* The best treatment of the subject will be found in W. P.
Pycraft's History of Birds, 1910.
The special problems of bird-evolution are as numerous and unsettled as those of the insects. There is the same dispute as to "protective colours" and "recognition marks", the same uncertainty as to the origin of such instinctive practices as migration and nesting. The general feeling is that the annual migration had its origin in the overcrowding of the regions in which birds could live all the year round. They therefore pushed northward in the spring and remained north until the winter impoverishment drove them south again. On this view each group would be returning to its ancestral home, led by the older birds, in the great migration flights. The curious paths they follow are believed by some authorities to mark the original lines of their spread, preserved from generation to generation through the annual lead of the older birds. If we recollect the Ice-Age which drove the vast majority of the birds south at the end of the Tertiary, and imagine them later following the northward retreat of the ice, from their narrowed and overcrowded southern territory, we may not be far from the secret of the annual migration.
A more important controversy is conducted in regard to the gorgeous plumage and other decorations and weapons of the male birds. Darwin, as is known, advanced a theory of "sexual selection" to explain these. The male peacock, to take a concrete instance, would have developed its beautiful tail because, through tens of thousands of generations, the female selected the more finely tailed male among the various suitors. Dr. Wallace and other authorities always disputed this aesthetic sentiment and choice on the part of the female. The general opinion today is that Darwin's theory could not be sustained in the range and precise sense he gave to it. Some kind of display by the male in the breeding season would be an advantage, but to suppose that the females of any species of birds or mammals had the definite and uniform taste necessary for the creation of male characters by sexual selection is more than difficult. They seem to be connected in origin rather with the higher vitality of the male, but the lines on which they were selected are not yet understood.
This general sketch of the enrichment of the earth with flowering plants, insects, and birds in the Tertiary Era is all that the limits of the present work permit us to give. It is an age of exuberant life and abundant food; the teeming populations overflow their primitive boundaries, and, in adapting themselves to every form of diet, every phase of environment, and every device of capture or escape, the spreading organisms are moulded into tens of thousands of species. We shall see this more clearly in the evolution of the mammals. What we chiefly learn from the present chapter is the vital interconnection of the various parts of nature. Geological changes favour the spread of a certain type of vegetation. Insects are attracted to its nutritious seed-organs, and an age of this form of parasitism leads to a signal modification of the jaws of the insects themselves and to the lavish variety and brilliance of the flowers. Birds are attracted to the nutritious matter enclosing the seeds, and, as it is an advantage to the plant that its seeds be scattered beyond the already populated area, by passing through the alimentary canal of the bird, and being discharged with its excrements, a fresh line of evolution leads to the appearance of the large and coloured fruits. The birds, again, turn upon the swarming insects, and the steady selection they exercise leads to the zigzag flight and the protective colour of the butterfly, the concealment of the grub and the pupa, the marking of the caterpillar, and so on. We can understand the living nature of to-day as the outcome of that teeming, striving, changing world of the Tertiary Era, just as it in turn was the natural outcome of the ages that had gone before.
CHAPTER XVII. THE ORIGIN OF OUR MAMMALS
In our study of the evolution of the plant, the insect, and the bird we were seriously thwarted by the circumstance that their frames, somewhat frail in themselves, were rarely likely to be entombed in good conditions for preservation. Earlier critics of evolution used, when they were imperfectly acquainted with the conditions of fossilisation, to insinuate that this fragmentary nature of the geological record was a very convenient refuge for the evolutionist who was pressed for positive evidence. The complaint is no longer found in any serious work. Where we find excellent conditions for preservation, and animals suitable for preservation living in the midst of them, the record is quite satisfactory. We saw how the chalk has yielded remains of sea-urchins in the actual and gradual process of evolution. Tertiary beds which represent the muddy bottoms of tranquil lakes are sometimes equally instructive in their fossils, especially of shell-fish. The Paludina of a certain Slavonian lake-deposit is a classical example. It changes so greatly in the successive levels of the deposit that, if the intermediate forms were not preserved, we should divide it into several different species. The Planorbis is another well-known example. In this case we have a species evolving along several distinct lines into forms which differ remarkably from each other.
The Tertiary mammals, living generally on the land and only coming by accident into deposits suitable for preservation, cannot be expected to reveal anything like this sensible advance from form to form. They were, however, so numerous in the mid-Tertiary, and their bones are so well calculated to survive when they do fall into suitable conditions, that we can follow their development much more easily than that of the birds. We find a number of strange patriarchal beasts entering the scene in the early Eocene, and spreading into a great variety of forms in the genial conditions of the Oligocene and Miocene. As some of these forms advance, we begin to descry in them the features, remote and shadowy at first, of the horse, the deer, the elephant, the whale, the tiger, and our other familiar mammals. In some instances we can trace the evolution with a wonderful fullness, considering the remoteness of the period and the conditions of preservation. Then, one by one, the abortive, the inelastic, the ill-fitted types are destroyed by changing conditions or powerful carnivores, and the field is left to the mammals which filled it when man in turn began his destructive career.
The first point of interest is the origin of these Tertiary mammals. Their distinctive advantage over the mammals of the Mesozoic Era was-the possession by the mother of a placenta (the "after-birth" of the higher mammals), or structure in the womb by which the blood-vessels of the mother are brought into such association with those of the foetus that her blood passes into its arteries, and it is fully developed within the warm shelter of her womb. The mammals of the Mesozoic had been small and primitive animals, rarely larger than a rat, and never rising above the marsupial stage in organisation. They not only continued to exist, and give rise to their modern representatives (the opossum, etc.) during the Tertiary Era, but they shared the general prosperity. In Australia, where they were protected from the higher carnivorous mammals, they gave rise to huge elephant-like wombats (Diprotodon), with skulls two or three feet in length. Over the earth generally, however, they were superseded by the placental mammals, which suddenly break into the geological record in the early Tertiary, and spread with great vigour and rapidity over the four continents.
Were they a progressive offshoot from the Mesozoic Marsupials, or Monotremes, or do they represent a separate stock from the primitive half-reptile and half-mammal family? The point is disputed; nor does the scantiness of the record permit us to tell the place of their origin. The placental structure would be so great an advantage in a cold and unfavourable environment that some writers look to the northern land, connecting Europe and America, for their development. We saw, however, that this northern region was singularly warm until long after the spread of the mammals. Other experts, impressed by the parallel development of the mammals and the flowering plants, look to the elevated parts of eastern North America.