conditions of life on each of these archipelagos are in great contrast to those which surround the faunas and floras of their adjacent continents. The species of birds, reptiles, and plants found on the Galapagos are, for the most part, such as exist nowhere else in the world. On the ordinary view, naturalists would say they must have been created there. But according to Mr. Darwin,1- and it was this fact which turned his mind into the channel of speculation with which his name is associated nearly all [these species] bore an American stamp. In the song of the mockingthrush, in the harsh cry of the carrion-hawk, in the great candlestick-like opuntias, I clearly perceived the neighborhood of America, though the islands were separated by so many miles of ocean from the main-land, and differed much from it in their geological constitution and climate. Still more surprising was the fact that most of the inhabitants of each separate island in this small archipelago were specifically different, though most closely related to each other." The animals. and plants on the Cape de Verde Islands have a corresponding affinity to those of Africa. The problem is to find, if possible, the bond of secondary causation which shall explain these complex phenomena. It must account for the

1 Animals and Plants under Domestication, Vol. i. p. 21. See also Origin of Species, pp. 353-356.

similarity under diverse conditions, and the diversity under similar conditions. Mr. Darwin believes that it is idle for us to search here for a "final cause." So far as there is truth in his remark it is, in our opinion, partially owing to the inadequate views current regarding the doctrine of final causes. Mr. Darwin supposes he has found a natural mode of accounting for the similarities and the difference of representative species, in the effect of diversity of condition acting on the descendants of a common ancestry. According to him, the facts delineated with regard to the relationship between the forms of life on the islands and those on the adjoining continental areas, point to community of descent in comparatively recent time; and no one can deny that there is great plausibility in this explanation.

In further illustration similar facts may be adduced regarding the island of Madagascar, where all the species of animals but one, and nearly all the genera, are different from those on the continent of Africa. Yet these genera and species resemble those in Africa more than they do those of any other province.

To get the significance of these facts they must be seen in their connections, and somewhat more in detail. As is now well known, there is a marked distinction between the life upon islands sur

1 Lyell's Principles of Geology, Vol. ii. p. 347.

rounded by deep seas and that upon islands separated from continents by shallow seas. True oceanic islands are such as rise from an ocean floor ten thousand feet or more in depth; of which the Bermudas, the Azores, the Galapagos, St. Helena, and the Sandwich Islands are examples. Upon such islands no native mammalia, nor even frogs and reptiles, are to be found; but their native inhabitants are limited to birds and insects, and such other animals and plants as could have been transported naturally from the neighboring continents over the intervening expanse of water. On the other hand, continental islands, or those surrounded by shallow seas, indicating a comparatively recent land connection with the continents, are inhabited by many species which could have migrated only on dry land. Such islands are Great Britain, Borneo, Java, Japan, Formosa, and Madagascar. In the case of all these an elevation of a few hundred, or, at the most, two or three thousand, feet would establish a continental connection. The sea separating Siam from Borneo and Java is only about three hundred feet, and that surrounding England is not over six hundred feet, in depth. Now the rule is, that the antiquity of the type of life present upon a continental island is proportionate to the depth of the sea separating it from the mainland. Of this principle Madagascar presents some

of the most interesting illustrations. It is probable, from the depth of the Mozambique Channel and from the distribution of shoals in the Indian Ocean, that Madagascar was, at an early period, joined to Africa, and that several islands, now submerged, existed in the direction of India; and the forms of life in Madagascar have affinities with those in India only in the case of such species as could migrate from island to island; while with Africa the affinities are such as to necessitate a continuous land connection, there being similitude in no less than sixty-six species of mammalia. But these affinities with Africa are not with the species now most characteristic of that continent. Madagascar does not contain monkeys, baboons, apes, lions, leopards, zebras, elephants, giraffes, and antelopes, which abound in Africa; but the larger part of its species belongs to the lemuroid family. This family is now represented by a few scattered species in West Africa, India, Ceylon, and the Malay Archipelago, all very unlike the species in Madagascar. The inference is that Madagascar was joined to Africa during an early extension of the lemuroid family, and became isolated before the advent in Africa of its present characteristic mammalia. To find that period, the geologist has to go back to the first portion of the Tertiary epoch, when lemurs and the other families. now characteristic of Madagascar, are

known to have been abundant in Europe. We know, also, that about that time, during the Eocene and Miocene periods, the northern portion of Africa was submerged, transforming the southern portion into an immense island.

The theory which explains the peculiar distribution of the lemuroid family and its present predominance in Madagascar is, that at the time just referred to Madagascar and Southern Africa were united, allowing lemurs to spread over the whole territory; but that before land connection was reestablished with Asia and Europe Madagascar had become separated from the main-land, and has ever since maintained its insular position, and its plants and animals have consequently been preserved from that competition with new northern forms of life which took place when, in later Tertiary times, the barriers to immigration were removed from the northern part of the continent. According to present evidence, the camel originated in America during the Tertiary period, and most of the characteristic larger mammalia of Africa, in Europe and central Asia during the Tertiary isolation of Africa. When, later, access was given them to Africa, they availed themselves of the opportunity and gradually crowded out the lemuroid family; but Madagascar was beyond their reach.

For another illustration we turn to the Canary Islands, which lie in close vicinity to the west