45P 



NATURE 



[Sept. 6, 1888 



during the Tertiary period the climatic conditions of the earth 

 did not permit their growth there. Arctic fossil floras of tem- 

 perate and therefore Miocene aspect are, in all probability, of 

 Eocene age, and what has been recognized in them as a newer 

 or Miocene facies is due to their having been first studied in 

 Europe in latitudes which only became fitted for them in Miocene 

 times. When stratigraphical evidence is absent or inconclusive, 

 this unexpected persistence of plant types or species throughout 

 the Tertiaries should be remembered, and the degrees of latitude 

 in which they are found should be well considered before 

 conclusions are published respecting their relative age." 



This view is consistent with that held by the leaders in botany 

 — Hooker, Dyer, Saporta, Dawson, and Asa Gray (whose 

 recent loss we so deeply deplore) — that the North Polar region 

 is the centre of dispersal, from which the Dicotyledons spread 

 over the northern hemisphere. If it be true — and I, for one, 

 am prepared to accept it — it will follow that for the co-ordination 

 of the subdivisions of the Tertiary strata in various parts of the 

 world the plants are uncertain guides, as they haye been shown 

 to be in the case of the Primary and Secondary rocks. In all 

 cases where there is a clash of evidence, such as in the Laramie 

 lignites, in which a Tertiary flora is associated with a Cretaceous 

 fauna, the verdict, in my opinion, must go to the fauna. They 

 are probably of the same geological age as the deposit at 

 Aix-la-Chapelle. 



I would remark, further, before we leave the floras behind us, 

 that the migration of new forms of plants into Europe and 

 America took place before the arrival of the higher types in the 

 fauna, after the break-up of the land at the close of the Car- 

 boniferous period, and after the great change in geography at 

 the close of the Neocomian.' The Secondary plants preceded 

 the Secondary vetebrates by the length of time necessary for 

 the deposit of the Permian rocks, and the Tertiary plants pre- 

 ceded the Tertiary vertebrates by the whole period of the Upper 

 Cretaceous. 



Let us now turn to the fauna. 



Prof. Huxley, in one of his many addresses which have left 

 their mark upon our science, has called attention to the persist- 

 ence of types revealed by the study of palaeontology, or, to put 

 it in other words, to the singularly little change which the 

 ordinal groups of life have undergone since the appearance of 

 life on the earth. The species, genera, and families present an 

 almost endless series of changes, but the existing orders are for 

 the most part sufficiently wide, and include the vast series of 

 fossils without the necessity of framing new divisions for their 

 reception. The number of these extinct orders is not equally 

 distributed through the animal kingdom. Taking the total 

 number of orders at 108, the number of extinct orders in the 

 Invertebrata amounts only to 6 out of 88, or about 7 per cent., 

 while in the Vertebrates it is not less than 12 out of 40, or 

 30 per cent. These figures imply that the amount of ordinal 

 change in the fossil Vertebrates stands to that in the Inverte- 

 brata in the ratio of 30 to 7. This disproportion becomes still 

 more marked when we take into account that the former had 

 less time for variation than the latter, which had the start by 

 the Cambrian and Ordovician periods. It follows also that as 

 a whole they have changed faster. 



The distribution of the extinct orders in the animal kingdom, 

 taken along with their distribution in the rocks, proves further 

 that some types have varied more than others, and at various 

 places in the geological record. In the Protozoa, Porifera, and 

 Vermes there are no extinct orders ; among the Ccelenterates 

 one — the Rugosa ; fin the Echinodermata three — Cystideans, 

 Edriasterida, and Blastoidea ; in the Arthropoda two — the 

 Trilobita and Eurypterida. All these, with the solitary ex- 

 ception of the obscure order Rugosa, are found only in the 

 Primary rocks. Among the Pisces there are none ; in the 

 Amphibia one ; the Labyrinthodonts ranging from the Car- 

 boniferous to the Triassic age. Among the Reptilia there are at 

 least six of Secondary age — Ple-iosauria, Ichthyosauria, Dicyno- 

 dontia, Pterosauria, Therioclontia, Deinosauria ; in the Aves 

 two— the Saururae and Odontornithes, also Secondary. In the 

 Mammalia the Amblypoda, Tillodontia, Condylarthra, and 

 Toxodontia represent the extinct orders — the three first Early 

 Tertiary, and the last Pleistocene. It is clear, therefore, that, 

 while the maximum amount of ordinal variation is presented by 

 the Secondary Reptilia and Aves, all the extinct orders in the 

 Tertiary are Mammalian. 



