622 D. T. MACDOUGAL 
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. 
