34 



When allowance is made for the dominance of the yellow cotyledon 

 and the round form of seed it will be found that there result from 

 these combinations 9 yr + 3 ya -f- 3 gr + 1 ga, and it has already 

 been noted that these were the approximate proportions obtained by 

 Mendel for two pairs of characters. 



Since the rediscovery of Mendel's paper in 1900 much work has 

 been done by many investigators on the same lines. Mendel's 

 results have been repeatedly verified, and the law formulated by him 

 found to hold good for a very large number of characters in both 

 animals and plants. Many facts which at first seemed opposed to 

 the law have since been explained away in a very simple fashion. 

 Thus the fact that what seem to be intermediate forms between 

 "round" and "wrinkled" peas sometimes occur, which might 

 show that complete segregation of the determining factors for these 

 characters cannot always take place, has been found to rest upon a 

 misconception of the nature of the characters "round" and 

 " wrinkled." On the one hand, certain indentations of the surface 

 giving rise to an appearance approaching to " wrinkled " have been 

 shown to constitute quite a distinct character from either round or 

 wrinkled, and on the other hand the very important discovery has 

 been made that the " roundness " and " wrinkledness " are only the 

 outward expression of the fact that the starch grains in the two forms 

 of peas are radically different. Owing to accidents in growth or in 

 picking and drying, it may happen that a " round " pea appears almost 

 like a "wrinkled" pea, but if the starch grains be examined it is 

 impossible to make any mistake as to the category into which it 

 should be placed. 



Again, the apparently contradictory results at first obtained with 

 mice have been explained in a very simple way by Cuenot, the 

 explanation at the same time ranking as a most important develop- 

 ment of Mendelian theory. It had been found that when coloured 

 and albino mice (both being pure races in the usual acceptation of 

 the term) were bred together, certain coat colours were produced in 

 the progeny which were not evident in the parents ; e. g. a pure black 

 mouse might be obtained from a pure grey and a pure white. Some 

 of the evidence seemed to point to the conclusion that it was the 

 albino which was responsible for the new colour, and Cuenot made 

 the bold assumption that an albino might contain in a latent con- 

 dition the factor for some particular colour (that of the ancestor 

 from which the albino stock was obtained), and that this colour 

 only showed itself when the albino was crossed with a coloured 

 form. He represented the constitution of the germ-plasm not as 

 being simply, say, G (grey) or a (albino), or a combination of the 

 two, but as involving two pairs of characters, namely, c (coloured) 

 and a (albino), and, say, G (grey) and b (black), the grey being the 

 actual colour of the coloured mouse and the black being the latent 

 colour in the albino. The characters of the first pair are to be looked 

 upon as constitutional, so to speak, the " coloured " constitution 



