THE SEVEN AGES OF PLANT LIFE 37 



grains in the flooded river of one epoch differs from 

 that brought down by the river of a succeeding 

 epoch— though it might chance that the sandgrains 

 were the same identical ones. And hence it is by the 

 remains of the plants and animals in a rock that we 

 can tell to which epoch it belonged. Unless, of course, 

 ready-formed fossils from an earlier epoch get mixed 

 with it, coming as pebbles in the river in flood — but 

 that is a subtle point of geological importance which 

 we cannot consider here. Such cases are almost always 

 recognizable, and do not affect the main proposition. 



From the various epochs, the plants which have been 

 preserved as fossils are in nearly all cases those which 

 had lived on the land, or at least on swamps and marshes 

 by the land. Of water plants in the wide sense, in- 

 cluding both those growing in fresh water and those 

 in the sea, we have comparatively few. This lack is 

 particularly remarkable in the case of the seaweeds, be- 

 cause they were actually growing in the very medium 

 in which the bulk of the rocks were formed, and which 

 we know from recent experiments acts as a preservative 

 for the tissues of land plants submerged in it. It must 

 be remembered, however, that almost all the plants 

 growing in water have very soft tissues, and are usually 

 of small size and delicate structure as compared with 

 land plants, and thus would stand less chance of being 

 preserved, and would also stand less chance of being 

 recognized to-day were they preserved. The mark on 

 a stone of the impression of a soft film of a waterweed 

 would be very slight as compared with that left by a 

 leathery leaf or the woody twig of a land plant. 



There are, of course, exceptions, and, as will be 

 noted later on (see Chapter XVII), there are fossil sea- 

 weeds and fossil freshwater plants, but we may take it 

 on the whole that the fossils we shall have to deal with 

 and that give important evidence, are those of the land 

 which had drifted out to sea, in the many cases when 

 they are found in rocks together with sea shells. 



