GEOLOGY OF THE ATLANTIC OCEAN. i 9 i 



age onward 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 like manner the early plants of 

 the Upper Silurian, Devonian, and Carboniferous present many iden- 

 tical species ; but this identity becomes less marked in the vegetation 

 of the more modern times. 



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 

 for great distances. In considering this, we can not fail to conclude 

 that the union of simple cryptogamous fructification with arboreal 

 stems of high complexity, so well illustrated by Dr. Williamson, had 

 a direct relation to the necessity for rapid and wide distribution of 

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

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

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

 and Pachytheca, were fitted for flotation. Further, the periods of 

 Arctic warmth permitted the passage round the northern belt of many 

 temperate species of plants, just as now happens with the Arctic flora ; 

 and when these were dispersed by colder periods they marched south- 

 ward along both sides of the sea on the mountain-chains. The same 

 remark applies to northern forms of marine invertebrates, which are 

 much more widely distributed in longitude than those farther south. 

 The late Mr. Gwyn Jeffreys, in one of his latest communications to 

 this Association, stated that fifty-four per cent of the shallow-water 

 mollusks of New England and Canada are also European, and of the 

 deep-sea forms thirty out of thirty-five ; these last, of course, enjoying 

 greater facilities for migration than those which have to travel slowly 

 along the shallows of the coasts in order to cross the ocean and settle 

 themselves on both sides. Many of these animals, like the common 

 muscle and sand-clam, are old settlers which came over in the Pleisto- 

 cene period, or even earlier. Others, like the common periwinkle, seem 

 to have been slowly extending themselves in modern times, perhaps 

 even by the agency of man. The older immigrants may possibly have 

 taken advantage of lines of coast now submerged, or of warm periods, 

 when they could creep around by the Arctic shores. 



Mr. Herbert Carpenter and other naturalists employed on the Chal- 

 lenger collections have made similar statements respecting other ma- 

 rine invertebrates, as, for instance, the Echinoderms, of which the 

 deep-sea crinoids present many common species, and my own collec- 

 tions prove that many of the shallow-water forms are common. Dall 

 and Whiteaves have shown that some mollusks and Echinoderms are 

 common even to the Atlantic and Pacific coasts of North Amei'ica ; a 

 remarkable fact, testifying at once to the fixity of these species, and 

 to the manner in which they have been able to take advantage of geo- 

 graphical changes. Some of the species of whelks common to the 



