74 THE PRINCIPLES OF HEREDITY 



never see a regular flower realize the structure of an irregular 

 one.'" 



129. " Striped flowers, though they can be propagated truly 

 by seed, have a latent tendency to become uniformly coloured, 

 but when once crossed by a uniformly coloured variety, they 

 ever afterwards fail to produce striped seedlings." 2 



1 Animals and Plants, vol. ii., pp. 32-33. 



2 Op. cit., pp. 45-6. Recent research renders it very probable that 

 some of the experiments on cross-breeding quoted above are incomplete. 

 They tell us of the reappearance of the common ancestral type ; but they 

 do not record the results of continued breeding from the reverted indi- 

 viduals. It has been shown by de Vries, Bateson, and other followers 

 of Mendel, that, even when the offspring of crossed varieties revert to 

 the common ancestral form, there is a strong tendency towards the 

 reappearance of the differentiated types among subsequent descendants. 

 Thus "the first cross of albino mice with black-and-white Japanese 

 waltzing mice gave grey house-mice resembling in size, colour, and 

 wildness the wild house-mouse. The first crosses, bred together, 

 gave albinos, grey mice, black-and-white, grey-and-white, and black 

 mice." (Von Guita, quoted by Mr. Bateson and Miss Sanders Report 

 to Evolution Committee of the Royal Society, 1903, p. 145.) In the 

 present state of the inquiry it is difficult to estimate to what extent 

 particulate and exclusive inheritance prevails among the offspring of 

 crossed forms ; but it appears that complete and permanent reversion to 

 the common ancestral type is by no means unusual. At any rate, it is 

 certain that while cross-breeding is frequently a cause of reversion, it 

 is never a cause of progression. The work done by Mendel and his fol- 

 lowers is of the highest interest and importance. Very probably it will 

 shed a clear light on some of the phenomena of heredity. But it must 

 be constantly borne in mind that the mating of distinct varieties is not 

 the same thing as breeding within the same variety. The former must 

 occur so very rarely in nature, that it can play no part in the adaptation 

 of species to their environments. In Mr. Bateson's words, " the central 

 phenomenon in Mendelian heredity is segregation." (Proceedings of the 

 Zoological Society of London, 1903, vol. ii., p. 85.) " The germ-cells or 

 gametes produced by cross-bred organisms may in respect of given 

 characters be of the pure parental types, and consequently incapable of 

 transmitting the opposite character." (Report of the Evolution Committee 

 of the Royal Society, 1902.) In the case of abnormalities also (e. g. a sixth 

 digit ) the inheritance tends to be exclusive ; indeed an abnormal indi- 

 vidual is in effect one who belongs to a new variety. As already indicated, 

 an abnormality (i. e. a deformity) can seldom increase the adaptation to 

 the environment. Even when not injurious (e. g. sixth digit) it is apt to 

 be bred out in the course of generations. The central fact of normal 

 breeding, that is, breeding between members of the same variety, is 

 blended inheritance, not segregation of the parental characters. It is 

 from the effects of normal breeding that we must endeavour to ascertain 

 the function of bi-parental reproduction in nature ; and very certainly 

 that function is to produce, not progressive but regressive variations. 



When two individuals of the same race vary somewhat as regards any 

 character differently from one another, the chances on the whole are that 

 both their variations are useless. It is therefore an advantage to the off- 

 spring that both variations should be eliminated by the regressive action 



