October 8, 1903] 



NATURE 



567 



ith extraordinary vigour and created a revolution in the 

 ^lant-world. Let us glance for a moment at the facts to 

 be gleaned from an examination of the records of this 

 critical period in the history of vegetation. 



I have already pointed out that we have as yet recog- 

 nised no Angiosperms in the Wealden floras of England, 

 Spitzbergen, Germany, France, Austria, Belgium, Russia, 

 and Japan ; but from plant-bearing rocks of Portugal, re- 

 garded as homotaxial with those which British geologists 

 speak of as Wealden, the late Marquis of Saporta named a 

 fragment of a leaf Alismacites primaevus, a determination 

 that, while possibly correct, cannot be accepted as con- 

 clusive testimony. In Virginia and Maryland there occurs 

 a thick series of strata known as the Potomac formation 

 from which a rich harvest of plant-remains has been 

 obtained. Prof. Lester Ward has recently shown that under 

 this title are included several floras, some of which are 

 undoubtedly homotaxial with the Wealden of Europe, while 

 others represent the vegetation of a later phase of the 

 Cretaceous era. From the older Potomac beds a few leaves 

 have been assigned to Dicotyledons and referred to such 

 genera as Ficophyllum, Myrica, Proteaephyllum, and others. 

 Some of these may well be small fronds of ferns with 

 venation characters like those of the Elk's Horn fern 

 (Platycerium), while others, though presenting a close re- 

 semblance to Dicotyledonous leaves, afford insufficient data 

 fo' accurate generic identification. In dealing with fossil 

 leaves of the dicotyledonous type, we must not forget that 

 the recent genus Gnetum — a gymnosperm of the section 

 Gnetales — possesses leaves that may be said to be in- 

 distinguishable in form and venation from those of certain 

 Dicotyledons. Before the close of the Potomac period these 

 few fragmentary relics of possible Dicotyledons are replaced 

 by a comparative abundance of specimens which must be 

 accepted as undoubted Angiosperms. Previous to the dis- 

 covery of the supposed Angiosperms in Wealden strata of 

 Portugal and North America, the earliest record of an 

 Angiosperm was represented by Heer's Populus primaeva 

 from Northern Greenland. This name was applied to a 

 fragmentary specimen which may be a true dicotyledonous 

 leaf. In 1897 Dr. White, of the Geological Survey of the 

 United States, stated that additional examples of dicoty- 

 ledonous leaves had been obtained during the visit of the 

 Peary Arctic expedition to the well-known locality in Green- 

 land where Heer's Populus primaeva was discovered in the 

 so-called Kome series. From strata known as the Atane 

 beds, which rest on the Kome series, unmistakable Angio- 

 sperms have been collected in abundance. 



Another indication of the sudden increase in the number 

 of dicotyledons is furnished by the Dakota flora of the 

 United States — in age somewhat more recent than the older 

 Potomac beds. In these plant-beds it is stated that Angio- 

 sperms constitute two-thirds of the vegetation. 



We may sum up the whole matter in a iew words. There 

 is some evidence of the existence of Angiosperms before the 

 close of the Wealden period. It may be added that the 

 Stonesfield Slate of England (a formation of approximately 

 the same age as the Inferior Oolite plant-beds of Yorkshire) 

 has afforded a single specimen of a leaf which in form and 

 venation has- as much claim to be referred to the dicoty- 

 ledons as many of the leaves from Wealden rocks. These 

 earliest records are, however, unsatisfactory, and the names 

 assigned to them are often misleading. As soon as we 

 ascend a stage higher in the geological series, not only do 

 the Angiosperms at once become abundant, but the whole 

 facies of the vegetation undergoes a striking change. The 

 Gymnosperms, especially the Cycads, are ousted from a 

 supremacy maintained through countless ages, and the 

 vegetation becomes essentially modern. Many of the earlier 

 angiospermous plants may be referred to existing genera 

 and present no features of special interest from a phylo- 

 genetic standpoint. 



One of our most pressing needs is a thoroughly critical 

 revision of the late Cretaceous and earlier Tertiary floras, 

 with the object both of determining the systematic position 

 of the older .Angiosperms and of mapping out with greater 

 accuracy the geographical distribution of the floras of the 

 world in post-Wealden periods. This is a task which is 

 sometimes said to be impossible or hardly worth the 

 attempt ; the available evidence is indeed meagre, and much 

 of it has been treated with more respect than it deserves. 



