Miscellaneous. 



570 



[Junb, 1909 



the behaviour of the black and white 

 factors is the simplest conceivable. We 

 have to picture the original black and 

 white-determining particles— whatever 

 their nature may be— to have multiplied 

 to such an extent that there is now a 

 pair— a black and a white— in every- 

 one of the cells which build up the 

 organism. Each of these cells has been 

 produced by the division into two equal 

 parts of a previously existing cell, and 

 in all ordinary cases the black factor 

 (for example) divides into two before 

 the cell divides, and half passes into 

 each of the new cells formed, But in 

 the division which leads to the forma- 

 tion of the reproductive cells the factors 

 do not split. On the contrary the 

 black and the white factors separate 

 from one another. And every repro- 

 ductive cell which arises contains either 

 one or the other— a black or a white— 

 but never both together and never a 

 blend of the two. Thus if we pay atten- 

 tion simply to this pair of characters, 

 black and white, we can represent the 

 repoductive cells of the cross-bred indi- 

 vidual by any number of single black 

 and white draughts— an equal number 

 of each ; for it appears that in both 

 sexes each of the two factors turns up 

 in very nearly the same number of repro- 

 ductive cells. 



Now suppose a pair of these cross-bred 

 individuals to be mated together. The 

 essential process of such mating con- 

 sists, of course, in the union of a pair 

 of reproductive cells, one derived from 

 either parent. So far as we can tell 

 these reproductive cells will meet 

 entirely at random. It is like an 

 experiment in which one draws a single 

 draughtsman out of each of two bags, 

 supposing each bag to contain an equal 

 number of blacks and whites. 



Thus we may draw :— 



(1) A black from the first and a black from the second 



(2) black „ white ., 



(3) white „ blaok „ 



(4) white „ white „ 



If this experiment is repeated a suffi- 

 cient number of times the average 

 result to be expected is 25 % BB, 50 % 

 BW, 25 % WW, or 1 BB, 2. BW, 1 

 WW, and in appearance they will be 

 3B, 1W. 



The accompanying diagram shows the 

 whole of the process of which a descrip- 

 tion has so far been attempted. A pure 

 black individual, producing exclusively 

 black reproductive cells (and this is 

 the test of its pureness), is mated with 

 a pure white. A black and a white germ- 

 cell meet, therefore, giving rise to an 

 individual which is black in appearance, 

 but which contains also the factor for 

 whiteness. This cross-bred individual 

 gives rise to an equal number of repro- 



ductive cells bearing the black and the 

 white factor respectively. When such 

 cross breds are mated together we get 

 three different kinds of offspring, of 

 which one kind is just twice as numer- 

 ous as either of the others. We have 

 pure blacks — just as pure as the mem- 

 bers of the original black strain— pure 

 whites like the original white strain, 

 and a third variety of individuals black 

 in appearance, which, nevertheless, con- 

 tain an element ot whiteness in addition, 

 and are thus of exactly the same nature 

 as their cross-bred parents. If these 

 last are again bred together they will 

 give rise to exactly the same series as 

 before ; whereas the pure blacks when 

 mated together can only give blacks, 

 and the pure whites bred together can 

 only give whites. 



In this particular instance the char- 

 acter of the cross-bred individual ex- 

 actly resembled that of one of the pure 

 parental strains, such a character being 

 said to be dominant over that of the 

 other parent which is said to be reces- 

 sive. The phenomenon of dominance is 

 not, however, by any means universal. 

 In some cases the cross-bred shows a 

 simple blend of the characters of its 

 parents. But even in such cases as 

 these it is possible for the parental 

 factors to separate completely when the 

 reproductive cells are formed, and the 

 actual process of heredity is just the 

 same as that previously described. In 

 other cases again the cross-bred may 

 show a character quite different from 

 that of either parent, and of these it 

 will be as well to take an example. 



There is a certain strain of fancy 

 fowls known as blue Andalusians which 

 have long been known to possess the 

 troublesome tendency of throwing a 

 large proportion of wasters, as they 

 are called. In every brood arising from 

 blue parents there are found a certain 

 number of nearly black birds, as well 

 as a certain number which are nearly 

 white. The reason of this was not in 

 the least understood until the question 

 was taken up by Professor Bateson 

 of Cambridge a few years ago. In the 

 first place Bateson showed, on raising 

 a large number of chicks from blue 

 parents, that blacks, blues and whites 

 appeared among the offspring in the 

 proportion of 1 : 2 : 1 on the average. 

 But he showed further that if a pair 

 of the black wasters were mated to- 

 gether they produced black offspring 

 only, and that if a pair of whites were 

 mated they produced only whites. If 

 on the other hand a black and a white 

 were mated together they produced 

 only blues. He thus arrived at the 

 apparently paradoxical result that th 



