XI.] SPECIFIC GEXESIS. 237 



similarity is sometimes directly induced by very obscure 

 conditions, at present quite inexplicable, e. g., by causes 

 immediately connected with geographical distribution ; as 

 in the loss of the tail in certain forms of Lepidoptera and 

 in simultaneous modifications of color in others, and in the 

 direct modification of young English oysters, when trans- 

 ported to the shore of the Mediterranean. 



Again, it has been asserted that certain groups of or- 

 ganic forms seem to have an innate tendency to remark- 

 able developments of some particular kind, as .beauty and 

 singularity of plumage in the group of birds of paradise. 



It has also been contended that these is- something to 

 be said in favor of sudden, as opposed to exceedingly 

 minute and gradual modifications, even if the latter are not 

 fortuitous. Cases were brought forward in Chapter IV., 

 such as the bivalve just mentioned, twenty-seven kinds of 

 American trees simultaneously and similarly modified, also 

 the independent production of pony breeds, and the case 

 of the English greyhounds in Mexico, the offspring of 

 which produced directly acclimated progeny. Besides 

 these, the case of the Normandy pigs, of Datura tatida, 

 and also of the black-shouldered peacock, have been spoken 

 of. The teeth of the labyrinthodon, the hand of the potto, 

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

 tendrils of some plants, etc., have also been adduced as 

 instances of structures, the origin and production of which 

 are probably 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 intermedi- 

 ate (as, e. g., the aye-aye) are really by no means so ; while 

 the general rule, that the progress of forms has been " from 

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

 present remarkable exceptions, as, e. g., Macrauchenia, the 

 Glyptodon, and the sabre-toothed tiger (Machairodus). 