If we turn from the extinct orders to the extinct species, it 

 will also be found that the maximum amount of variation is 



presented by the plants, and all the animals, excapting the 

 Mammalia, in the Primary and Secondary periods. 



The general impression left upon my mind by these facts is 

 that, while all the rest of the animal kingdom had ceased to 

 present important modifications at the close of the Secondary 

 period, the Mammalia, which presented no great changes 

 in the Secondary rocks, were, to quote a happy phrase 

 of Prof. Gaudry, "en pleine evolution" in the Tertiary 

 age. And when, further, the singular perfection of the record 

 allows us to trace the successive and gradual modifications 

 of the Mammalian types from the Eocene ta the close 

 of the Pleistocene age, it is obvious that they can be 

 used to mark subdivisions of the Tertiary period, in the 

 same way as the reigns of kings are used to mark periods in 

 human history. In my opinion they mark the geological 

 horizon with greater precision than the remains of the lower 

 members of the animal kingdom, and in cases such as that of 

 Pikermi, where typical Miocene forms, such as Deinotheria, are 

 found in a stratum above an assemblage of marine shells of 

 Pliocene age, it seems to me that the Mammalia are of greater 

 value in classification than the Mollusca, some of the species of 

 which have been living from the Eocene down to the present day. 



Vet another important principle must be noted. The fossils 

 are to be viewed in relation to those forms now living in their 

 respective geographical regions. The depths of the ocean have 

 been where they are now since the earliest geological times, 

 although continual geographical changes have been going on at 

 their margins. In other words, geographical provinces must have 

 existed even in the earlier geological periods, although there is 

 reason to believe that they did not differ so much from each other 

 as at the present day. It follows from this that the only just 

 standard for comparison in dealing with the fossils, and especially 

 of the later rocks, is that which is offered by the fauna and flora of 

 the geographical province in which they are found. The non- 

 recognition of this principle has led to serious confusion. The 

 fauna, for example, of the Upper Sivalik formation has been 

 very generally viewed from the European stand-point and placed 

 in the Miocene, while, judged by the stand-point of India, it is 

 really Pliocene. A similar confusion has followed from taking 

 the Miocene flora of Switzerland as a standard for the Tertiary 

 flora of the whole of the northern hemisphere. 



It now remains for us to see how these principles may be 

 applied to the co-ordination of Tertiary strata in various parts 

 of the world. In 1880 I proposed a classification of the Euro- 

 pean Tertiaries, in which, apart from the special characteristic 

 fossils of each group, stress was laid on the gradual approxima- 

 tion of various groups to the living Mammalia. The definitions 

 are the following : — 



Divisions. 



Characteristics. 



1. Eocene, or that in which the Extinct orders, 

 higher Mammalia (Eutheria) now on Living orders and families, 

 the earth were represented by allied No living genera, 

 forms belonging to existing orders 

 and families. 



Oligocene. 



2. Miocene, in which the alliance 

 between fossil and living Mammals 

 is closer than before. 



Living genera. 

 No living species. 



3. Pliocene, in which living Living species few. 

 species of Mammals appear. Extinct species predo- 



minant. 



4. Piistocene, in which living 

 species of Mammals preponderate. 



Living species abundant. 

 Extinct species present. 

 Man present. 



5. Prehistoric, or that period out- Man abundant. 



side history in which Man has Domestic animals present, 



multiplied exceedingly on the earth Wild Mammals in retreat. 



and introduced the domestic animals. One extinct Mammal. 



6. Historic, in which the events Records, 

 are recorded in history. 



These definitions are of more than European significance. 

 The researches of Leidy, Marsh, and Cope prove that they 

 apply equally to the Tertiary strata of North America. The 



