CANNON: Stupies In Pianr Hysrrps 139 
their offspring being recessive, one fourth dominant and one half 
mixed. 
The regularity in the variation as just described in the second 
and later generations is accounted for by supposing that the hybrids 
of the first generation organize germ-cells which are of pure descent, 
and that these unite in fertilization according to the laws of chance. 
Taking a specific case by way of illustration, we can imagine the 
following to take place, when the sex-cells of say the second 
generation hybrids (A(a)) meet each other in fecundation. The 
pollen, which is of pure descent, unites with the egg, which also is 
of pure descent, and the chances of union may be thus expressed : 
AA;Aa;aA;aa. So that it happens, since the anther forms 
two sorts of germ-cells and the ovaries also two sorts, that in this 
way one half of the hybrids of say the third generation will be 
of mixed descent, and one half pure, the latter being equally reces- 
sive and dominant. The results, as calculated by the laws of 
chance, are thus seen to be precisely the same as what is found 
empirically to occur. 
Such may perhaps be considered the more essential facts and 
conclusions of the discovery of Mendel, and upon them are based 
the two so-called “laws’’ of Mendel, namely, the law of domi- 
nance and that of the splitting of the hybrid race. 
Mendel gives also, and in much detail, the results when the 
pure parents of hybrids are separated by more than one character, 
as, for instance, by ¢wo or three. But his experiments are too ex- 
tensive to present here and I can not do better than to quote from 
the account of them by Bateson (/. c.g). ‘In both sets of experi- 
ments the numbers of individuals and their constitution * * * wore 
consistent with the hypothesis arrived at in the case of varieties 
differing in respect of one character, namely, that each male and 
female cell of the cross-bred is pure in respect of one character. of 
each pair of characters, and is capable of transmitting this char- 
acter to the exclusion of the opposite character; that the repro- 
ductive cells are, in the cross-breds, of as many kinds as there are 
possible combinations of pure characters (taken two or sae 
together, as the case may be); and, finally, that each kind is 
represented in the cross-breds on the average in equal num- 
bers.” 
