按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
ost primitive phylum of the Vascular Cryptogams。 There is nothing in the fossil record to disprove this view； but neither is there anything to support it； for this class so far as we know is no more ancient than the megaphyllous Cryptogams； and its earliest representatives show no special simplicity。 If the indications of affinity with Sphenophylls are of any value the Lycopods are open to suspicion of reduction from a megaphyllous ancestry； but there is no direct palaeontological evidence for such a history。
The general conclusions to which we are led by a consideration of the fossil record of the Vascular Cryptogams are still very hypothetical； but may be provisionally stated as follows：
The Ferns go back to the earliest known period。 In Mesozoic times practically all the existing families had appeared； in the Palaeozoic the class was less extensive than formerly believed； a majority of the supposed Ferns of that age having proved to be seed…bearing plants。 The oldest authentic representatives of the Ferns were megaphyllous plants； broadly speaking； of the same type as those of later epochs； though differing much in detail。 As far back as the record extends they show no sign of becoming merged with other phyla in any synthetic group。
The Equisetales likewise have a long history； and manifestly attained their greatest development in Palaeozoic times。 Their oldest forms show an approach to the extinct class Sphenophyllales； which connects them to some extent； by anatomical characters； with the Lycopods。 At the same time the oldest Equisetales show a somewhat megaphyllous character； which was more marked in the Devonian Pseudoborniales。 Some remote affinity with the Ferns （which has also been upheld on other grounds） may thus be indicated。 It is possible that in the Sphenophyllales we may have the much…modified representatives of a very ancient synthetic group。
The Lycopods likewise attained their maximum in the Palaeozoic； and show； on the whole； a greater elaboration of structure in their early forms than at any later period； while at the same time maintaining a considerable degree of uniformity in morphological characters throughout their history。 The Sphenophyllales are the only other class with which they show any relation； if such a connection existed； the common point of origin must lie exceedingly far back。
The fossil record； as at present known； cannot； in the nature of things； throw any direct light on what is perhaps the most disputed question in the morphology of plantsthe origin of the alternating generations of the higher Cryptogams and the Spermophyta。 At the earliest period to which terrestrial plants have been traced back all the groups of Vascular Cryptogams were in a highly advanced stage of evolution； while innumerable Seed…plantspresumably the descendants of Cryptogamic ancestorswere already flourishing。 On the other hand we know practically nothing of Palaeozoic Bryophyta； and the evidence even for their existence at that period cannot be termed conclusive。 While there are thus no palaeontological grounds for the hypothesis that the Vascular plants came of a Bryophytic stock； the question of their actual origin remains unsolved。
III。 NATURAL SELECTION。
Hitherto we have considered the palaeontological record of plants in relation to Evolution。 The question remains； whether the record throws any light on the theory of which Darwin and Wallace were the authorsthat of Natural Selection。 The subject is clearly one which must be investigated by other methods than those of the palaeontologist； still there are certain important points involved； on which the palaeontological record appears to bear。
One of these points is the supposed distinction between morphological and adaptive characters； on which Nageli； in particular； laid so much stress。 The question is a difficult one； it was discussed by Darwin （〃Origin of Species〃 （6th edition）； pages 170…176。）； who； while showing that the apparent distinction is in part to be explained by our imperfect knowledge of function； recognised the existence of important morphological characters which are not adaptations。 The following passage expresses his conclusion。 〃Thus； as I am inclined to believe； morphological differences； which we consider as importantsuch as the arrangement of the leaves； the divisions of the flower or of the ovarium； the position of the ovules； etc。first appeared in many cases as fluctuating variations； which sooner or later became constant through the nature of the organism and of the surrounding conditions； as well as through the inter…crossing of distinct individuals； but not through natural selection； for as these morphological characters do not affect the welfare of the species； any slight deviations in them could not have been governed or accumulated through this latter agency。〃 （Ibid。 page 176。）
This is a sufficiently liberal concession； Nageli； however； went much further when he said： 〃I do not know among plants a morphological modification which can be explained on utilitarian principles。〃 （See 〃More Letters〃； Vol。 II。 page 375 （footnote）。） If this were true the field of Natural Selection would be so seriously restricted； as to leave the theory only a very limited importance。
It can be shown； as the writer believes； that many typical 〃morphological characters；〃 on which the distinction between great classes of plants is based； were adaptive in origin； and even that their constancy is due to their functional importance。 Only one or two cases will be mentioned； where the fossil evidence affects the question。
The pollen…tube is one of the most important morphological characters of the Spermophyta as now existingin fact the name Siphonogama is used by Engler in his classification； as expressing a peculiarly constant character of the Seed…plants。 Yet the pollen…tube is a manifest adaptation； following on the adoption of the seed…habit； and serving first to bring the spermatozoids with greater precision to their goal； and ultimately to relieve them of the necessity for independent movement。 The pollen…tube is constant because it has proved to be indispensable。
In the Palaeozoic Seed…plants there are a number of instances in which the pollen…grains； contained in the pollen…chamber of a seed； are so beautifully preserved that the presence of a group of cells within the grain can be demonstrated； sometimes we can even see how the cell…walls broke down to emit the sperms； and quite lately it is said that the sperms themselves have been recognised。 （F。W。 Oliver； 〃On Physostoma elegans； an archaic type of seed from the Palaeozoic Rocks〃； 〃Annals of Botany〃； January； 1909。 See also the earlier papers there cited。） In no case； however； is there as yet any satisfactory evidence for the formation of a pollen…tube； it is probable that in these early Seed…plants the pollen… grains remained at about the evolutionary level of the microspores in Pilularia or Selaginella； and discharged their spermatozoids directly； leaving them to find their own way to the female cells。 It thus appears that there were once Spermophyta without pollen…tubes。 The pollen…tube method ultimately prevailed； becoming a constant 〃m