Mendelism 265 



"expected" results agree with those actually obtained in 

 breeding experiments? Obviously the larger the number of 

 individuals the greater the probability of agreement between 

 expectation and realization, and when the former is small, 

 say 1 :64 or 256, a very large number of tests may be neces- 

 sary before it can be realized. While the correspondence be- 

 tween expectation and realization is seldom exact, the agree- 

 ment is nevertheless generally close enough to furnish a sub- 

 stantial basis for the theory. A few random examples may 

 be cited. In the common weed, the shepherd's purse (Bursa 

 bursa-pastoris) there is a variety with round and another 

 with triangular fruits. The latter dominates the former and 

 is determined by two factors. Therefore the expectation for 

 round vs. triangular fruits is 1 in 16. In a total of 2907 sec- 

 ond generation hybrids Shull found 2782 with triangular and 

 125 with round fruits, a ratio of 23.3 to 1, as compared with 

 an expectation of 2725 of the former and 182 of the latter, a 

 ratio of 16 to 1. In crosses between quadroons and whites 

 Davenport found out of 99 children there were 42 "near- 

 whites," 56 quadroons and 1 mulatto, whereas the expectation 

 was an equal number of "near-whites" and quadroons and 

 no mulattoes. In another series of matings between quadroons 

 he found out of a total of 134 children, 24 "near-whites," 

 87 quadroons and 23 mulattoes, the expectation being 3.5, 67 

 and 33.5 respectively. 



Any rabbit breeder knows what a mixture of colors and 

 markings he may expect in his product. Professor Castle, 

 who has recently analyzed the color varieties of rabbits, clas- 

 sifies them as follows: gray, black, yellow (with white belly 

 and tail), sooty (a variety of yellow with the belly and tail 

 colored like the rest of the body), and white. The first four 

 of these may in turn be modified by intensity of pigment (dark 

 or light), by its uniformity, or lack of uniformity (spotting), 

 and the white may be either wholly so or cream colored with 

 black nose, ears, feet and tail (the so-called "Himalayan" 

 of the fanciers). This makes a total of eighteen varieties 

 in all, which when interbred can theoretically produce 243 

 different varieties, different, that is, from the viewpoint of 

 their hereditary structure, not in their external appearance, 

 for things "are (very often) not what they seem" in genetics. 

 Many of these varieties have been obtained, others still re- 

 main to be "created." There are thirty-two possibilities in 

 gray rabbits, many of which are already known. As a com- 

 parison of the results realized with those expected when one 

 variety of these grays is crossed with itself, the following 

 table from Professor Castle 's paper is of interest : 



