In other cases, as in that of the common Ash-tree mentioned in the Introduction, the stamens are rudimentary in some individuals, the pistils in others, others again remaining as hermaphrodites. Here the modification of the two sets of organs appears to have occurred simultaneously, as far as we can judge from their equal state of abortion. If the hermaphrodites were supplanted by the individuals having separated sexes, and if these latter were equalised in number, a strictly dioecious species would be formed.
There is much difficulty in understanding why hermaphrodite plants should ever have been rendered dioecious. There would be no such conversion, unless pollen was already carried regularly by insects or by the wind from one individual to the other; for otherwise every step towards dioeciousness would lead towards sterility. As we must assume that cross-fertilisation was assured before an hermaphrodite could be changed into a dioecious plant, we may conclude that the conversion has not been effected for the sake of gaining the great benefits which follow from cross-fertilisation. We can, however, see that if a species were subjected to unfavourable conditions from severe competition with other plants, or from any other cause, the production of the male and female elements and the maturation of the ovules by the same individual, might prove too great a strain on its powers, and the separation of the sexes would then be highly beneficial. This, however, would be effected only under the contingency of a reduced number of seeds, produced by the females alone, being sufficient to keep up the stock.
There is another way of looking at the subject which partially removes a difficulty that appears at first sight insuperable, namely, that during the conversion of an hermaphrodite into a dioecious plant, the male organs must abort in some individuals and the female organs in others. Yet as all are exposed to the same conditions, it might have been expected that those which varied would tend to vary in the same manner. As a general rule only a few individuals of a species vary simultaneously in the same manner; and there is no improbability in the assumption that some few individuals might produce larger seeds than the average, better stocked with nourishment. If the production of such seeds were highly beneficial to a species, and on this head there can be little doubt, the variety with the large seeds would tend to increase. (7/1. See the facts given in 'The Effects of Cross and Self-fertilisation' page 353.) But in accordance with the law of compensation we might expect that the individuals which produced such seeds would, if living under severe conditions, tend to produce less and less pollen, so that their anthers would be reduced in size and might ultimately become rudimentary. This view occurred to me owing to a statement by Sir J.E. Smith that there are female and hermaphrodite plants of Serratula tinctoria, and that the seeds of the former are larger than those of the hermaphrodite form. (7/2. 'Transactions of the Linnean Society' volume 8 page 600.) It may also be worth while to recall the case of the mid-styled form of Lythrum salicaria, which produces a larger number of seeds than the other forms, and has somewhat smaller pollen-grains which have less fertilising power than those of the corresponding stamens in the other two forms; but whether the larger number of seeds is the indirect cause of the diminished power of the pollen, or vice versa, I know not. As soon as the anthers in a certain number of individuals became reduced in size in the manner just suggested or from any other cause, the other individuals would have to produce a larger supply of pollen; and such increased development would tend to reduce the female organs through the law of compensation, so as ultimately to leave them in a rudimentary condition; and the species would then become dioecious.
Instead of the first change occurring in the female organs we may suppose that the male ones first varied, so that some individuals produced a larger supply of pollen.