第122章

Vol. 199, B. 1908.) The seeds of the two genera are differently constructed, and evidently had an independent origin. Here, then, we have seeds arising casually, as it were, at different points among plants which otherwise retain all the characters of their cryptogamic fellows; the seed is not yet a morphological character of importance. To suppose that in these isolated cases the seed sprang into being in obedience to a Law of Advance ("Vervollkommungsprincip"), from which other contemporary Lycopods were exempt, involves us in unnecessary mysticism. On the other hand it is not difficult to see how these seeds may have arisen, as adaptive structures, under the influence of Natural Selection. The seed-like structure afforded protection to the prothallus, and may have enabled the embryo to be launched on the world in greater security. There was further, as we may suppose, a gain in certainty of fertilisation. As the writer has pointed out elsewhere, the chances against the necessary association of the small male with the large female spores must have been enormously great when the cones were borne high up on tall trees. The same difficulty may have existed in the case of the herbaceous Miadesmia, if, as Miss Benson conjectures, it was an epiphyte. One way of solving the problem was for pollination to take place while the megaspore was still on the parent plant, and this is just what the formation of an ovule or seed was likely to secure.

The seeds of the Pteridosperms, unlike those of the Lycopod stock, have not yet been found in statu nascendi--in all known cases they were already highly developed organs and far removed from the cryptogamic sporangium.

But in two respects we find that these seeds, or some of them, had not yet realised their possibilities. In the seed of Lyginodendron and other cases the micropyle, or orifice of the integument, was not the passage through which the pollen entered; the open neck of the pollen-chamber protruded through the micropyle and itself received the pollen. We have met with an analogous case, at a more advanced stage of evolution, in the Bennettiteae, where the wall of the gynaecium, though otherwise closed, did not provide a stigma to catch the pollen, but allowed the micropyles of the ovules to protrude and receive the pollen in the old gymnospermous fashion. The integument in the one case and the pistil in the other had not yet assumed all the functions to which the organ ultimately became adapted. Again, no Palaeozoic seed has yet been found to contain an embryo, though the preservation is often good enough for it to have been recognised if present. It is probable that the nursing of the embryo had not yet come to be one of the functions of the seed, and that the whole embryonic development was relegated to the germination stage.

In these two points, the reception of the pollen by the micropyle and the nursing of the embryo, it appears that many Palaeozoic seeds were imperfect, as compared with the typical seeds of later times. As evolution went on, one function was superadded on another, and it appears impossible to resist the conclusion that the whole differentiation of the seed was a process of adaptation, and consequently governed by Natural Selection, just as much as the specialisation of the rostellum in an Orchid, or of the pappus in a Composite.

Did space allow, other examples might be added. We may venture to maintain that the glimpses which the fossil record allows us into early stages in the evolution of organs now of high systematic importance, by no means justify the belief in any essential distinction between morphological and adaptive characters.