XI.] SPECIFIC GENESIS. 237 



similarly is sometimes directly indueecl by very obscure 

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

 immediately connected with geographical distribution; as 

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

 in simultaneous modifications of color in others, and in the 

 diicct 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 devclojmicnts of some particular kind, as beauty and 

 singularity of plumage in the group of birds of paradise. 



It has also been contended that there 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 Chaj)ter IV., 

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

 American trees simultaneously and similarly modified, also 

 the independent jiroduction of pony breeds, and the case 

 of the English greyhounds in Mexico, the ofTspring of 

 which produced directly acclimated progeny. Besides 

 these, the case of the Normandy pigs, of Dixtxira tatula^ 

 and also of the black-shouldered j)eacock, have been sjwken 

 of. The teeth of the lai)yrinth()don, 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 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 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, tho 

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