NO. 1 77 1, VOL. 68] 



but it is at least a praiseworthy aim, not to say a duty, to 

 take stock of our material and to compile lists of plants 

 that may bear the scrutiny of experienced systematists. We 

 are profoundly ignorant of the means by which Nature pro- 

 duced this new creation ; we can only emphasise the fact 

 that in the early days of the Cretaceous era a new type was 

 evolved which no sooner appeared than it swept all before 

 it and by its overmastering superiority converted the past 

 into the present. 



Conclusion. 



In conclusion, I would urge the importance of taking 

 stock of our accumulated facts, and of so recording our 

 observations that they may be safely laid under contribution 

 as aids to broad generalisations. Detailed descriptions and 

 the enumeration of small collections are a necessity, but 

 there is danger of the student neglecting the application 

 of his results to problems of far-reaching import. We 

 may borrow a saying of a great artist in regard to atten- 

 tion to detail — " I see it, but I prefer to construct the 

 synthesis." 



There is no more fascinating task than to follow the 

 onward march of the plant-world from one stage to another 

 and to watch the fortunes of the advancing army. We see 

 from time to time war-worn veterans dropping from the 

 ranks and note the constant addition of recruits, some of 

 whom march but a short distance and fall by the way ; 

 while others, better equipped, rise to a position of im- 

 portance. 



At long intervals the formation is altered and the con- 

 stitution of the advancing and increasing host is suddenly 

 changed ; familiar leaders are superseded by new-comers 

 who mark their advent by drastic reorganisation. To 

 change the metaphor, we may compare the stages of plant- 

 evolution to the records of changing architectural styles re- 

 presented in Gothic buildings. The simple Norman arch 

 and massive pier are replaced, with apparent suddenness, 

 by the pointed arch and detached shafts of the thirteenth 

 century ; the latter style, which marked an architectural 

 phase characterised by local variations subordinated to a 

 uniformity in essential features, was replaced by one in 

 which simplicity was superseded by elaboration, and new 

 elements were added leading to greater complexity and a 

 modification of plan. Similarly the Palaeozoic faCies of 

 vegetation passes with almost startling suddenness into that 

 which monopolised the world in the Mesozoic era, and was 

 in turn superseded by the more highly elaborated and less 

 hornogeneous vegetation of the Cretaceous and Tertiary 

 periods. In taking a superficial view of architectural styles 

 we are apt to lose sight of the signs of gradual transition 

 bv which one period passes into the next ; so, too, in our 

 retrospect of the changing scenes which mark the progress 

 of plant-evolution, we easily overlook the introduction of 

 new types and the gradual substitution of new for old. The 

 invention of a new principle in the construction of build- 

 ings is soon followed by its wide adoption ; new conceptions 

 become stereotyped, and in a comparatively few years the 

 whole style is altered. As a new and successful type of 

 plant-architecture is produced it rapidly comes into 

 prominence and acts as the most potent factor in changing 

 the facies of a flora. Making due allowances for the im- 

 perfection of the Geological record, we cannot escape from 

 the conclusion, which is by no means opposed to our ideas 

 of the operation of the laws governing evolutionary forces, 

 that the state of equilibrium in the vegetable kingdom was 

 rudely shaken during two revolutionary periods. The 

 earlier transitional period occurred when Conifers and 

 Cycads became firmly established, while for the second re- 

 volution the introduction of the Angiospermous type was 

 mainly responsible. As in the half-effaced documents 

 accessible to the student of architecture " the pedigrees of 

 English Gothic can still be recovered," so also we are able 

 to trace in the registers imprinted on the rocks the 

 genealogies of existing botanical types. 



In the course of this address I have given but scant 

 attention to the lessons we have learnt and are still to learn 

 as to the family-history of plants. As Prof. Coulter says : 

 " The most difficult as well as the most fascinating problem 

 in connection with any group is its phylogeny. The data 

 upon which we base opinions concerning phylogeny are 

 never sufficient, but such opinions usually stimulate research 

 and are necessary to progress." 



