No. 484] CLOSELY RELATED SPECIES 



215 



comb is a different unit, but they are not sharply separable. Crest 

 and no crest are units, but they run into each other in hybridizing. 

 Unit characters may show transitions, and, if so, they may have 

 originated gradually, so far as I see. It does not follow that 

 they must have originated gradually" — Davenport (:06, p. 80). 



Castle and Forbes's results with guinea pigs indicate the 

 same modifiability of unit characters. These authors (:0C, p. 

 13) say: "From the foregoing observations it is clear that, while 

 the long-haired and short-haired conditions are sharply alterna- 

 tive to each other in heredity, the gametes formed by cross-breds 

 are not in all cases pure. Frequently they consist of a blend 

 or a mixture of the two alternative conditions, constituting in 

 effect a new condition intermediate ])etween the other two. A 

 study of other characters alternative in iKMCility yields roults 

 somewhat similar, 



"Albinism is, in heredity, the most sliarply alu'riintivc uf char- 

 acters, yet cross-ln-eedino- hvUxwu albin.. and pi-nuMiicd ouinra- 

 pigs may modify tlie cliavacler both of the albino racr and of die 

 pigmented one. This modification may take on a variety of 

 forms, as has elsewhere been pointed onl i( "a>tle, :().") i. It may 

 result in the production of mosaics ( piunuMiteil aniinai> spotted 

 with white), or of albinos with a modified peripheral pigmenta- 

 tion, or of albinos visibly like their ancestors hnt transnn'ttini;- a 

 different set of latent characters. A,n-ain, die ron-h or ro>etted 

 coat of certain races of n-uinea-pius is sharply ahernaiive to smooth 

 coat, yet cross-breeding of ron,uii with smooth races may indnce 

 curious modifications of the ronuli character or prodnce smooth 

 individuals bearing the merest trace of the roni:h cliaracter. 



"All these facts are in harmony with the hypothesis, for which 

 there is strong evidence on the cytolouical side, that each sepa- 

 rately heritable character is represented by a differeiiT strnctnral 

 element in the germ (egg or spermatozoon!. In fertilization 

 the paternal and maternal representatives of a chaiacter become 

 more or less closely united, this nnion persistin-- ihroii-h all 



sexual elements. At that time the paternal and maternal repre- 

 sentatives of a character separate from each other and pa.ss into 

 different cells. 



