2 



ROYAL HORTICULTURAL SOCIETY. 



reversion to forms authorized by nature is not universal ; nothing 

 is more common, in fact, than to find in hybrids of the same 

 origin and of the second generation, or of a generation more 

 advanced, in company with individuals which revert to the form 

 of the parent species, a more or less numerous residue of indivi- 

 duals, which do not retrograde, or which even differ more from 

 these last, than they differ from hybrids of the first generation. 

 What sort of physiognomy do these refractory hybrids present, 

 and what are their descendants ? This is the question which I 

 purpose to examine in the present memoir. 



In 18G2 I made numerous crosses, all of them successful, be- 

 tween Datura Icevis^fer ox Stramonium, and quercifolia, four species 

 perfectly distinct, between which there are no known interme- 

 diates, and which moreover do not appear susceptible of variation. 

 Nevertheless, though very distinct, these species are sufficiently 

 closely related to admit of reciprocal impregnation, and to give 

 rise to hybrids, which, though sterile at first, become very fertile 

 at a more advanced period. They were then in the most favour- 

 able condition for the object which I proposed — the observation 

 of their hybrids during at least two consecutive generations. 



In order to apprehend properly the facts which follow, I ought to 

 state that the species of Datura of the subgeneric group to which 

 these four species belong, may be divided into two groups, one in 

 which the plants have green stems and white flowers, the other in 

 which the stems are more or less brown, or blackish purple, 

 and the flowers violet. For brevity I shall call them the white 

 and violet groups. D. Stramonium, Icevis, tm&ferox belong to the 

 first ; D. Tatula, quercifoUa, and some others to the second. 



As I have just remarked, I have made numerous crosses be- 

 tween these species, all?of which have succeeded, and under such 

 conditions of insolation that I could have no doubt of the results 

 which I obtained. I will not speak here of all these experiments, 

 which I reserve for a more extended memoir ; I wish only at pre- 

 sent to lay before the Academy the very remarkable phenomena of 

 variation which have been elicited by these crosses, and to point 

 out the consequences which appear to me to result from them. 



Datura Icevis andferox, the two species which differ the most 

 m the white group, having been fecundated the one by the other, 

 and in both directions, I was able in 1863, by help of the seeds 

 derived from this double intercrossing, to raise sixty individuals 

 of Datura Icevi-ferox, and seventy of D. feroci-lcevis, in all 130 

 hybrid plants, derived from the same parents, which had alter- 



