S6 THE ORIQIN OF SPECIES 



not corresponding in height with the pistil, differs much 

 in degree, up to absolute and utter sterility; just in the 

 same manner as occurs in crossing distinct species. As 

 the degree of sterility in the latter case depends in an 

 eminent degree on the conditions of life being more or 

 less favorable, so I have found it with illegitimate unions. 

 It is well known that if pollen of a distinct species be 

 placed on the stigma of a flower, and its own pollen 

 be afterward, even after a considerable interval of time, 

 placed ou the same stigma, its action is so strongly pre- 

 potent that it generally annihilates the e£fect of the 

 foreign pollen; so it is with the pollen of the several 

 forms of the same species, for legitimate pollen is 

 strongly prepotent over illegitimate pollen, when both 

 are placed on the same stigma. I ascertained this by 

 fertilizing several flowers, first illegitimately, and twenty- 

 four hours afterward legitimately, with pollen taken from 

 a peculiarly colored variety, and all the seedlings were 

 similarly colored; this shows that the legitimate pollen, 

 though applied twenty-four hours subsequently, had 

 wholly destroyed or prevented the action of the pre- 

 viously applied illegitimate pollen. Again, as in making 

 reciprocal crosses between the same two species, there is 

 occasionally a great difference in the result, so the same 

 thing occurs with trimorphic plants; for instance, the 

 mid-styled form of Lythrum salicaria was illegitimately 

 fertilized with the greatest ease by pollen from the longer 

 stamens of the short-styled form, and yielded many 

 seeds; but the latter form did not yield a single seed 

 when fertilized by the longer stamens of the mid-styled 

 form. 



In all these respects, and in others which might be 



