258 Fertilisation and Hybridisation 



If we apply this reasoning to our conception of the 

 arrangement of the units in rows on the nuclear threads, 

 the immediate result would be that their cooperation must 

 be disturbed. The threads no longer fit, neither during 

 fertilization and in vegetative life, nor later when the units 

 are exchanged before the formation of the sexual cells. 



If we imagine two corresponding chromosomes of the 

 two pronuclei placed exactly side by side, and in such a 

 way that every unit of the one has the corresponding unit 

 of the other for a neighbor, this will occur in a species- 

 cross only as far as the point of difference. Here one nu- 

 clear thread has one unit more than the other. The latter 

 has, so to say, a gap. 



The greater the number of points of difference, the 

 more numerous are these gaps, and the more will the co- 

 operation of the two nuclei be interferred with. And this 

 must diminish the vitality of the germ or at least the nor- 

 mal development of all characters. 



If the differences between the two parents are too nu- 

 merous, a crossing, as is well known, remains quite with- 

 out effect. Crossings between species belonging to dif- 

 ferent genera succeed in very rare cases only, indeed 

 within by far the most genera even the ordinary system- 

 atic species are not fertile when united. Genera such as 

 Nicotiana, Dianthus, Salix, and others, which are rich in 

 hybrids, are, as a rule the very ones in which the species 

 are exceedingly closely related to each other. 



Even if the agreement of two species is great enough 

 for mutual fertilization, the life of the hybrid is by no 

 means assured thereby. Some of them die as seeds with- 

 in the unripe fruit, as has been specially described by 

 Strasburger for the hybrid seeds of Orchis Morio after 

 fertilization with 0. fusca. 



