Darwinism (1889) Part 31

During a few weeks spent recently in the Rocky Mountains, I was struck by the great scarcity of monocotyledons and ferns in comparison with dicotyledons--a scarcity due apparently to the dryness and rarity of the atmosphere favouring the higher groups. If we compare Coulter's _Rocky Mountain Botany_ with Gray's _Botany of the Northern (East) United States_, we have two areas which differ chiefly in the points of alt.i.tude and atmospheric moisture. Unfortunately, in neither of these works are the species consecutively numbered; but by taking the pages occupied by the two divisions of dicotyledons on the one hand, monocotyledons and ferns on the other, we can obtain a good approximation. In this way we find that in the flora of the North-Eastern States the monocotyledons and ferns are to the dicotyledons in the proportion of 45 to 100; in the Rocky Mountains they are in the proportion of only 34 to 100; while if we take an exclusively Alpine flora, as given by Mr. Ball, there are not one-fifth as many monocotyledons as dicotyledons. These facts show that even at the present day elevated plateaux and mountains are more favourable to dicotyledons than to monocotyledons, and we may, therefore, well suppose that the former originated within such elevated areas, and were for long ages confined to them. It is interesting to note that their richest early remains have been found in the central regions of the North American continent, where they now, proportionally, most abound, and where the conditions of alt.i.tude and a dry atmosphere were probably present at a very early period.

[Ill.u.s.tration: FIG. 34.--Diagram ill.u.s.trating the Geological Distribution of Plants.]

The diagram (Fig. 34), slightly modified from one given by Mr. Ward, will ill.u.s.trate our present knowledge of the development of the vegetable kingdom in geological time. The shaded vertical bands exhibit the proportions of the fossil forms actually discovered, while the outline extensions are intended to show what we may fairly presume to have been the approximate periods of origin, and progressive increase of the number of species, of the chief divisions of the vegetable kingdom.

These seem to accord fairly well with their respective grades of development, and thus offer no obstacle to the acceptance of the belief in their progressive evolution.

_Geological Distribution of Insects._

The marvellous development of insects into such an endless variety of forms, their extreme specialisation, and their adaptation to almost every possible condition of life, would almost necessarily imply an extreme antiquity. Owing, however, to their small size, their lightness, and their usually aerial habits, no cla.s.s of animals has been so scantily preserved in the rocks; and it is only recently that the whole of the scattered material relating to fossil insects and their allies have been brought together by Mr. Samuel H. Scudder of Boston, and we have thus learned their bearing on the theory of evolution.[194]

The most striking fact which presents itself on a glance at the distribution of fossil insects, is the completeness of the representation of all the chief types far back in the Secondary period, at which time many of the existing families appear to have been perfectly differentiated. Thus in the Lias we find dragonflies "apparently as highly specialised as to-day, no less than four tribes being present." Of beetles we have undoubted Curculionidae from the Lias and Trias; Chrysomelidae in the same deposits; Cerambycidae in the Oolites; Scarabaeidae in the Lias; Buprestidae in the Trias; Elateridae, Trogositidae, and Nitidulidae in the Lias; Staphylinidae in the English Purbecks; while Hydrophilidae, Gyrinidae, and Carabidae occur in the Lias. All these forms are well represented, but there are many other families doubtfully identified in equally ancient rocks. Diptera of the families Empidae, Asilidae, and Tipulidae have been found as far back as the Lias. Of Lepidoptera, Sphingidae and Tineidae have been found in the Oolite; while ants, representing the highly specialised Hymenoptera, have occurred in the Purbeck and Lias.

This remarkable ident.i.ty of the families of very ancient with those of existing insects is quite comparable with the apparently sudden appearance of existing genera of trees in the Cretaceous epoch. In both cases we feel certain that we must go very much farther back in order to find the ancestral forms from which they were developed, and that at any moment some fresh discovery may revolutionise our ideas as to the antiquity of certain groups. Such a discovery was made while Mr.

Scudder's work was pa.s.sing through the press. Up to that date all the existing orders of true insects appeared to have originated in the Trias, the alleged moth and beetle of the Coal formation having been incorrectly determined. But now, undoubted remains of beetles have been found in the Coal measures of Silesia, thus supporting the interpretation of the borings in carboniferous trees as having been made by insects of this order, and carrying back this highly specialised form of insect life well into Palaeozoic times. Such a discovery renders all speculation as to the origin of true insects premature, because we may feel sure that all the other orders of insects, except perhaps hymenoptera and lepidoptera, were contemporaneous with the highly specialised beetles.

