JULY 



1909] 



NA TURE 



117 



adapt the vascular system of the stem to the supply of the 

 large and compound leaves, the polystelic type of struc- 

 ture was assumed, i.e. the single vascular cylinder (still 

 to be recognised in some of the earlier members of the 

 group) became broken up, in various ways, into a number 

 of distinct cyiinaers, only connected at intervals. So far 

 the change was in the same general direction as in the 

 evolution of the higher ferns ; the fossil family, however, 

 was not content with a complex primary vascular system, 

 hut must have secondary growth as well. Now if you 

 !i:ive a number of vascular columns in the same stem, each 

 flowing continuously in thickness on its own account, it 

 is evident that very special arrangements will be necessary 

 to avoid overcrowding. The difficulty was overcome, and 

 the McduUoseie for some time flourished among the 

 'l.iininant families — the Permian formation represents their 

 iiolden age. But one is tempted to think that the system 

 was too complicated to last ; at any rate, it seems not 

 to have lasted, for these elaborate stems have not been 

 found in any later rocks. Either, as Mr. Worsdell sup- 

 poses, the meduUosean stem became reduced and simplified 

 to form the cycadean type of stem of later days, or, as 

 I am more inclined to believe, the family died out 

 altogether. Even here, though we seem to have an 

 instance of a cumbrous mechanism, over-reaching itself in 

 elaboration, yet it worked well enough for a time, and it 

 would be difficult to say exactly what the conditions were 

 that led to its being superseded. 



The hypothesis of " a gradual development from the 

 simpler to the more complex " is not borne out by the 

 facts of palajobotany — the real course of events was 

 infinitely niore involved. On a general view, as Darwin 

 himself recognised, " the geological record does not extend 

 far enough back to show with unmistakable clearness 

 that within the known history of the world organisation 

 has largely advanced."' This wise saying has been too 

 often overlooked by those who have tried to popularise 

 evolution — it is eminently true of the geological history 

 of plants. Though there is no doubt a balance on the 

 side of advance, due chiefly to the increasing complexity 

 of the inter-relations among the organisms themselves, 

 the general progress since Paleozoic days is by no means 

 so great as has often been assumed, and we may be 

 sure that as our knowledge of the older plants increases 

 we shall come to form a still higher estimate than we do 

 now of their adaptive organisation. 



It has been alleged that it is the fact of the gradual 

 appearance of higher forms which enables us to determine 

 the relative age of strata by their fossils. So far as plants 

 are concerned, this statement is only true to a very limited 

 extent. A fossil angiosperm, no doubt, would be evidence 

 of an age not earlier than the Cretaceous, but, on the 

 other hand, a lycopod of much higher organisation than 

 at present would establish a strong presumption of 

 Paleozoic age ; so would the higher forms of the equise- 

 tales ; a cycadophyte with a fructification far more elaborate 

 than that of recent Cycadaceai would afford sure proof 

 that the bed containing it belonged to the Lower Mesozoic. 



Of course, much depends on the meaning we give to 

 the words "higher" and "lower." If by "higher" we 

 mean nearer to the recent types, then it is merely a truism 

 to say that the higher forms are characteristic of the 

 later rocks; but if by " higher " we mean more elaborately 

 differentiated, then the statement quoted is, in any general 

 sense, untrue. If, again, we imply by the word " higher " 

 more perfectly adapted to the existing conditions, then it 

 would be very difficult to prove any advance, for, as I 

 have endeavoured to show, adaptation has in every age 

 been fully adequate in relation to the then conditions. If 

 organisms have grown in complexity, it is only where the 

 conditions of their life have become more complex. The 

 most striking examples of high organisation in relation to 

 organic environment are presented by the characteristic 

 modern subkingdom, the angiosperms, in the evolution of 

 which, as Saporla pointed out, insect fertilisation has been 

 the chief determining factor, leading to an infinite variety 

 in the special adaptations of the flower, and no doubt 

 indirectly affecting the mode of life of the whole plant. 

