darwin and modern science-第53章
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primitive known seed…plants it was a highly compound leaf or pinna； its reduction has gone on in the Conifers and modern Cycads； as well as in the Angiosperms； though in different ways and to a varying extent。
The seed offers another striking example； the Palaeozoic seeds （if we leave the seed…like organs of certain Lycopods out of consideration） were always； so far as we know； highly complex structures； with an elaborate vascular system； a pollen…chamber； and often a much…differentiated testa。 In the present day such seeds exist only in a few Gymnosperms which retain their ancient charactersin all the higher Spermophytes the structure is very much simplified； and this holds good even in the Coniferae； where there is no countervailing complication of ovary and stigma。
Reduction； in fact； is not always； or even generally； the same thing as degeneration。 Simplification of parts is one of the most usual means of advance for the organism as a whole。 A large proportion of the higher plants are microphyllous in comparison with the highly megaphyllous fern… like forms from which they appear to have been derived。
Darwin treated the general question of advance in organisation with much caution； saying： 〃The geological record。。。does not extend far enough back； to show with unmistakeable clearness that within the known history of the world organisation has largely advanced。〃 （〃Origin of Species〃； page 308。） Further on （Ibid。 page 309。） he gives two standards by which advance may be measured： 〃We ought not solely to compare the highest members of a class at any two periods。。。but we ought to compare all the members； high and low； at the two periods。〃 Judged by either standard the Horsetails and Club Mosses of the Carboniferous were higher than those of our own day； and the same is true of the Mesozoic Cycads。 There is a general advance in the succession of classes； but not within each class。
Darwin's argument that 〃the inhabitants of the world at each successive period in its history have beaten their predecessors in the race for life； and are； in so far； higher in the scale〃 （〃Origin of Species〃； page 315。） is unanswerable； but we must remember that 〃higher in the scale〃 only means 〃better adapted to the existing conditions。〃 Darwin points out （Ibid。 page 279。） that species have remained unchanged for long periods； probably longer than the periods of modification； and only underwent change when the conditions of their life were altered。 Higher organisation； judged by the test of success； is thus purely relative to the changing conditions； a fact of which we have a striking illustration in the sudden incoming of the Angiosperms with all their wonderful floral adaptations to fertilisation by the higher families of Insects。
II。 PHYLOGENY。
The question of phylogeny is really inseparable from that of the truth of the doctrine of evolution； for we cannot have historical evidence that evolution has actually taken place without at the same time having evidence of the course it has followed。
As already pointed out； the progress hitherto made has been rather in the way of joining up the great classes of plants than in tracing the descent of particular species or genera of the recent flora。 There appears to be a difference in this respect from the Animal record； which tells us so much about the descent of living species； such as the elephant or the horse。 The reason for this difference is no doubt to be found in the fact that the later part of the palaeontological record is the most satisfactory in the case of animals and the least so in the case of plants。 The Tertiary plant…remains； in the great majority of instances； are impressions of leaves； the conclusions to be drawn from which are highly precarious； until the whole subject of Angiospermous palaeobotany has been reinvestigated； it would be rash to venture on any statements as to the descent of the families of Dicotyledons or Monocotyledons。
Our attention will be concentrated on the following questions； all relating to the phylogeny of main groups of plants： i。 The Origin of the Angiosperms。 ii。 The Origin of the Seed…plants。 iii。 The Origin of the different classes of the Higher Cryptogamia。
i。 THE ORIGIN OF THE ANGIOSPERMS。
The first of these questions has long been the great crux of botanical phylogeny； and until quite recently no light had been thrown upon the difficulty。 The Angiosperms are the Flowering Plants； par excellence； and form； beyond comparison； the dominant sub…kingdom in the flora of our own age； including； apart from a few Conifers and Ferns； all the most familiar plants of our fields and gardens； and practically all plants of service to man。 All recent work has tended to separate the Angiosperms more widely from the other seed…plants now living； the Gymnosperms。 Vast as is the range of organisation presented by the great modern sub…kingdom； embracing forms adapted to every environment； there is yet a marked uniformity in certain points of structure； as in the development of the embryo…sac and its contents； the pollination through the intervention of a stigma； the strange phenomenon of double fertilisation （One sperm fertilising the egg； while the other unites with the embryo…sac nucleus； itself the product of a nuclear fusion； to give rise to a nutritive tissue； the endosperm。）； the structure of the stamens； and the arrangement of the parts of the flower。 All these points are common to Monocotyledons and Dicotyledons； and separate the Angiosperms collectively from all other plants。
