 - in The Origin of Species

TRANSITIONAL VARIETIES 179 leads the bee to make cells, and which has practically antici- pated 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 miscel- laneous objections in the following chapter; Instinct and Hybridism in the two succeeding chapters. On the Absence or Ranty 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 con- venient to discuss this question in the chapter on the Imper- fection of the Geological Record; and I will here only state / that I believe the answer mainly lies in the record being in- ^ parably 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. But it may be urged that when several closely-allied species inhabit the same territory, we surely ought to find at the present time many transitional forms. Let us take a simple case : in travelling from north to south over a conti- nent, we generally meet at successive intervals with closely allied or representative species, evidently filling nearly the same place in the natural economy of the land. Those represen- tative species often meet and interlock; and as the one be- comes rarer and rarer, the other becomes more and more frequent, till the one replaces the other. But if we compare these species where they intermingle, they are generally as ab-