OCR Text |
Show 94 ON THE ADVANTAGE [CHAP. IV. limited sense. I believe this objection to be valid, but that nature has largely provided against it by giving to trees a strong tendency to bear flowers with separated sexes. When the sexes are separated, although the n1ale and female flowers may be produced on the same tree, we can see that pollen must be regularly carried from flower to flower ; and this will give a better chance of pollen being occasionally carried fro1n tree to tree. That trees belonging to all Orders have their sexes more often separated than other plants, I find to be the case in this country ; and at my request Dr. Hooker tabulated the trees of New Zealand, and Dr. Asa Gray those of the United States, and the result was as I anticipated. On the other hand, Dr. Hooker has recently informed me that he finds that the rule does not hold in Australia ; and I have made these few remarks on the sexes of trees sin1ply to call attention to the subject. Turning for a very brief space to animals : on the land there are some hermaphrodites, as land-mollusca and earth-worms; but these all pair. As yet I have not found a single case of a terrestrial animal which fertilises itself. We can understand this remarkable fact, which offers so strong a contrast with terrestrial plants, on the view of an occasional cross being indispensable, by considering the medium in which terrestrial animals live, and the nature of the fertilising element ; for we know of no means, analogous to the action of insects and of the wind in the case of plants, by which an occasional cross could be effected with terrestrial animals without the concurrence of two individuals. Of aquatic animals, there are many self-fertilising hermaphrodites; but here currents in the water offer an obvious means for an occasional cross. And, as in the case of flowers, I have as yet failed, after consultation with one of the highest authorities, namely, Professor Huxley, to discover a single case of an hermaphrodite animal with the organs of reproduction so perfectly enclosed within the body, that access from without and the occasional influence of a distinct individual can be shown to be physically impossible. Cirripedes long appeared to me to present a case of very great difficulty CHAP. IV.] OF INTERCROSSING. 95 under this point of view; but I have been enabled b tfhor tunha tbe cthha nce, ell sewh.e r. e. to prove that tw · d' ·'a Y1a . oug o are se f-fertihsing hermaphrodi0t eIsl l dl VI U a S' t1mes cross. ' o some- 1 ~t mu~t hahe struck most naturalists as a strange anom~ fyth at, In tf: e ~ase of both animals and plants, species ~ same amily and even of the same genus thou ·h agrem.ng .closely with each other in almost their wll!Ie o~ganisation, yet are not rarely, some of them herma hrodites, and s~me of them unisexual. But if, in facf all ~~r~aphrodites. do occasionally intercross with othe; in- IVI uals, t~e drfference between hermaphrodites and unisexual sp 1 e 1 c1es, as far as function is concerned becomes very sma . ' From these several considerations and from th special facts which I have collected but h' hIe many h bl t · ' w Ic am not b e~he ~ teh o give, I am strongly inclined to suspect t.hat 0 In e vegetable and animal kin do . ' intercross with a dist' t . d' 'd 1g. ms, an occasional I II h Inc In IVI ua Is a law of nature ofa md'f fwi e It aware t at the~ e are, on thI'S V·I ew, many cases· F' ll c~h y, some of which I am trying to investigate . Ina y en, we may conclude that in many or anic be~ ~ftgs~ a croh b.et"wee~ two individuals is an obviots necesat 1o:r ;a~ byt~ ;bIn ~any others it occurs perhaps only 1. t' g n erv~ s ' ut In none, as I suspect can self-ferti- Isa Ion go on lOr perpetuity. ' . Oircumstances.fruv_ourable to Natural .Selection.-Thi Is an extremely mtriCate subi ect A 1 s · h · t bl d · · J • arge amount of bn l.eri a e an. d~v~rsrfied variability is favourable but I A {eve mere IndiVI~u~l .differences suffice for th; work for ~~~e :umber of lll~l~Iduals, by giving a better chanc~ . t' ppe~rance Withm any given period of profitable bilU; fu_n~a';h11 r:d~fcJensre ~r. a lesse! amount of varia. important element f ua ' an IThs, I beheve, an extremely . d . 0 success. ough nature grants vast ~~~1~r=n~f ti.I?edffl ~he wo~k of natural selection, she does are . . an ~n e rute period ; for as all organic bein s ~c strivin~> It may be said, to seize on each place in tfe onomy o nature, if any one species does not become |