CAMBRIAN AND LOWER SILURIAN ROCKS. 555 



inferior classes, yet they are not the lowest types in those classes, but 

 often show evidences of considerable progression in development. 

 On this account I have often expressed the opinion that we were far 

 from the beginning of this type of life even in the earliest Cambrian 

 faunas, and that the forms had already undergone many changes 

 previous to this period. It is easy now to see how these changes 

 could have taken place, and moreover how it was that new forms 

 so frequently appeared at certain stages highly developed and 

 with no previous evidence in the rocks as to the changes they had 

 undergone. The home of the earliest forms of life seems to have 

 been somewhere towards the south-west, and possibly not far from 

 the equator ; and it is from here that the various forms seem to 

 have migrated to the areas in which they were subsequently en- 

 tombed. The migrations seem to have taken place towards the North- 

 American continent very much about the same time as towards the 

 European; and the sea-encroachments along that continent seem to 

 have been in a direction from south-east to north-west, so that the 

 lines indicating the two depressions would meet in mid- Atlantic. 

 This accounts for the great similarity in the two faunas, and for the 

 general resemblance offered by the order of succession of these early 

 rocks in the two continents. The higher lands in America would be 

 to the west and north-west, and the higher lands in Europe to the 

 east and north-east ; so that the last lands submerged would approach 

 each other and occupy the same region of the globe. This land would 

 doubtless be clothed with plants before it was submerged ; but subse- 

 quent marine denudation would remove all traces of vegetation. 

 The plant-remains preserved in the Upper Silurian and Devonian are 

 indications of the general character and the state of progression of 

 the vegetation at that time. In the Cambrian rocks the evidence of 

 land vegetation is imperfect, though I am of the opinion that some 

 markings now visible on these rocks were produced by land plants 

 which were then washed from the pree-Cambrian lands as the waters 

 encroached upon them. It is impossible in any other way to 

 account for the rich vegetation of the Carboniferous period or for the 

 progression which had then taken place in vegetation. The con- 

 ditions which would allow the waters to teem with life in earlier 

 periods, would also enable progression to take place in animal and 

 vegetable life on the land. When, therefore, the European continent 

 again appeared above the water, which took place by the level- 

 ling of a large surface of the globe through the combined action of 

 denudation and the heaping up of material, aided in some cases 

 by upward movements in the earth's crust, the vegetation spread 

 rapidly, and enormous areas were soon covered with forests, and 

 tenanted by air-breathing invertebrates and vertebrates, which mi- 

 grated thither from regions not then reached by the water. 



If I am correct in my suppositions that the sea encroached from 

 the west, that the first faunas were brought in from that direction, 

 and that the subsequent faunas in these areas also migrated along 

 the same line, then, especially if it is true that the whole of the 

 Cambrian and Lower Silurian rocks form a truly conformable series, 



