ORIGIN OF SPECIES 405 



of complexity of structure undergone by the germ and embryo 

 of a highly organized animal in its progress to maturity. He 

 speculates on the influence of premature birth, or on a some- 

 what prolonged fcetation, in establishing the beginning of a 

 specific form different from that of the parent. 



Mr. Wallace,* assuming that varieties may arise in a wild 

 species, shows how such deviations from type may tend to 

 adapt a variety to some changes in surrounding conditions, 

 under which it is better calculated to exist, than the type- 

 form from which it deviated. 



ISTo doubt the type-form of any species is that which is 

 best adapted to the conditions under which such species at 

 the time exists ; and as long as those conditions remain 

 unchanged, so long will the type remain ; all varieties 

 departing therefrom being in the same ratio less adapted to 

 the environing conditions of existence. But, if those con- 

 ditions change, then the variety of the species at an antece- 

 dent date and state of things will become the type-form of the 

 species at a later date, and in an altered state of things. 



Mr. Charles Darwin had, previously to Mr. Wallace, 

 pondered over and worked at this principle, which he 

 illustrates by ingenious suppositions, of which I select the 

 following : — " To give an imaginary example from changes 

 in progress on an island : — let the organization of a canine 

 animal which preyed chiefly on rabbits, but sometimes 

 on hares, become slightly plastic ; let these same changes 

 cause the number of rabbits very slowly to decrease, and the 

 number of hares to increase ; the effect of this would be that 

 the fox or dog would be driven to try to catch more hares ; 

 his organization, however, being slightly plastic, those indi- 

 viduals with the lightest forms, longest limbs, and best eye- 

 sight, let the difference be ever so small, would be slightly 

 favoured, and would tend to live longer, and to survive during 



* Proceedings of the Linnsean Society, August 1858, p. 57. 



