322 THE BIOLOGY OF FLOWERING PLANTS 



derived from different individuals or stocks ; though this 

 introduces new factors. Undoubted weakening does occur 

 in some species if close inbreeding is persisted in through 

 many generations. This is the case in human beings and, 

 among plants, in the maize. In this plant the crossing of 

 two races leads to the production of vigorous offspring ; it 

 is a case of " hybrid vigour." The factors involved have 

 been investigated recently by East and Jones (19 19). They 

 have shown that vigour is produced by a number of in- 

 dependent dominant hereditary factors : the greater the 

 number of these present in an individual the better its 

 growth. When two individuals of different races are 

 crossed an addition of such factors takes place. Theo- 

 retically it is possible to breed an individual containing all 

 these dominant factors in homozygous condition, and such 

 an individual if inbred would show no weakening. Prac- 

 tically this is extremely difficult or even impossible because 

 of linkages which are only rarely broken (cp. Morgan, 

 19 1 9). Other cases of hybrid vigour in plants are described 

 by Darwin (1876), and these are probably to be explained 

 in the same way. There are many plants and animals in 

 which inbreeding is the rule, and in these no degeneracy 

 results. This more advanced type of sexuality which is 

 represented by outbreeding is therefore without a primary 

 effect on stamina, and this makes it all the more difficult 

 to believe in a rejuvenation resulting from the more primi- 

 tive fusion of two sister cells, or of cells derived from the 

 same individual. We shall return to the question of 

 the possibility of prolonged vegetative existence in another 

 connection, and may here note simply that the evidence in 

 favour of the rejuvenation theory of sex is not convincing. 



(c) The case of the maize, however, brings us to a third 

 possible explanation of the importance of sexuality. The 

 zygote is remarkable in that it contains a contribution from 

 each of two cells, and very often these two cells are derived 

 from different individuals or even races. In the zygote the 

 characters of two parents are brought together, and the 

 individual which arises from it has also a dual set of 



