March 29, 1877] 



NATURE 



469 



small area of the Purbeck beds revealed their presence in numbers ; 

 had this spot not been quarried it would have been supposed for 

 years that mammalia had made their appearance in Eocene 

 times. Some causes, tending to make the preservation of 

 dicotyledons difficult were discussed in Nature, vol. xv. p. 281, 

 and need not be further alluded to here. Mere localised patches 

 of plant remains are not an unerring index of the character of 

 a flora at any period. At Bournemouth there are patches just 

 underlying the lowest marine beds, which are crowded with ferns 

 only ; other patches contain nothing but ferns, aroids, and 

 gymnosperms. Had these patches been isolated, inferences of a 

 most misleading character would have been drawn. 



The upper Cretaceous floras are known to us principally from 

 Aix-la-Chapelle and from Ainerica; but as in both these — -indeed 

 in most cases — the supposed Cretaceous beds containing plant re- 

 mains rest on palaeozoic rocks, their relative age is a matter of 

 uncertainty. M. Barrois fixes it as contemporaneous with his 

 zone of Belemnitella, but whether he is right in this supposition 

 or nof, the flora contains ferns and other plants which seem iden- 

 tical widi those of the Bournemouth beds. In America, in the 

 Dakota group, we have leaf beds 400 feet thick of the supposed 

 age of our gray chalk, but the associated marine beds have, mixed 

 with decidedly Cretaceous forms, shells approaching very closely 

 those of our London clay. It seems more logical to determine 

 the age of a rock by the incoming of new types than by the 

 lingering of old, and the whole paLieontological evidence shows 

 that these beds are at most intermediate in age between our 

 Eocene and Chalk, the enormous gap between which is probably 

 tilled up here by some 2,200 feet of strata. American geologists 

 are not agreed as to their age. It would be out of place to dis- 

 cuss this subject at length, but enough is said to show that the 

 relative ages of these floras is not definitely known, and that no 

 series of arguments based on their relative sequence is, at pre- 

 sent, entitled to any weight. M. Lesquereux finds evidence in 

 support of evolution in the flora of Dakota, " in the remarkable 

 disproportion of genera compared to species ; " and in the same- 

 ness ot the leaves, which are "mostly entire, coarsely veined, and 

 coriaceous, the difficulty of separating them into distinct group?, 

 by fixed characters, the numerous forms of leaf which, seen sepa- 

 rately, represent diff^erent species, or even genera, and which, 

 considered in series or groups, appear undividable into sections." 

 When, however, he theorises, we see that he makes use of the 

 same arguments against evolution as those put^'/orward by Mr. 

 Carruthers. Von Ettingshausen, on the other hand, who has paid 

 much attention to the subject, states that he is able to trace the 

 ancestry of our present floras back to simple elements in Tertiary 

 time?, and these to still simpler and more united types in Cre- 

 taceous times. In his works a number of examples are given. 

 The flora of Sezanne, whose age as Lower Eocene may be 

 accepted, is closely analagous with that of Bournemouth. 



Now let us examine the manner in which determinations of 

 fossil leaves from these earlier rocks have been made, and see 

 whether they are sufficiently reliable to entitle us to form any 

 theories whatever as to the simultaneous appearance of the three 

 divisions of Dicotyledons. Let us take the flora of Dakota. 



Of I'olypetala we have Liriodendron, founded on two frag- 

 ments, and Magnolia on two fragments. These fragments are 

 of simple leaves and possess no character whatever in themselves, 

 upon which they can be determined. Magnolia, for instance, 

 is determined from the similarity in form to leaves described as 

 Magnolia by Heer in the Flora of Greenland, which themselves 

 are supposed to be Magnolia because they resemble (not specifi- 

 cally) Magnolias from the Miocene of Europe. In Menespermites, 

 the third genus, the name indicates that its affinities are vague, 

 and we accordingly see that it had been formerly described as 

 Dombeyopsis, Acer, Populites. The Gainopetalce are represented 

 by three genera. Of these Andromeda is determined on 

 two fragments and one indistinct leaf of simple lanceolate 

 form ; Diospyros, formerly described as Quercus, is determined 

 from one simple and ovate leaf resembling Laurus, the other a 

 round and simple leaf ; while Brumelia is siill more unsatisfactory, 

 and has been previously thought to be either Laurus or Quercus. 

 The determinations have been changed, as we see by the position 

 of the plates and the figures on the plates, many times during the 

 progress of the work, and it is not too much to say that all the 

 determinations of leaves of Polypetalne and Gamopetake from 

 this flora are vague and unsatisfactory, and no one would be 

 more ready to acknowledge this than Mr. Carruthers himself. 

