species are crossed and they produce numerous seeds, we expect as a general rule that their hybrid offspring will be moderately fertile; but if the parent species produce extremely few seeds, we expect that the hybrids will be very sterile. But there are marked exceptions, as shown by Gartner, to these rules. So it is with illegitimate unions and illegitimate offspring. Thus the mid- styled form of Lythrum salicaria, when illegitimately fertilised with pollen from the longest stamens of the short-styled form, produced an unusual number of seeds; and their illegitimate offspring were not at all, or hardly at all, sterile. On the other hand, the illegitimate offspring from the long-styled form, fertilised with pollen from the shortest stamens of the same form, yielded few seeds, and the illegitimate offspring thus produced were very sterile; but they were more sterile than might have been expected relatively to the difficulty of effecting the union of the parent sexual elements. No point is more remarkable in regard to the crossing of species than their unequal reciprocity. Thus species A will fertilise B with the greatest ease; but B will not fertilise A after hundreds of trials. We have exactly the same case with illegitimate unions; for the mid-styled Lythrum salicaria was easily fertilised by pollen from the longest stamens of the short-styled form, and yielded many seeds; but the latter form did not yield a single seed when fertilised by the longest stamens of the mid-styled form. Another important point is prepotency. Gartner has shown that when a species is fertilised with pollen from another species, if it be afterwards fertilised with its own pollen, or with that of the same species, this is so prepotent over the foreign pollen that the effect of the latter, though placed on the stigma some time previously, is entirely destroyed. Exactly the same thing occurs with the two forms of a heterostyled species. Thus several long-styled flowers of Primula veris were fertilised illegitimately with pollen from another plant of the same form, and twenty-four hours afterwards legitimately with pollen from a short- styled dark-red polyanthus which is a variety of P. veris; and the result was that every one of the thirty seedlings thus raised bore flowers more or less red, showing plainly how prepotent the legitimate pollen from a short-styled plant was over the illegitimate pollen from a long-styled plant.