THE HISTORY OF THE NORTH ATLANTIC 85 



the Cambrian age onwards there were, on the two sides of the 

 ocean, many species of invertebrate animals which were either 

 identical or so closely allied as to be possibly varietal forms, in- 

 dicating probably the shallowness of the ocean in these periods. 

 In like manner, the early plants of the Upper Silurian, Devo- 

 nian, and Carboniferous present many identical species ; but 

 this identity is less marked in more modern times. Even in 

 the latter, however, there are remarkable connections between 

 the floras of oceanic islands and the continents. Thus the 

 Bermudas, altogether recent islands, have been stocked by 

 the agency chiefly of the ocean currents and of birds, with 

 nearly 150 species of continental plants; and the facts col- 

 lected by Helmsley as to the present facilities of transmission, 

 along with the evidence afforded by older oceanic islands 

 which have been receiving animal and vegetable colonists 

 for longer periods, go far to show that, time being given, 

 the sea actually affords facilities for the migration of the in- 

 habitants of the land, comparable with those of continuous 

 continents. 



In so far as plants are concerned, it is to be observed that 

 the early forests were largely composed of cryptogamous 

 plants, and the spores of these in modern times have proved 

 capable of transmission from great distances. In considering 

 this, we cannot fail to conclude, that the union of simple cryp- 

 togamous fructification with arboreal stems of high complexity, 

 so well illustrated by Dr. Williamson, had a direct relation to 

 the necessity for a rapid and wide distribution of these ancient 

 trees. It seems also certain that some spores, as, for example, 

 those of the Rhizocarps, 1 a type of vegetation abundant in 

 the Palaeozoic, and certain kinds of seeds, as those named 

 AZthoetesta and Pachytheca, were fitted for flotation. Further, 

 the periods of Arctic warmth permitted the passage around 



1 See paper by the author on Palaeozoic Rhizocarps, Chicago Trans., 

 1886. 



