194 PLANT HYBRIDIZATION BEFORE MENDEL 



ment. It is sometimes the case, that the pollen tubes of A have 

 a greater sexual attraction to the germinal vesicle of B, than the 

 pollen tubes of B to the germinal vesicle of A. Hybrids are stated 

 to possess one character in common, that they are much more in- 

 clined to variation than are the pure forms. This variability, ac- 

 cording to Nageli, in the case of variety-hybrids, occurs in the 

 first generation ; in the case of species-hybrids, in the second or 

 later generations. Sometimes, Nageli proceeds, the offspring re- 

 semble, not the parents but the grandparents, and characters 

 sometimes come into appearance in a later generation, which were 

 present in a previous generation, but which afterwards disap- 

 peared. The organism may, at the same time, harbor several ten- 

 dencies, of which some attain to development sooner, others later, 

 and others not at all. He continues : 



"It is now comprehensible that pre-eminently two tendencies are lo- 

 cated in the hybrid, the one that it should resemble the father, the other 

 that it should resemble the mother. Correspondingly, the changes in the 

 second and following generations consist especially in this, that forms 

 develop which are very similar to the two parent forms." (p. 285.) 



The tendency of cultivated plants to vary more than wild 

 plants may, according to Nageli, have a double cause. On the one 

 hand, through the long operation of partly unnatural conditions, 

 the balance is seriously disturbed, and hence a stimulation is 

 given to inner changes. More important is the circumstance that 

 natural selection does not take place, or only in a direction cor- 

 responding to the demands of cultivation. In the wild condi- 

 tion, the incipient new varieties perish, since, in the struggle for 

 existence, only the most advantageous variation persists. In cul- 

 tivation, on the other hand, all individual variations, so far as 

 they form seeds and do not run counter to the demands of culti- 

 vation, reproduce and form new indi.vidual modifications through 

 crossing with other variations. 



A physiological question discussed by Nageli, is that of the 

 relative infertility of species-hybrids. For example, according to 

 Gartner's experiment which Nageli cites, the hybrids between Lo- 

 belia cardinalis Linn, and Lobelia fulgens Willd. ripened 900 

 seeds per capsule; the parents, on the other hand, 1,100 to 1,200. 

 Lychnis-diurria Sibth. X L. vespertina Sibth. produced 90 to 125; 

 the parents 150 to 190 seeds. Datura stramonium X -0. tatula 



