54 THE EVOLUTION THEORY 



fertilized by their own joollen are either purely paternal or purely 

 maternal. There cannotj therefore, be free combination of the 

 iclants at the reducing (Jivisions ; the idants of the two parent- 

 forms separate from one another and do not combine. It is doubtful, 

 however, whether the same thing occurs within a race, for instance, 

 in the case of reproduction within a human race. 



In Man reversion to a grandparent occurs not infrequently, and 

 we may explain it thus : the id-group which controlled the type of 

 the grandfather was also contained in the germ-cell which gave rise 

 to the existence of the father, but it did not dominate the type in 

 that case because a more powerful id-group was opposed to it in 

 the gerrn-cell of the grandmother. When, later on, at the reducing 

 divisions of the germ-cells of the father, this id-group again arrived 

 in the sperm-cells of the father, it would predominately control 

 the type of the child, that is, of the third generation, provided 

 that the egg-cell Avith which it combined contained a weaker 

 id-group. 



In the case of ordinary plant-hybrids what are designated rever- 

 sions can only be called so in a wide sense, for the ancestral characters 

 are contained visibly in the parent, although mingled with those of 

 the other parent. In human families, however, there are undoubted 

 cases in which one or more characters of the grandparent reappear 

 in the child which were not in any visible way expressed in the 

 parent, and must therefore have been contained in the parent's 

 germ-plasm in a latent state. And there are both in animals and 

 plants reversions to ancestors lying much further back, to characters 

 and groups of characters which have not been visible for many 

 generations, and the occurrence of these can only be explained on 

 the assumption that certain groups of ancestral determinants have 

 been carried on in the germ-plasm in too small a number to be able 

 to give rise ordinarily to the relevant character. Such isolated 

 determinants may, however, in certain circumstances be strengthened 

 by the amphimixis of two germ-cells both containing small groups 

 of them, and thus augmented they may gain a controlling influence. 

 In this case the chances of the reducing divisions have a part to 

 play, since they bring together the old unvaried ancestral determinants 

 which, as we have seen, may persist in the germ- plasm of any species 

 through a long series of generations. This would, of course, only 

 suffice to bring about a reversion if the determinants of the ancestral 

 species were still contained in the germ-plasm in comparative abun- 

 dance. If this is no longer the case, something more is necessary, and 

 that is the relative weakness of the more modern determinants. 