 The advent of the angiosperms seems to have been almost 



^ " Origin of Specips." ^ixth edition, p. 308. 



NO. 2073, VOL. 81] 



simultaneous with that of the higher families of insects, 

 which now, at all events, are chiefly concerned in pollina- 

 tion. It would be difiicult to overestimate the importance 

 of these relations in their effect on the flora of the world. 

 If the vegetation of our own epoch appears, on the whole, 

 definitely more advanced than that ■ of earlier geological 

 periods, this is probably due in a greater, degree to the 

 contemporary insect life than to any other cause. 



I have discussed the subject of reduction in evolution 

 elsewhere,' and will only briefly allude to it here. In 

 many groups (lycopods, equisetales, cycadophytes) there 

 has been a lowering of the standard of organisation, partly 

 due to direct reduction, partly to the extinction of the 

 higher forms in each group. There are, however, many 

 other cases in which the simplification of particular organs 

 means a real advance. 



Taking into account all the causes which make for 

 simplification, the question suggests itself whether; when 

 we find a simple type of structure existing at the present 

 day, there is any presumption in favour of its primitive 

 nature. It has sometimes been urged that such a pre- 

 sumption exists (except when direct evidence of reduction 

 can be adduced) on the ground that the general course of 

 evolution must have been from the simpler to the more 

 complex, a rule, as we have seen, subject to so many 

 exceptions that, within the limited period to which the 

 palseontological record extends, it has practically no 

 validity. My own conviction is that in such cases there 

 is no presumption of primitiveness at all, and that we 

 should demand very strong evidence before admitting that 

 a given simple structure is primitive. Of course, it may 

 happen that a primitive simple type, or at least an old 

 simple type, may have survived to our own day ; this may 

 have been the case in decaying families, where the less 

 advanced members have had the best chance of evading 

 the competition of ascendant races ; but, on the whole, it 

 is very unlikely that, among all the changes and chances 

 of the world's history, a really primitive simplicity should 

 have been preserved. " The eternal ages are long," and 

 there has been time enough for many ups and downs on 

 every line of descent. 



The subject of reduction, so essential a clue in any 

 attempt to trace the course of evolution, suggests a refer- 

 ence to the question of the simpler angiospermous flowers. 

 While the older morphologists were wont to interpret such 

 flowers (e.g. those of Aroidese, Piperaceae, Cupuliferse) as 

 reductions from more " perfect " types, there has been a 

 tendency in more recent times to accept the simpler flowers 

 as primitive structures from which more elaborate forms 

 have been evolved. Quite lately, however, a reaction has 

 set in, due to the discovery by Dr. Wieland of the 

 wonderful bisexual flowers of the Mesozoic cycadophyta, 

 which are constructed on the same plan (though, of course, 

 with many differences in detail) as the more perfect angio- 

 spermous flowers, such as those of Magnoliaceas. If the 

 angiospermous flower was derived from a source allied to 

 the Bennettiteae, its evolution, as suggested by Wieland, 

 must have been essentially a process of reduction. I only 

 wish to point out that this view is not inconsistent with 

 the great relative antiquity of simple and, ex hypolhesi, 

 reduced forms, for which, 'in the case of the Amentiferae, 

 there seems to be good geological evidence. Reduction 

 appears to have often been a rapid, indeed a comparatively 

 sudden, change, as shown by the frequent occurrence of 

 much-simplified forms in the same family in which the 

 prevailing structure is typically complete. It appears quite 

 probable'that some groups with very simple flowers, though 

 not " primitive," may be very ancient, tracing their origin 

 from forms which in' quite early days underwent reduction 

 (as a means of specialisation) from the highly developed 

 flowers which probably characterised the first autonomous 

 angiosperms. 



The tentative and somewhat fragmentary observations 

 which I have here stated tend to the following con- 

 clusions : — 



(i) That at all known stages of the past history of 

 plants there has been a thoroughly eflicient _ degree of 

 adaptation to the conditions existing at each period. 



1 "Darwin and Modern Science." XII. The Palajontological Rfcord. 11. 

 Plants. (1909.) 



