SEX AND HEREDITY 477 



will thus be indestructible, provided it be not injurious. If it is 

 injurious it will be eliminated by Natural Selection. The effect will 

 be the survival of harmless or beneficial mutations. In the case 

 of hybrids this appears in the tendency they show to revert to one 

 of the original forms. In practice Mendelian segregation has already 

 been used in the establishment of pure varieties of cultivated plants 

 resistant to disease, and conspicuously in the case of Wheats. But 

 the practical application of Mendelian methods is still in its infancy, 

 and will probably be slow, because of the extreme complexity of the 

 questions which arise, except in the simplest cases. 



Sexuality appears to bring as a consequence the distribution and 

 perpetuation of inheritable characters. Mendelian segregation is 

 not in itself a constructive process. It is a distributive agency. 

 Attention will then be centred not on the agency, however interesting 

 and impressive its working may be, but upon the origin of the factors 

 which it distributes. The central question of Evolution comes finally 

 to be the origin of the Heritable Mutations. Of this as little is positively 

 known at the moment as of the constitution of the protoplasm that 

 gives rise to them. 



IRREGULAR PROPAGATION. 



Some Plants may eliminate normal sexual propagation, substituting for it 

 in various ways other means of increase in numbers. Thus they forego the 

 advantages which follow from sexuality, but not unfrequently they secure 

 greater certainty of propagation. The commonest cases are where vegetative 

 propagation replaces partially or completely the reproduction by seed : a 

 condition common in Nature, and seen in special degree in cultivated plants, 

 such as the Potato, Jerusalem Artichoke, Sugar-Cane, Banana, and Pine- 

 Apple (Chapter XII.). In the viviparous habit of Alpine Plants the substi- 

 tution of vegetative buds for flowers is probably a biological accommodation 

 to the shortness of the Alpine summer. Frequently, however, there may be 

 an apparent maturing of good seeds, though the embryos within them are 

 not sexually produced. In Funkia, Coelebogyne, and others, numerous em- 

 bryos arise by adventitious budding from the tissue of the nucellus, and they 

 project like normally produced embryos into the embryo-sac. The nucellar 

 tissue in such cases was already diploid : so that here there is neither reduc- 

 tion nor sexual fusion. They are peculiar examples of sporophytic budding. 

 But as they involve a loss of sexuality, they may be described under the 

 general term of Apogamy, or better, of Apomixis, by which is meant quite 

 generally the absence of syngamy where it would normally occur. 



Somewhat similar states, which however involve the contents of the 

 embryo-sac, are found in Alchemilla, Thalictrum, Taraxacum, and Hieracium. 

 In them embryos may be formed from an ovum without fertilisation. But 

 here the egg itself has been found to be diploid, for reduction had been omitted 

 in the development of the embryo-sac. Technically this has been described 



