254 TUE GENESIS OF SPECIES. [Chap. 



the whalebone of whales, the wings of birds, the climbing 

 tendrils of some plants, &c. have also been adduced as 

 instances of structures, the origin and production of which 

 are prol)ably due rather to considerable modifications than 

 to minute increments. 



It has also been shown that certain forms which were 

 once supposed to be especially transitional and inter- 

 mediate (as, e.g., the aye-aye) are really not so; while 

 the ordinary rule, that the progress of forms has been 

 "from the more general to the more special," has been 

 shown to have remarkalde exceptions, as, c.(j. IMa- 

 crauchenia, the Glyptodon, and the sabre-toothed tiger 

 (Machairodus). 



Xext, 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 presented by 

 the inorganic world, and perhaps also by the lowest forms 

 of the organic world. It has also been seen that witli 

 regard to minute spontaneous variations in races, there is 

 a rapidly increasing difficulty in intensifying them, in any 

 one direction, by ever such careful breeding. ^Moreover, it 

 has appeared that different species show a tendency to 

 variability in definite directions, and probably in different 

 degrees, 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 

 proclivity to change when he speaks of " a whole organiza- 

 tion seeming to have become j)lastic, and tending to depart 

 from the parental type." 



We have seen also that a marked proclivity to reversion 



