134 



A bsence or Rarity Chap. Vfc 



objections in the following chapter ; Instinct and Hybridism in the 

 two succeeding 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-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 per- 

 fection 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 

 Eecord ; 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. 



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 continent, 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. These representative species often meet and interlock ; 

 and as the one becomes 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 absolutely 

 distinct from each other in every detail of structure as are specimens 

 taken from the metropolis inhabited by each. By my theory these 

 allied species are descended from a common parent ; and during the 

 process of modification, each has become adapted to the conditions 

 of life of its own region, and has supplanted and exterminated its 

 original parent-form and all the transitional varieties between its 

 past and present states. Hence we ought not 'to expect at the 

 present time to meet with numerous transitional varieties in each 

 region, though they must have existed there, and may be embedded 

 there in a fossil condition. But in the intermediate reo-ion, having 

 intermediate conditions of life, why do we not now find closely- 

 lmkmg intermediate varieties? This difficulty for a long time 



