254 Fertilisation and Hybridisation 



their inequality, cooperate in order to regulate the evolu- 

 tion ? This question amounts to the same as asking, what 

 is the sum of the influence of an active and a latent unit ? 

 At first glance one would expect that this influence would 

 correspond to half the value of a pair composed of two 

 active units. Previously this opinion was rather gener- 

 ally accepted, and there was an inclination to regard plants 

 with intermediate characters as hybrids. Especially many 

 plants with pale red or pale blue flowers were regarded 

 as such. But the experience of later years has decided 

 differently. 



Variety-hybrids generally bear the characteristic of 

 the species, sometimes fully developed, sometimes more 

 or less weakened^ but this for the most part only so little 

 that superficial observation sees no difference. An active 

 and a latent unit are not essentially different in their co- 

 operation from two active ones ; a fact which may prob- 

 ably be best explained by the assumption that two cannot 

 accomplish more than one already does. This conception 

 finds a very strong support in the results of the most 

 recent investigations by Boveri on dispermia, which we 

 have already partly discussed. By fertilizing one egg 

 with two spermatozoa the composition of the structure 

 of the nuclear threads can be altered in different ways, 

 for instance, in such a manner that in one nucleus there 

 lie not two, but three pieces of any one of its chromo- 

 somes. It might then be expected that the given charac- 

 ters would be very strongly developed, to about one and 

 one-half of their intensity. But, as far as can be judged 

 from Boveri 's experiments, this is not the case, and the 

 influence of the three equivalent units is not noticeably 

 greater than that of two. 



We come now to the progeny of hybrids, and we, of 



