i 9 2 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 Coronula, or whale-barnacle, the 

 C. regince 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 numerous 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 we 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 tbese. 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 



