238 THE GENESIS OF SPECIES. [CHAP. 



Next, as to specific stability, it has been seen that there 

 may be a certain limit to normal variability, and that if 

 changes take place they may be expected a priori to be 

 marked and considerable ones, from the facts of the inor- 

 ganic world, and perhaps also of the lowest forms of the 

 organic world. It has also been seen that with regard to 

 minute spontaneous variations in races, there is a rapidly- 

 increasing difficulty in intensifying them, in any one di- 

 rection, by ever such careful breeding. Moreover, it has 

 appeared that different species show a tendency to varia- 

 bility in special directions, and probably in different de- 

 grees, and that at any rate Mr. Darwin himself concedes 

 the existence of an internal barrier to change when he 

 credits the goose with " a singularly inflexible organiza- 

 tion ; " also, that he admits the presence of an internal pro- 

 clivity to change when he speaks of " a whole organization 

 seeming to have become plastic, and tending to depart 

 from the parental type." 



We have seen also that a marked tendency to reversion 

 does exist, inasmuch as it sometimes takes place in a 

 striking manner, as exemplified in the white silk fowl in 

 England, in spite of careful selection in breeding. 



Again, we have seen that a tendency exists in nature 

 to eliminate hybrid races, by whatever means that elimi- 

 nation is effected, while no similar tendency bars the way 

 to an indefinite blending of varieties. This has also been 

 enforced by statements as to the prepotency of certain pol- 

 len of identical species, but of distinct races. 



To all the preceding considerations have been added 

 others derived from the relations of species to past time. 

 It has been contended that we have as yet no evidence 

 of minutely intermediate forms connecting uninterruptedly 

 together undoubtedly distinct species. That while even 

 " horse ancestry " fails to supply such a desideratum, in very 

 strongly-marked and exceptional kinds (such as the Ichthy- 



