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 Archeopteryx 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 ages. 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, Silurian. 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.1 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. Notwithstanding these irregularities and gaps in the record, the accompanying table, summarising our actual knowledge of the geological distribution of the five classes of vertebrata, GEOLOGICAL DISTRIBUTION OF MAMMALIA. Fishes Mammalia 1 For fuller details, see the author's Geographical Distribution of Animals, and Heilprin's Geographical and Geological Distribution of Animals. 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 classes 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 paleontologists 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 noble science, when properly understood, affords clear and weighty evidence of evolution. 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 intelligenceSemper 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. HAVING now set forth and illustrated at some length the most important of the applications of the development hypothesis in the explanation of the broader and more generally interesting phenomena presented by the organic world, we propose to discuss some of the more fundamental problems and difficulties which have recently been adduced by eminent naturalists. It is the more necessary to do this, because there is now a tendency to minimise the action of natural selection in the production of organic forms, and to set up in its place certain fundamental principles of variation or laws of growth, which it is urged are the real originators. of the several lines of development, and of most of the variety of form and structure in the vegetable and animal kingdoms. These views have, moreover, been seized upon by popular writers to throw doubt and discredit on the whole theory of evolution, and especially on Darwin's presentation of that theory, to the bewilderment of the general public, who are quite unable to decide how far the new views, even if well established, tend to subvert the Darwinian theory, or whether they are really more than subsidiary parts of it, and quite powerless without it to produce any effect whatever. The writers whose special views we now propose to consider are: (1) Mr. Herbert Spencer, on modification of structures arising from modification of functions, as set forth in his Factors of Organic Evolution. (2) Dr. E. D. Cope, who advocates similar views in detail, in his work entitled The Origin of the Fittest, and may be considered the head of a school of American naturalists who minimise the agency of natural selection. (3) Dr. Karl Semper, who has especially studied the direct influence of the environment in the whole animal kingdom, and has set forth his views in a volume on The Natural Conditions of Existence as they Affect Animal Life. (4) Mr. Patrick Geddes, who urges that fundamental laws of growth, and the antagonism of vegetative and reproductive forces, account for much that has been imputed to natural selection. We will now endeavour to ascertain what are the more important facts and arguments adduced by each of the above writers, and how far they offer a substitute for the action of natural selection; having done which, a brief account will be given of the views of Dr. Aug. Weismann, whose theory of heredity will, if established, strike at the very root of the arguments of the first three of the writers above referred to. Mr. Herbert Spencer's Factors of Organic Evolution. Mr. Spencer, while fully recognising the importance and wide range of the principle of natural selection, thinks that sufficient weight has not been given to the effects of use and disuse as a factor in evolution, or to the direct action of the environment in determining or modifying organic structures. As examples of the former class of actions, he adduces the decreased size of the jaws in the civilised races of mankind, the inheritance of nervous disease produced by overwork, the great and inherited development of the udders in cows and goats, and the shortened legs, jaws, and snout in |