HEREDITY AND VARIATION 587 



on the balance the species is like .1 living organism. At .ill I 



organization life depends on the maintenance ol 

 among its factors." Further it may he noted that : " If the population 

 size is small, favourable mutations may actually be lost, and evolution- 

 ary changes may proceed against the pressure and diro tion oi -«-|. 



tion. In large populations, however, selection will act to an amount 

 approximately proportional to its intensity." 



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 infrequently 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 XIII.). 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. In other cases there may be an 

 apparent maturing of good seeds, though the embryos within them arc 

 not sexually produced. Thus in Funkia, Coelebogyne, and others, numerous 

 embryos 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 Apowixis, 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 embi 

 sac, are found in Alchemilla, Thalictrum, Taraxacum, and Hieracium. In them 

 embryos may be formed from an ovum without fertilisation. But here the i 

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

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

 *' somatic parthenogenesis," which implies that the embryo springs from the 

 ovum but the ovum was itself diploid. A like condition h 

 Marsilia Drummondi, and in Athyrium filix-foemina, var. dan In ^ich 



cases again no fertilisation is necessary to arrive at the diploid state. A n 

 rare condition is that where an egg that is really haploid develops as though rt 

 had been fertilised. This is rare in Seed-Plants, but it has been 

 It has also been found to occur in Chara crintia. 



Contrasted with these cases of apomixis. where no sexual fanon 

 are various conditions which may be rank- **•** * > ' ■ 



nuclear fusion is seen, with consequences like those following on normal 

 syngamy But the nuclei involved are not produced in the normal m 

 Examples have already been described for N*pkrodHum ■ ■ '• 



dactvlum (p. 508. Fig." 4 o3) I and in thai unusual x^ 

 which occurs in the formation of the fruit In the Uregineae (p. 43*. ' '- *W- 



