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PEOF. ERNEST W. HACBRIDE, M.A., F.R.S., ON 



chance in four that a green ovule will meet a green pollen grain, 

 and one chance in four that a yellow ovule will meet a yellow 

 pollen grain, but there is also one chance in four that either a 

 yellow pollen grain will meet a green ovule, or that a green 

 pollen grain will meet a yellow ovule. These two latter unions 

 produce exactly the same result ; the resulting pea looks yellow, 

 but is a hybrid which in the next generation will give rise, when 

 self-fertilized, to yellow and green peas. Therefore, out of four 

 peas produced by the first hybrid, one is a pure yellow, one is a 

 pure green, and two are yellow hybrids, which look like the pure 

 yellow, so that the proportion, three yellow to one green, is 

 accounted for. 



Now suppose that we cross a plant producing round yellow 

 peas with one which produces green angular peas. The 

 resulting peas are round and yellow. Thus roundness dominates 

 over angularity and yellowness over greenness. But when the 

 plants raised from these peas are self-fertilized, three-fourths 

 of the peas produced are yellow and one-fourth green as before,, 

 and three-fourths are round and one-fourth angular ; but these 

 two sets of qualities are distributed through the peas indepen- 

 dently of each other ; that is to say, it is exactly as if one had a 

 bag of billiard balls and one were to select at random three-fourths 

 and paint them black, and if one were then to put the balls 

 back into the bag, shake them up, and again select at random 

 three-fourths and mark them with a red dot. As a consequence- 

 some balls would have the black paint and the red dot and 

 some would have neither, and some would have the black paint 

 only and some the red dot only. These balls may be taken to 

 represent the ovules and pollen grains of the hybrid. So 

 amongst our peas produced by the self-fertilization of the 

 hybrid we have not only green and angular and yellow and 

 round peas, but two new varieties make their appearance, viz., 

 yellow and angular and green and round. On an average out 

 of sixteen peas nine (i.e., f x f ) will be yellow and round, one 

 will be green and angular (i.e., \ x ^), whilst three will be round 

 and green (f x J), and three angular and yellow (f x \). Of 

 these two new varieties, however, only one-third, i.e., one in 

 sixteen of the whole progeny of the original hybrid, will breed 

 true. The two-thirds reveal themselves as hybrid with respect 

 to one character (the dominant one). 



These* examples illustrate the laws of heredity discovered 

 by Mendel, and little new in principle has been added since. 

 It has, however, been shown that the difference between two 

 races consists in most cases in the presence in one of some 



