3i8 SYSTEMATIC DISTRIBUTION [ch. 



bered that the entire area of the fresh waters of the globe is very- 

 small as compared with the land surfaces, and that thus the 

 aquatic Angiosperms occupy a much more restricted field than 

 their terrestrial compeers. 



The mode of systematic distribution of aquatics among the 

 Angiosperms shows every possible variety. In the earlier part 

 of this chapter we have pointed out that among the Dicotyledons 

 there are cases in which one species of a terrestrial genus is 

 aquatic (e.g. Polygonum amphibium)^ and others in which a num- 

 ber of species in a genus are hydrophytic while some are terres- 

 trial (e.g. Ranunculus with its aquatic sub-genus Batrachium). 

 Again, an entire genus of an otherwise terrestrial family may be 

 aquatic (e.g. Hottonia among the Primulaceae) or several genera 

 of a land family may be aquatic (e.g. Jussiaea^ Ludwigia^ etc. 

 among the Onagraceae, and Limosella^ Hydrotriche^ etc. among 

 the Scrophulariaceae). Finally, an entire family may be aquatic 

 and contain no terrestrial forms (e.g. Podostemaceae). A family 

 given over wholly to aquatic life may include a number of 

 genera (e.g. Nymphaeaceae and Podostemaceae) or a single 

 genus (e.g. Ceratophyllaceae and Callitrichaceae). Among the 

 Monocotyledons, on the other hand, we meet with more cases 

 of entire families leading a water life (e.g. Lemnaceae, Pontede- 

 riaceae, and various families belonging to the Helobieae), but 

 there are fewer instances of individual aquatic genera and 

 species belonging to families which are mainly terrestrial, though 

 these occasionally occur (e.g. Glyceria aquatica of theGramineae). 



When one genus or species in an otherwise terrestrial family 

 has taken to aquatic life, this may well be held to indicate that 

 the habit is a recent one; but when a whole family, containing 

 a number of genera, is found to be hydrophytic, it is hardly 

 possible to avoid the conclusion that the differentiation of the 

 genera has occurred since the adoption of the aquatic habit, 

 which, on this view, must be very ancient. The only other alter- 

 native, namely that all the genera have been evolved in the 

 course of terrestrial life, and that they have all subsequently 

 and independently taken to the water, seems too far-fetched to 



