210 NICOTIANA TABACUM. CnAF.VI. 



certain individuals are sexually incompatible, and will not 

 produce offspring, although fertile with other individuals. But 

 Kolreuter has recorded a case * which bears more closely on our 

 present one, as it shows that in the genus Nicotiana the varieties 

 differ in their sexual affinities. He experimented on five 

 varieties of the common tobacco, and proved that they were 

 varieties by showing that they were perfectly fertile when re- 

 ciprocally crossed ; but one of these varieties, if used either as 

 the father or the mother, was more fertile than any of the others 

 when crossed with a widely distinct species, N. glutinosd. As the 

 different varieties thus differ in their sexual affinities, there is 

 nothing surprising in the individuals of the same variety differ- 

 ing in a like manner to a slight degree. 



Taking the plants of the three generations altogether, the 

 crossed show no superiority over the self-fertilised, and I can 

 account for this fact only by supposing that with this species, 

 which is perfectly self-fertile without insect aid, most of the indi- 

 viduals are in the same condition, as those of the same variety 

 of the common pea and of a few other exotic plants, which 

 have been self-fertilised for many generations. In such cases a 

 cross between two individuals does no good ; nor does it in any 

 case, unless the individuals differ in general constitution, either 

 from so-called spontaneous variation, or from their progenitors 

 having been subjected to different conditions. I believe that 

 this is the true explanation in the present instance, because, as 

 we shall immediately see, the offspring of plants, which did not 

 profit at all by being crossed with a plant of the same stock, 

 profited to an extraordinary degree by a cross with a slightly 

 different sub-variety. 



The Effects of a Cross with afresh Stock. I procured some seed 

 of N. tabacum from Kew and raised some plants, which formed 

 a slightly different sub- variety from my former plants ; as the 

 flowers were a shade pinker, the leaves a little more pointed, and 

 the plants not quite so tall. Therefore the advantage in height 

 which the seedlings gained by this cross cannot be attributed to 

 direct inheritance. Two of the plants of the third self-fertilised 

 generation, growing in Pots II. and V. in Table LXXXVIL, 

 which exceeded in height their crossed opponents (as did their 

 parents in a still higher degree) were fertilised with pollen 

 from the Kew plants, that is, by a fresh stock. The seedlings 



* ' Das Geschlecht der Pflanzen, Zweite Fortsetzung,' 1764, p. 55-0. 



