CHAP. VI.] TRANSITIONAL VARIETIES. 12.D 



sterile and producing sterile offspring, whereas, when varieties 

 are crossed, their fertility is unimpaired 1 



The two first heads will here be discussed ; some miscellaneous 

 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 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; 

 hut 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 ars 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 sup- 

 planted 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 nie^t 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 region, having intermediate conditions of 

 life, why do we not now find closely-linking intermediate varieties 1 