The less highly organised terrestrial arthropoda--the Arachnida and Myriapoda--are, as might be expected, much more ancient. A fossil spider has been found in the Carboniferous, and scorpions in the Upper Silurian rocks of Scotland, Sweden, and the United States. Myriapoda have been found abundantly in the Carboniferous and Devonian formations; but all are of extinct orders, exhibiting a more generalised structure than living forms.

Much more extraordinary, however, is the presence in the Palaeozoic formations of ancestral forms of true insects, termed by Mr. Scudder Palaeodictyoptera. They consist of generalised c.o.c.kroaches and walking-stick insects (Orthopteroidea); ancient mayflies and allied forms, of which there are six families and more than thirty genera (Neuropteroidea); three genera of Hemipteroidea resembling various h.o.m.optera and Hemiptera, mostly from the Carboniferous formation, a few from the Devonian, and one ancestral c.o.c.kroach (Palaeoblattina) from the Middle Silurian sandstone of France. If this occurrence of a true hexapod insect from the Middle Silurian be really established, taken in connection with the well-defined Coleoptera from the Carboniferous, the origin of the entire group of terrestrial arthropoda is necessarily thrown back into the Cambrian epoch, if not earlier. And this cannot be considered improbable in view of the highly differentiated land plants--ferns, equisetums, and lycopods--in the Middle or Lower Silurian, and even a conifer (Cordaites Robbii) in the Upper Silurian; while the beds of graphite in the Laurentian were probably formed from terrestrial vegetation.

On the whole, then, we may affirm that, although the geological record of the insect life of the earth is exceptionally imperfect, it yet decidedly supports the evolution hypothesis. The most specialised order, Lepidoptera, is the most recent, only dating back to the Oolite; the Hymenoptera, Diptera, and h.o.m.optera go as far as the Lias; while the Orthoptera and Neuroptera extend to the Trias. The recent discovery of Coleoptera in the Carboniferous shows, however, that the preceding limits are not absolute, and will probably soon be overpa.s.sed. Only the more generalised ancestral forms of winged insects have been traced back to Silurian time, and along with them the less highly organised scorpions; facts which serve to show us the extreme imperfection of our knowledge, and indicate possibilities of a world of terrestrial life in the remotest Palaeozoic times.

_Geological Succession of Vertebrata._

The lowest forms of vertebrates are the fishes, and these appear first in the geological record in the Upper Silurian formation. The most ancient known fish is a Pteraspis, one of the bucklered ganoids or plated fishes--by no means a very low type--allied to the sturgeon (Accipenser) and alligator-gar (Lepidosteus), but, as a group, now nearly extinct. Almost equally ancient are the sharks, which under various forms still abound in our seas. We cannot suppose these to be nearly the earliest fishes, especially as the two lowest orders, now represented by the Amphioxus or lancelet and the lampreys, have not yet been found fossil. The ganoids were greatly developed in the Devonian era, and continued till the Cretaceous, when they gave way to the true osseous fishes, which had first appeared in the Jura.s.sic period, and have continued to increase till the present day. This much later appearance of the higher osseous fishes is quite in accordance with evolution, although some of the very lowest forms, the lancelet and the lampreys, together with the archaic ceratodus, have survived to our time.

The Amphibia, represented by the extinct labyrinthodons, appear first in the Carboniferous rocks, and these peculiar forms became extinct early in the Secondary period. The labyrinthodons were, however, highly specialised, and do not at all indicate the origin of the cla.s.s, which may be as ancient as the lower forms of fishes. Hardly any recognisable remains of our existing groups--the frogs, toads, and salamanders--are found before the Tertiary period, a fact which indicates the extreme imperfection of the record as regards this cla.s.s of animals.

