152 MENDEL'S LAWS 



white fowls is wholly black or wholly white. Particulate inherit- 

 ance may be regarded as exclusive inheritance occurring in patches. 

 (3) Blended inheritance, as in the skin colour of a mulatto. 

 Blended inheritance! may be regarded as particulate inheritance 

 occurring in a very fine mosaic. Galton thinks, " There are pro- 

 bably no heritages that perfectly blend or that absolutely exclude 

 one another, but all heritages have a tendency in one or the other 

 direction, and the tendency is often a strong one." 1 There can be 

 no doubt, however, that small progressive variations (fluctuations) 

 present in one parent, but absent in the other, tend to disappear 

 absolutely in the offspring. In such cases, therefore, there is no 

 blending. 2 



252. The greater part of modern experimental work follows 

 the lines laid down by Gregor Mendel, a monk who became Abbot 

 of Briinn, and who about the middle of the last century crossed 

 many varieties of the edible pea Pisum sativum. Peas are 

 normally self-fertilized, the pollen-grains and ovules of the same 

 flower uniting. But the plants have conspicuous flowers, which, 

 presumably, underwent evolution because they attracted insects 

 and so secured cross-fertilization. We must suppose, therefore, that 

 peas have descended from ancestors that interbred. If the anthers 

 of a flower be amputated before the pollen is ripe, and pollen from 

 another plant be conveyed to the pistil, it is still possible to secure 

 cross-fertilization. 



253. Mendel worked with a number of strongly contrasted 

 varieties 'tall' and 'dwarf plants, for example. From the 

 mongrel seeds, thus obtained, only tall plants developed. They 

 were allowed to self-fertilize themselves, as were their descendants. 

 About one-quarter of the offspring the grandchildren of the 

 original cross developed as dwarfs and three-quarters as tall plants. 

 No plants were intermediate. Thereafter the dwarfs continued to 

 breed ' true ' as long as the experiments were continued, producing 

 no tall plants in succeeding generations. It seemed evident that 

 the race was purged of the influence of the tall variety. On the 

 average, a similar number of tall plants behaved in like manner, 

 producing only tall descendants. Their race, also, had become 

 1 pure.' But the remaining plants (fifty per cent, of the whole) 

 behaved like their cross-bred parents, producing tall and dwarf 

 offspring in the old proportion, the dwarfs being all pure, and the 

 tall plants pure and impure. This process, also, continued until 

 the experiments ceased. 



l Qp. fit,, p. 12. 2 See 194. 



