32 REPORT— 1886. 



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 when young, which they lose when 

 adult, and that others may have special means of transit. 1 once found 

 at Gaspo a specimen of the Pacific species of Coronula, or whale-barnacle, 

 the 0. reginm of Darwin, attached to a whale taken in the Gulf of St. 

 Lawrence, and which had probably succeeded in making that passage 

 around 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 numerous species 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 we find such genera as the horse, the 

 camel, and the elephant appearinj^ on the two sides of the Atlantic ; but 

 even then the species seem difierrnt, except, in the case of a few northern 

 forms. 



Some of the longer-lived mollnsks of the Atlantic furnish suggestions 

 which remarkably illustrate the biological aspect of these questions. 

 Our familiar 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 Cref ceous, 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 Cre- 

 taceous 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 

 terms which are so variable and yet so similar that one might suppose all 

 the so-called species to be mere varieties. Did the oyster originate 

 separately en 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 r Who can tell ? This is one of the cases where 

 causation and development — the two scientific factors which jonstitute the 



' White, Report U.S. Qeol. Survey, 1882-83. 



