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Show 262 CONCLUDING REMARKS CHAP. VI. thus varied, it would never have been rendered heterostyled, as this state would then have been superfluous. But the parent-species of our several existing heterostyled plants may have been, and probably were (judging from their present constitution) in some degree self-sterile ; and this would have made regular crossfertilisation still more desirable. Now let us take a highly varying species with most or all of the anthers exserted in some individuals, and in others seated low down in the corolla; with the stigma also varying in position in like manner. Insects which visited such flowers would have different parts of their bodies dusted with pollen, and it would be a mere chance whether this were left on the stigma of the next flower which was visited. If all the anthers could have been placed on the same level in all the plants, then abundant pollen would have adhered to the same part of the body of the insects which frequented the flowers, and would afterwards have been deposited without loss on the stigma, if it likewise stood on the same unvarying level in all the flowers. But as t4e stamens and pistils are supposed to have already varied much in length and to be still varying, it might well happen that they could be reduced much 1nore easily through natural selection into two sets of different lengths in different individuals, than all to the same length and level in all the individuals. We know from innumerable instances, in which the two sexes and the young of the same species differ, that there is no difficulty in two or more sets of individuals being formed which inherit different characters. In our particular case the law of co1npensation or balanceInent (which is ad1nitted by many botanists) would tend to cause the pistil to be reduced in those incli- CHAP. VI. ON HETEROSTYLED PLANTS. 263 viduals in which the stamens were greatly developed, and to be increased in length in those which had their stamens but little developed. Now if in our varying species the longer stamens were to be nearly equalised in length in a considerable body of individuals, with the pistil more or less reduced; and in another body, the shorter stamens to be similarly equalised, with the pistil more or less increased in length, cross-fertilisation would be secured with little loss of pollen; and this change would be so highly beneficial to the species, that there is no difficulty in believing that it could be effected through natural selection. Our plant would then make a close approach in structure to a heterosty led dim. orphic species ; or to a trimorphic species, if the sta1nens wer~ reduced to two lengths in the same flower in correspondence with that of the pistils in the other two fonns. But we have not as yet even touched on the chief difficulty in understanding how heterostyled species could have originated. A completely self-sterile plant or a dichogamous one can fertilise and be fertilised by any other individual of the same species; whereas the essential character of a heterostyled plant is that an individual of one form cannot' fully fertilise or be fertilised by an individual of the same form, but only by one belonging to another form. H. Muller has suggested* that ordinary or homo ·styled plants may have been rendered heterostyled merely through the effects of habit. Whenever pollen from one set of anthers is habitually applied to a pistil of particular length in a varying species, he believes that at last the. possibility of fertilisation in any other * ' Die Befruahtung der Blumen,' p. 352. |