True reptiles have not been found till we reach the Permian where Prohatteria and Proterosaurus occur, the former closely allied to the lizard-like Sphenodon of New Zealand, the latter having its nearest allies in the same group of reptiles--Rhyncocephala, other forms of which occur in the Trias. In this last-named formation the earliest crocodiles--Phytosaurus (Belodon) and Stagonolepis occur, as well as the earliest tortoises--Chelytherium, Proganochelys, and Psephoderma.[195]

Fossil serpents have been first found in the Cretaceous formation, but the conditions for the preservation of these forms have evidently been unfavourable, and the record is correspondingly incomplete. The marine Plesiosauri and Ichthyosauri, the flying Pterodactyles, the terrestrial Iguanodon of Europe, and the huge Atlantosaurus of Colorado--the largest land animal that has ever lived upon the earth[196]--all belong to special developments of the reptilian type which flourished during the Secondary epoch, and then became extinct.

Birds are among the rarest of fossils, due, no doubt, to their aerial habits removing them from the ordinary dangers of flood, bog, or ice which overwhelm mammals and reptiles, and also to their small specific gravity which keeps them floating on the surface of water till devoured.

Their remains were long confined to Tertiary deposits, where many living genera and a few extinct forms have been found. The only birds yet known from the older rocks are the toothed birds (Odontornithes) of the Cretaceous beds of the United States, belonging to two distinct families and many genera; a penguin-like form (Enaliornis) from the Upper Greensand of Cambridge; and the well-known long-tailed Archaeopteryx from the Upper Oolite of Bavaria. The record is thus imperfect and fragmentary in the extreme; but it yet shows us, in the few birds discovered in the older rocks, more primitive and generalised types, while the Tertiary birds had already become specialised like those living, and had lost both the teeth and the long vertebral tail, which indicate reptilian affinities in the earlier Mammalia have been found, as already stated, as far back as the Trias formation, in Europe in the United States and in South Africa, all being very small, and belonging either to the Marsupial order, or to some still lower and more generalised type, out of which both Marsupials and Insectivora were developed. Other allied forms have been found in the Lower and Upper Oolite both of Europe and the United States. But there is then a great gap in the whole Cretaceous formation, from which no mammal has been obtained, although both in the Wealden and the Upper Chalk in Europe, and in the Upper Cretaceous deposits of the United States an abundant and well-preserved terrestrial flora has been discovered. Why no mammals have left their remains here it is impossible to say. We can only suppose that the limited areas in which land plants have been so abundantly preserved, did not present the conditions which are needed for the fossilisation and preservation of mammalian remains.

When we come to the Tertiary formation, we find mammals in abundance; but a wonderful change has taken place. The obscure early types have disappeared, and we discover in their place a whole series of forms belonging to existing orders, and even sometimes to existing families.

Thus, in the Eocene we have remains of the opossum family; bats apparently belonging to living genera; rodents allied to the South American cavies and to dormice and squirrels; hoofed animals belonging to the odd-toed and even-toed groups; and ancestral forms of cats, civets, dogs, with a number of more generalised forms of carnivora.

Besides these there are whales, lemurs, and many strange ancestral forms of proboscidea.[197]

The great diversity of forms and structures at so remote an epoch would require for their development an amount of time, which, judging by the changes that have occurred in other groups, would carry us back far into the Mesozoic period. In order to understand why we have no record of these changes in any part of the world, we must fall back upon some such supposition as we made in the case of the dicotyledonous plants.

Perhaps, indeed, the two cases are really connected, and the upland regions of the primeval world, which saw the development of our higher vegetation, may have also afforded the theatre for the gradual development of the varied mammalian types which surprise us by their sudden appearance in Tertiary times.

[Ill.u.s.tration: GEOLOGICAL DISTRIBUTION OF MAMMALIA.]

Notwithstanding these irregularities and gaps in the record, the accompanying table, summarising our actual knowledge of the geological distribution of the five cla.s.ses of vertebrata, exhibits a steady progression from lower to higher types, excepting only the deficiency in the bird record which is easily explained. The comparative perfection of type in which each of these cla.s.ses first appears, renders it certain that the origin of each and all of them must be sought much farther back than any records which have yet been discovered. The researches of palaeontologists and embryologists indicate a reptilian origin for birds and mammals, while reptiles and amphibia arose, perhaps independently, from fishes.

_Concluding Remarks._

The brief review we have now taken of the more suggestive facts presented by the geological succession of organic forms, is sufficient to show that most, if not all, of the supposed difficulties which it presents in the way of evolution, are due either to imperfections in the geological record itself, or to our still very incomplete knowledge of what is really recorded in the earth's crust. We learn, however, that just as discovery progresses, gaps are filled up and difficulties disappear; while, in the case of many individual groups, we have already obtained all the evidence of progressive development that can reasonably be expected. We conclude, therefore, that the geological difficulty has now disappeared; and that this n.o.ble science, when properly understood, affords clear and weighty evidence of evolution.

