CHAPTER XVIII.— SUCCESSION OF VEGETABLE LIFE. 399 



tics which now belong to groups quite distinct from each other. 

 For example, the Lepidodendrids of the Devonian and Carbon- 

 iferous, while, on the whole, more nearly related to the Club- 

 mosses than to any other modern type, had affinities also with 

 the Conifers, not only in the structure of their leaves and stems, 

 but in the fact that they were heterosporous. We may well 

 believe that both Conifers and Club-mosses may have been 

 developed from them by gradual differentiation. 



Against the evolution view has been urged the apparent sud- 

 denness with which certain types of plants have made their 

 appearance. The considerations already given, showing why the 

 record must be incomplete and fragmentary, afford a sufficient 

 answer to most objections of this kind. But the apparently 

 abrupt advent of Dicotyledons in great numbers in the latter 

 part of the Cretaceous calls for a different explanation, for 

 between the underlying rocks and these, there is no record of a 

 lost interval ; the one lie conformably upon the other. 



Nor does the theory that the process of evolution goes on 

 much more rapidly during periods of great change in physical 

 geography afford an adequate explanation, for the period does 

 not appear to have been one of great disturbance. The most 

 plausible theory offered in explanation is, that the new flora, so 

 unrelated to that found in the underlying rocks, was an invasion 

 from the north, similar to the arctic invasion that occurred 

 during the ice epoch, and produced by similar causes, only the 

 change of temperature which caused the migration was probably 

 less extreme. The immediate predecessors of this flora are, 

 therefore, to be sought not in the same localities but in the now 

 frozen and inaccessible north. 



It is theoretically probable that the first portions of our globe 

 to become sufficiently cooled to support life would be the poles, 

 and it is not improbable that living beings may have swarmed 

 here in myriads, before the now temperate and tropical regions 

 of the world were sufficiently cool to be habitable. From here, 

 as the earth became cooler, they migrated toward the equator. 

 The theory, however, is not unsupported by facts. There is 

 strong evidence derived from the study of the present distribu- 

 tion of plants, in favor of the view that our present floras, at least, 

 originated at the north. Not only do we find in the rocks of 



