ORIGINATION OF SELF-GENERATING MATTER 297 



In all attempts to correlate ancient floras and interpret the climate 

 of formations, especially with regard to aridity, the following features 

 are to be taken into account: 



Vegetation of diverse lower types might cover moist lowlands, 

 make a profuse growth along streams, or clog extensive stretches 

 of shallow waters in seas and lakes, but only seed-plants could 

 occupy dry land. It is to be borne in mind that the forms represent- 

 ing this advanced type must have constituted a small proportion of 

 the vegetation for a long period after their origination. Their 

 present predominance must be a very modern feature. Furthermore, 

 the dissemination of new forms proceeds somewhat slowly and it is 

 by no means to be taken for granted that the existence of seed- 

 plants, as denoted by fossil remains, is to be taken as an indication 

 that such plants occupied or covered great continental areas. Soil 

 conditions would be a very important factor in such distribution. 



The distinction between the vegetation of a region in alternating 

 moist and arid epochs may not easily be made, since as has been 

 pointed out the fossilization of the flora of the Arizona Sonora desert 

 would probably result in material richer in moisture-requiring plants 

 than in xerophytes The morphological features of the forms pre- 

 served would offer the most valuable evidence, and the presence of 

 a single xerophyte among a hundred forms requiring moisture would 

 be of great significance. 



The final stages in the differentiation of the land flora, by which 

 spinose and succulent xerophytes have come into existence, seems to 

 have been reached within very recent times. No fossil remains of 

 desert plants have yet been recovered. Some of the forms which 

 have the aspect of xerophytes must have grown in moist regions by 

 reason of their method of reproduction. Some of the cycads and 

 the conifers may be regarded as being most suitable of the older 

 types for existence under arid conditions. The fitness of these 

 plants is due almost wholly to features of the shoot, and the known 

 features of their root-systems offer nothing suggestive of adapta- 

 bility for the characteristic soil conditions of the desert. 



