ORIGIN OF BIRDS.

121

the aberrant ostrich seemed to be described in large archaic characters; a little while and these faded into what could just be read off as pertaining to the sea-turtle, or most probably the rhynchosaurian lizard and not the sea-turtle, whilst underlying the whole the fish could be traced in morphological hieroglyphics."

With this picture before us let us inquire how the wings of birds may have been developed, as this has been considered one of the difficulties of the hypothesis of evolution. From the amphibian to the rhynchosaurian is no long stride. Extending into the cold arctic and antarctic regions, such creatures may have become clothed with downy feathers; and using their fore limbs as paddles, these may have become developed, by the formation of a short patagium, into something very like the wings of the penguins. Such birds, dwelling on the shores of a great southern continent, may have spread towards the tropic on to the land, and, losing their incipient power of flight, have acquired the type of the ostrich; whilst those of more southern or northern zones, gaining greater and greater strength and power in their paddle-like wings, first used for progression in the water only (as the short-winged auks and penguins and many of the diving ducks still use theirs), may ultimately have taken first short and then longer flights.

G

Indeed the steamer duck of the Straits of Magellan exhibits such a transitional condition at the present day, some of the individuals only using their wings for aërial progression.

Mr. Ray Lankester has suggested the term homoplastic for structures which are alike, but which have probably been independently evolved. Such structures undoubtedly exist in great numbers; and the acquirement of a patagium or flyingmembrane appears to be such a phenomenon. Bats amongst the mammalia, pterodactyles amongst reptiles, and birds are all characterized by the presence of such a membrane. It is not a little remarkable that a patagium occasionally occurs, although very rarely, as an abnormal result of development, just as webbing of the digits does. When we remember that probably the limbs are formed at first as mere thickenings of the edges of the blastoderm*, from which they ultimately become freed, just as the fingers and toes are formed in the broad paddle-like extremity of the primitive limb, it is not a matter for surprise that the intermediate blastoderm should sometimes become converted into a patagium, just as that between the digits often persists as a web. Such a modification may readily be believed to have occurred at the same time that the outer layer of the blastoderm * Catalogue of Malformations in Hunterian Museum, p. 79.

THE ORIGIN OF WINGS.

123

was undergoing modification to form the amniotic sac (which is characteristic of all these higher vertebrates), although now its occurrence is exceedingly rare. The existence of a patagium in the pterodactyles, in lizards, in some mammals, and, to a less degree, in birds may have been due to such a variation in development.

Modification of the sternum to form a keel is probably another example of homoplastic change, probably resulting from the action of muscles favouring, or perhaps even causing, the deposition of bone in their septa―a view borne out by the occurrence of a keeled sternum in such diverse forms as pterodactyles, ichthyornid and carinate birds, by its absence in the ostriches, as well as by its development in other birds being proportionate to their power of flight*. The modification of the rhomboidal sternum of lizards in the flying and diving birds may be the result of disuse, exosteal ossifications replacing the primitive cartilaginous basis of the sternum of the ancestral form in these.

The facts that the dragonflies amongst insects are aquatic, and that some Hymenoptera use their wings for diving, tend by analogy to confirm the hypothesis that wings for aërial flight were useful

* See a quotation from Mr. Wallace on the skulls of anthropoid apes, supra, page 79.

in their rudimentary state as paddles for progression in the water.

With all the complex laws of variation giving rise to adaptive, correlated, and indefinite modifications, checked and guided by natural selection, it is impossible, with our present knowledge at least, to account for all the varied modifications which have arisen; but enough has been done to show that even the highest types have been evolved. Without the hypothesis, how inexplicable are the facts! with it they conform to the great unity which all science seeks.

SUBAËRIAL DENUDATION.

125

CHAPTER XV.

GEOLOGICAL RECORD.

THE crust of the globe is a great fragmentary treatise on evolution, the imperfection of which, as a record, has been so ably treated by Mr. Darwin that little remains to be said; but the objections to the theory of evolution on geological grounds are of so serious a nature, if we believe that the fragmentary chapters which we see are a continuous history of the past ages of our globe, that it is most important to review this subject in extenso.

It is now very generally admitted by geologists. that every stratum is formed from the detritus of an earlier bed of rock, and that every rock, whether it be hard or soft, is not only undergoing degradation by the action of the waves along the shore line, or by the torrents and currents of rivers, but also by the continuous action of air, of alternating frosts and suns, and of rain upon its surface. These agencies produce the phenomena known as subaërial denudation, the older rocks being frequently laid bare by the wearing away of all newer deposits. The vast effects of subaërial denudation