XI.] SPECIFIC GENESIS. 255 



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 found that a tendency exists in nature 

 to eliminate hybrid races, by whatever means that elimina- 

 tion is effected, while no similar tendency bars the way to 

 an indefinite blending of varieties. This has also been en- 

 forced by statements as to the prepotency of certain pollen 

 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. It has also been 

 maintained that while even "horse ancestry" fails to 

 supply such a desideratum, in very strongly marked and 

 exceptional kinds (such as the Ichthyosauria, Chelonia, 

 and Anoura), the absence of links is both important and 

 significant. For if every species, without exception, has 

 arisen by minute modifications, it seems incredible that a 

 small percentage of such transitional forms should not 

 have been preserved. This, of course, is especially the 

 case as regards the marine Ichthyosauria and Plesiosauria, 

 of which so very many remains have been discovered. 



Sir William Thomson's great authority has been seen to 

 oppose itself to " Natural Selection," by limiting, on astro- 

 nomical and physical grounds, the duration of life on this 

 planet to about one hundred million years. This period, 

 it has been contended, is not nearly enough, on the one 

 hand, for the evolution of all organic forms by the exclusive 

 action of mere minute, fortuitous variations ; on the other 

 hand, for the deposition of all the strata which must have 