FOOTNOTES:

[Footnote 183: The reader who desires to understand this subject more fully, should study chap. x. of the _Origin of Species_, and chap. xiv.

of Sir Charles Lyell's _Principles of Geology_.]

[Footnote 184: On "Stagonolepis Robertsoni and on the Evolution of the Crocodilia," in _Q.J. of Geological Society_, 1875; and abstract in _Nature_, vol. xii. p. 38.]

[Footnote 185: From a paper by Messrs. Scott and Osborne, "On the Origin and Development of the Rhinoceros Group," read before the British a.s.sociation in 1883.]

[Footnote 186: American Addresses, pp. 73-76.]

[Footnote 187: Lecture on the Introduction and Succession of Vertebrate Life in America, _Nature_, vol. xvi. p. 471.]

[Footnote 188: _Nature_, vol. xxv. p. 84.]

[Footnote 189: See _The Mammalia in their Relation to Primeval Times_, p. 102.]

[Footnote 190: For a brief enumeration and description of these fossils, see the author's _Geographical Distribution of Animals_, vol. i. p.

146.]

[Footnote 191: Sketch of Palaeobotany in Fifth Annual Report of U.S.

Geological Survey, 1883-84, pp. 363-452, with diagrams. Sir J. William Dawson, speaking of the value of leaves for the determination of fossil plants, says: "In my own experience I have often found determinations of the leaves of trees confirmed by the discovery of their fruits or of the structure of their stems. Thus, in the rich cretaceous plant-beds of the Dunvegan series, we have beech-nuts a.s.sociated in the same bed with leaves referred to _f.a.gus_. In the Laramie beds I determined many years ago nuts of the _Trapa_ or water-chestnut, and subsequently Lesquereux found in beds in the United States leaves which he referred to the same genus. Later, I found in collections made on the Red Deer River of Canada my fruits and Lesquereux's leaves on the same slab. The presence of trees of the genera _Carya_ and _Juglans_ in the same formation was inferred from their leaves, and specimens have since been obtained of silicified wood with the microscopic structure of the modern b.u.t.ternut.

Still we are willing to admit that determinations from leaves alone are liable to doubt."--_The Geological History of Plants_, p. 196.]

[Footnote 192: Sir J. William Dawson's _Geological History of Plants_, p. 18.]

[Footnote 193: "On the Origin of the Flora of the European Alps," _Proc.

of Roy. Geog. Society_, vol. i. (1879), pp. 564-588.]

[Footnote 194: Systematic Review of our Present Knowledge of Fossil Insects, including Myriapods and Arachnids (_Bull. of U.S. Geol.

Survey_, No. 31, Was.h.i.+ngton, 1886).]

[Footnote 195: For the facts as to the early appearance of the above named groups of reptiles I am indebted to Mr. E. Lydekker of the Geological Department of the Natural History Museum.]

[Footnote 196: According to Professor Marsh this creature was 50 or 60 feet long, and when erect, at least 30 feet in height. It fed upon the foliage of the mountain forests of the Cretaceous epoch, the remains of which are preserved with it.]

[Footnote 197: For fuller details, see the author's _Geographical Distribution of Animals_, and Heilprin's _Geographical and Geological Distribution of Animals_.]

CHAPTER XIV

FUNDAMENTAL PROBLEMS IN RELATION TO VARIATION AND HEREDITY

Fundamental difficulties and objections--Mr. Herbert Spencer's factors of organic evolution--Disuse and effects of withdrawal of natural selection--Supposed effects of disuse among wild animals--Difficulty as to co-adaptation of parts by variation and selection--Direct action of the environment--The American school of evolutionists--Origin of the feet of the ungulates--Supposed action of animal intelligence--Semper on the direct influence of the environment--Professor Geddes's theory of variation in plants--Objections to the theory--On the origin of spines--Variation and selection overpower the effects of use and disuse--Supposed action of the environment in imitating variations--Weismann's theory of heredity--The cause of variation--The non-heredity of acquired characters--The theory of instinct--Concluding remarks.