In geological history the Angiosperms first appear in the Lower Cretaceous； and by Upper Cretaceous times had already swamped all other vegetation and seized the dominant position which they still hold。 Thus they are isolated structurally from the rest of the Vegetable Kingdom； while historically they suddenly appear； almost in full force； and apparently without intermediaries with other groups。 To quote Darwin's vigorous words： 〃The rapid development； as far as we can judge； of all the higher plants within recent geological times is an abominable mystery。〃 （〃More Letters of Charles Darwin〃； Vol。 II。 page 20； letter to J。D。 Hooker； 1879。） A couple of years later he made a bold suggestion （which he only called an 〃idle thought〃） to meet this difficulty。 He says： 〃I have been so astonished at the apparently sudden coming in of the higher phanerogams； that I have sometimes fancied that development might have slowly gone on for an immense period in some isolated continent or large island； perhaps near the South Pole。〃 （Ibid； page 26； letter to Hooker； 1881。） This idea of an Angiospermous invasion from some lost southern land has sometimes been revived since； but has not； so far as the writer is aware； been supported by evidence。 Light on the problem has come from a different direction。
The immense development of plants with the habit of Cycads； during the Mesozoic Period up to the Lower Cretaceous； has long been known。 The existing Order Cycadaceae is a small family； with 9 genera and perhaps 100 species； occurring in the tropical and sub…tropical zones of both the Old and New World； but nowhere forming a dominant feature in the vegetation。 Some few attain the stature of small trees； while in the majority the stem is short； though often living to a great age。 The large pinnate or rarely bipinnate leaves give the Cycads a superficial resemblance in habit to Palms。 Recent Cycads are dioecious； throughout the family the male fructification is in the form of a cone； each scale of the cone representing a stamen； and bearing on its lower surface numerous pollen… sacs； grouped in sori like the sporangia of Ferns。 In all the genera； except Cycas itself； the female fructifications are likewise cones； each carpel bearing two ovules on its margin。 In Cycas； however； no female cone is produced； but the leaf…like carpels； bearing from two to six ovules each； are borne directly on the main stem of the plant in rosettes alternating with those of the ordinary leavesthe most primitive arrangement known in any living seed…plant。 The whole Order is relatively primitive； as shown most strikingly in its cryptogamic mode of fertilisation； by means of spermatozoids； which it shares with the maidenhair tree alone； among recent seed…plants。
In all the older Mesozoic rocks； from the Trias to the Lower Cretaceous； plants of the Cycad class （Cycadophyta； to use Nathorst's comprehensive name） are extraordinarily abundant in all parts of the world； in fact they were almost as prominent in the flora of those ages as the Dicotyledons are in that of our own day。 In habit and to a great extent in anatomy； the Mesozoic Cycadophyta for the most part much resemble the recent Cycadaceae。 But； strange to say； it is only in the rarest cases that the fructification has proved to be of the simple type characteristic of the recent family； the vast majority of the abundant fertile specimens yielded by the Mesozoic rocks possess a type of reproductive apparatus far more elaborate than anything known in Cycadaceae or other Gymnosperms。 The predominant Mesozoic family； characterised by this advanced reproductive organisation； is known as the Bennettiteae； in habit these plants resembled the more stunted Cycads of the recent flora； but differed from them in the presence of numerous lateral fructifications； like large buds； borne on the stem among the crowded bases of the leaves。 The organisation of these fructifications was first worked out on European specimens by Carruthers； Solms…Laubach； Lignier and others； but these observers had only more or less ripe fruits to deal with； the complete structure of the flower has only been elucidated within the last few years by the researches of Wieland on the magnificent American material； derived from the Upper Jurassic and Lower Cretaceous beds of Maryland； Dakota and Wyoming。 （G。R。 Wieland； 〃American Fossil Cycads〃； Carnegie Institution； Washington； 1906。） The word 〃flower〃 is used deliberately； for reasons which will be apparent from the following brief description； based on Wieland's observations。
The fructification is attached to the stem by a thick stalk； which； in its upper part； bears a large number of spirally arranged bracts； forming collectively a kind of perianth and completely enclosing the essential organs of reproduction。 The latter consist of a whorl of stamens； of extremely elaborate structure； surrounding a central cone or receptacle bearing numerous ovules。 The stamens resemble the fertile fronds of a fern； they are of a compound； pinnate form； and bear very large numbers of pollen…sacs； each of which is itself a compound structure consisting of a number of compartments in which the pollen was formed。 In their lower part the stamens are fused together by their stalks； like the 〃monadelphous〃 stamens of a mallow。 The numerous ovules borne on the central receptacle are stalked； and are intermixed with sterile scales； the latter are expanded at their outer ends； which are united to form a kind of pericarp or ovary…wall； only interrupted by the protruding micropyles of the ovules。 There is thus an approach to the closed pistil of an Angiosperm； but it is evident that the ovules received the pollen directly。 The whole fru