 We do not find fault so much with the determinations themselves, 

 which are probably the best that could be m^de from such ma- 

 terial, but we think it premature to base any theories upon 



them as to the simultaneous appearance with the Apetalae of 

 the more highly organised Dicotyledons. 



In the Eocene and Miocene we have, however, richer mate- 

 rials, and the variety and completeness of the fossil flora become 

 conspicuous ; the forms, as Lyell says, "were perfect, changing, 

 but always becoming more and more like, generically and speci- 

 fically, to those now living." Von Ettingshausen has traced the 

 direct descent of many living species back to the Miocene, 

 sometimes two or more species to a common parent stock. 



5. With regard to the persistence of Salix folatis, it appears 

 to be simply a case of a plant becoming thoroughly adapted to 

 certain conditions of life which were met with in England 

 during the glacial period, and are present now in extreme northern 

 regions. Why Siplix polaris should have varied since glacial 

 times more than mollusca and other animal life is not apparent. 

 The intermediate forms which should connect willows and poplars 

 have not been found, but as poplar- like leaves have been met 

 with in lower cretaceous rocks, it is probable that the order of 

 Salicaceae is an extremely ancient one, and the single generalised 

 form must be sought for in remoter times even than the Creta- 

 ceous. 



Our general broad knowledge of the succession of plant life, as 

 testified by the rocks, is too well known to need recapitulating 

 here. Schimper enters in detail into its hiitory. In the Silurian, 

 Algoe ; in the Devonian ferns and Lycopods, reaching their apogee 

 of development in the Carboniferous ; and in the Permian the 

 conifers first take an important position. The Triassic indicates 

 a great gap, and may be considered the reign of gymnosperms, 

 whilst the incoming of the phanerogams is placed beyond doubt. 

 The Jurassic presents another hiatus, and but little is known of 

 its flora. ^ Heer, however, infers, from the entomological fauna, 

 that there were no leafy trees in the Lias. The oolitic rocks con- 

 tain abundance of cycads. The Wealden and Neocomian vege- 

 tation has left us little more than gymnosperms and ferns. 

 With the upper cretaceous period dicotyledons are abundant, but 

 their incoming is traced to older rocks. The Eocene contains 

 rich assemblages of dicotyledons, principally apetalous, and the 

 Miocene, better known, a still greater variety. We see the same 

 plan of development in the individual ; and, as Prof. Huxley re- 

 cently stated in a lecture at South Kensington, " we can trace 

 living plants from the most gigantic and complicated tree, step 

 by step down through many gradations to the lowest alg?e, the 

 lichens, and on down to a piece of animal jelly." 



Thus we find on reviewing the evidence that has been brought 

 forward, that other interpretations may be put upon the facts 

 presented to us by Mr. Carruthers. J. S. G. 



In an article in this month's Ctntemporary, entitled "Evolu- 

 tion and the Vegetable Kingdom," Mr. Carruthers refers inci- 

 dentally to a question that deserves the careful consideration of 

 all who accept the doctrine of evolution ; viz., whether the 

 earliest type of flower was hermaphrodite or unisexual. Alluding 

 to the abundance and variety of palaeozoic gymnosperms, as 

 evidenced by the numerous fruits that have been discovered in 

 the carboniferous measures, he lays stress on the fact that " they 

 all belong to the Taxineous group of conifers .... that the 

 plants of this section are all dicecious, i.e. having the sexes on 

 different plants. If the occurrence of the germ and sperm elemeiits 

 in different organs, and even in different individuals, is evidence, as 

 it is held, of higher development in phanerogams, then it is im- 

 portant to notice the order of appearance of dioecious and monce- 

 cious groups in relation to thOiC with hermaphrodite flowers. 

 Advocates of evolution hold that dimorphic plants are now in a 

 transition stage progressing towards a dioecious condition. The 

 conifers attanied to the highest known development as regards 

 this element of their structure on their first appearance." 



If Mr. Darwin be regarded as an exponent of the views held 

 by "advocates of evolution," we find that he expresses himself 

 very differently. From the following passages in his recently pub- 

 lished work on " The Effects of Cross and Self- fertilisation in the 

 Vegetable Kingdom," he would seem to consider the primordial 

 condition to be unisexual. " There is good reason to believe 

 that the first plants which appeared on this earth were crypto- 



gamic As soon as plants became phanerogamic and 



grew on the dry ground, if they were to intercross, it would be 

 indispensable that the male fertilising element should be trans- 

 ported by some means through the air ; and the wind is the 



' An extensive Jurassic flora has been described by Heer in Mhtt. de 

 I'Acad. Imp. des Sciences de Si. PHersbourg, vii*-" scrie, tome xxii. No. 12, 



1876. 



