HYBRIDITY 



375 



hybrid possessed two sets of reproductive cells, initials of the red-flowered form 

 and initials of the white-flowered form, in equal number. Fusion between two 

 sexual-cells with the same initials might then be as frequent as fusion between 

 sexual-cells with different initials. In lOO fertilizations we shall get on an average 

 fifty cases of fusion of similar initials — twenty-five fusions of cells bearing the 

 initials of white with white, twenty-five fusions of red with red — while fifty 

 fusions of dissimilar initials might also take place. Whether the red fuses with 

 red, or red with white is immaterial ; since red is dominant, 75 per cent, of the 

 second generation will show red flowers, and only the 25 per cent, in which 

 white has united with white wiU exhibit white flowers. But these 25 per cent, 

 have for ever lost the power of producing red flowers, and in this fact lies 

 the evidence for the support of the theory that the sexual-cells of the hybrid 

 contain only one kind of initial, while the vegetative-cells contain both types 

 of initial. There is thus in the formation of sex-cells a segregation of the 

 initials. Amongst the 75 per cent, of red-flowered forms only 25 per cent, 

 have red initials, and these are marked off from the fifty others which carry 

 white also, when sexual-cells are first formed. The twenty-five again form 

 only one kind of sex-cell, while the remaining fifty again segregate. Let us 

 imagine a case in which each plant produced only four offspring. We may then 

 construct the following scheme of the numerical relationships between the forms 

 with white, red, and mixed (red + white) initials in five generations. 



Generation I. 



Generation II. 



I white — > 



Generation III. 



4 white — 3 



/ 2 white — > 



I red ■(- white 2 red + white -( 4 red + white 



I red 



2 red 



4 red 



Generation IV. 



16 white — > 

 8 white — ^ 

 4 white — > 



8 red + white 



4 red — » 



8 red — > 



16 red — * 



Generation V. 



64 white 

 32 white 

 16 white 



8 white 

 16 red + white 



8 red 

 16 red 

 32 red 

 64 red 



Thus we see that the ' white-flowered ' character, lost apparently in the first 

 generation, reappears in the second generation in 25 per cent, of the forms, and 

 rapidly increases, until by the fifth generation there is but little difference in 

 number between the red and the white-flowered individuals. 



Mendel's law of segregation is, however, not of universal application. 

 There are hybrids which do not segregate and also others which segregate in 

 different proportions. Segregation may occur in one character while another 

 does not segregate. The segregation or non-segregation of characters does not 

 depend on whether the characters in the formation of hybrids are homodynamous 

 or heterodynamous. Notwithstanding the great interest attached to the 

 behaviour of races differing in two or more characters we must content our- 

 selves with what has been already said, and in conclusion raise the question as 

 to how far these facts suggested by the study of hybrids bear on the general 

 problem of fertilization. 



Bearing in mind the increased power of growth of many hybrids and Dar- 

 win's statement that seeds derived from cross-fertilization produced more 

 vigorous seedlings than those resulting from self-fertilization, it may be said 

 that cross-fertilization brings about a rejuvenescence or renewal of youth in the 

 protoplasm. Further, we may assume that protoplasm without such rejuve- 

 nescence may finally become, by continued vegetative reproduction, senile. 

 Although it is certain that the hybrid frequently exhibits more vigorous powers 

 of growth still any conclusion founded on that fact is insecure. It has not been 

 shown that the increased vigour depends on the fusion of the two kinds of 

 initials from which the hybrid results, and it seems almost more probable that 

 the very stimulus which removes from the egg its inability to develop may be also 



