192 THE POPULAR SCIENCE MONTHLY.
Gulf of St. Lawrence and the Pacific are animals which have no special locomotive powers even when young, but they are northern forms not proceeding far south, so that they may have passed through the Arctic seas.
In this connection it is well to remark that many species of animals have powers of locomotion in youth, which they lose when adult, and that others may have special means of transit. I once found at Gaspe a specimen of the Pacific species of Coronida, or whale-baraacle, the C. regincB of Darwin, attached to a whale taken in the Gulf of St. Law- rence, and which had probably succeeded in making that passage round the north of America which so many navigators have essayed in vain. It is to be remarked here that while many plants and marine inverte- brates are common to the two sides of the Atlantic, it is different "with land-animals, and especially vertebrates.
I do not know that any fossil insects or land-snails or millipedes of Europe and America are specifically identical, and of the ni;merous spe- cies of batrachians of the Carboniferous and reptiles of the Mesozoic all seem to be distinct on the two sides. The same appears to be the case with the Tertiary mammals, until in the later stages of that great period w^e find such genera as the horse, the camel, and the elephant appearing on the two sides of the Atlantic ; but even then the species seem different, except in the case of a few northern forms. Some of the longer-lived mollusks of the Atlantic furnish suggestions which remarkably illustrate the biological aspect of these questions. Our fa- miliar friend the oyster is one of these. The first-known oysters appear in the Carboniferous in Belgium and in the United States of America. In the Carboniferous and Permian they are few and small, and they do not culminate till the Cretaceous, in which there are no less than ninety- one so-called species in America alone ; but some of the largest known species are found in the Eocene. The oyster, though an inhabitant of shallow water, and very limitedly locomotive when young, has sur- vived all the changes since the Carboniferous age, and has spread itself over the whole northern hemisphere. I have collected fossil oysters in the Cretaceous clays of the coulees of Western Canada, in the Lias shales of England, in the Eocene and Cretaceous beds of the Alps, of Egypt, of the Red Sea coast, of Judea, and the heights of Lebanon. Everywhere and in all formations they present forms which are so variable and yet so similar that one might suppose all the so- called species to be mere varieties. Did the oyster origiate separately on the two sides of the Atlantic, or did it cross over so promptly that its appearance seems to be identical on the two sides ? Are all the oysters of a common ancestry, or did the causes, whatever they were, which introduced the oyster in the Carboniferous, act over again in later periods ? "Who can tell ?
This is one of the cases where causation and development — the two scientific factors which constitute the bases of what is vaguely called