406 D. J. SCOURFIELD ON MENDELI8M AND MICROSCOPY. 



obtained results of exactly the same kind, and explicable in the 

 same way. 



By a slight extension of Cuenot's hypothesis — namely, by 

 -conceiving of the "pairs" of characters as consisting in some 

 caseB of the presence of a character as opposed to its absence — it 

 lias l>een found possible to explain some extremely peculiar results 

 which seemed at first to be quite outside the pale of Mendel's 

 law in its original form. Thus the remarkable phenomena of the 

 production of the " walnut " (Malay) comb in fowls, by the mating 

 of individuals possessing " pea " and " rose " combs, and the 

 subsequent production by the "walnut" forms not only of 

 " walnut," " rose," and " pea," but also of " single " combs, have 

 been brought into line with ordinary Mendelian inheritance by 

 such a " presence and absence " theory. Briefly the idea is as 

 follows. The " pea " and " rose " factors are probably distinct 

 mutations of, or additions to, the factor for " single " comb, and 

 belong to different pairs of characters or allelomorphs, as they 

 are now often called. They can, therefore, exist side by side in 

 the same germ-cell. Their co-existence, however, produces a new 

 type of comb (" walnut "), a type which could not be predicted 

 simply from our knowledge of "pea" and "rose." When the 

 germ-cells of the " walnuts " are formed, in addition to all of 

 them containing " single," half the number will contain the factor 

 for "pea" and half the number the "rose" factor. Owing to 

 the fact, however, that "pea" and "rose" can occur together, 

 they will so occur on the average just as often as " pea " occurs 

 by itself and " rose " by itself. But for every case where " pea " 

 and "rose" occur together, there must be a case where there is 

 neither " pea" nor "rose," and therefore "single" is exposed to 

 view. Equal numbers of the four kinds of germ-cell — " single," 

 " pea," " rose," and " pea + rose " ( = " walnut ") — will, therefore, 

 be produced, and from these the results obtained by experiment 

 cau be explained. It is worthy of remark that here also the 

 influence of characters belonging to distinct allelomorphic pairs 

 upon one another, while present in the same individual, is 

 aecessarily involved in the above explanation of the "walnut" 

 comb, and in a much more pronounced form than merely pro- 

 moting or repressing the appearance of one of the pairs. 



Another conception that has been added to Mendel's original 

 idea Is that of "compound allelomorphs." A certain amount of 



