108 ABSENCE OR EAEITY Cii-vp. VI. 



Fourthl}', how can -wc account for species, Vv'hen crossed, 

 being sterile and jiroducins;^ sterile offsprinp-, "vvhcreas, when 

 varieties are crossed, their fertility is unimpaired? 



Tiie tlrst two heads shall be here discussed — Instinct and 

 Hybridism in separate chapters. 



On the Absence or Harlty of Transltio7ial 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 Avhich it comes into competition. Thus extinction and 

 natural selection go hand in hand. Hence, if we look at each 

 species as descended from some other iniknown 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 tliis question in the chapter on the Imperfection of 

 the Geological liccord ; and I will here only st:ite that I be- 

 lieve 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 imper- 

 fectly made, and only at long intervals of time. 



But it may be lu-ged that when several closely-allied species 

 inhabit the same teixitory, we surely ought to find at the pres- 

 ent 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 representa- 

 tive species, (Tvidently tilling neaily the same place in the nat- 

 ural economy of the land. These reiM-esentativc species often 

 meet and interlock ; and as the one l)ecomcs rarer and rarer, 

 the other becomes more and more fret(uent, till the one re- 

 places the other. But if we compare these species Avhere they 

 intermingle, they are generally as absolutel}" distinct from each 

 other in every detail of structiu'c as are specimens taken from 

 the metropolis inhalntcd by each. By my tlieory these allied 

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

 jHoccss of modilication, each has become adapted to the con- 

 ditions of lift^ of its own region, and has suj^planted and ex- 



