xxv] AQUATICS, NEW AND OLD 321 



Sympetalous series, contains apparently only four aquatic mem- 

 bers^. Exactly the same is true of all the earlier G^horts of 

 Engler's Archichlamydeae, which, on the present writer's view, 

 represent the more advanced and reduced forms of the Series. 

 The families which are generally known as Polypetalae (the 

 later Cohorts of Engler's Archichlamydeae), and which, on the 

 view here adopted, include all the more primitive Dicotyledons, 

 are markedly richer in aquatic types. It would hardly be going 

 too far to say that independent aquatic families are chiefly 

 characteristic of the Ranalean plexus, and of its derivatives 

 both Dicotvledonous and Monocotvledonous while among the 

 more advanced Polypetalae, and the Sympetalae, the sporadic 

 occurrence of aquatic types and their close relation to terrestrial 

 forms, indicate that the water-habit has been acquired com- 

 paratively recentlv. It is always possible that those individual 

 genera and species among the Sympetalae which are hydro- 

 phvtic at the present day, may each, in some future age, be 

 represented by an entire aquatic family, for such groups as the 

 Helobieae, Xymphaeaceae and Podostemaceae may owe their 

 richness in genera and species partly to their ancient birth and to 

 the length of time that has elapsed since they took to the water. 

 But, on the other hand, a member of the Sympetalae embarking- 

 at the present day upon an aquatic career, may possibly be 

 handicapped, as a potential ancestor, by the high degree of 

 specialisation it has attained in its pre\'ious terrestrial Hfe. The 

 members of the primaeval Ranalean plexus mav have possessed 

 a greater plasticity in correlation with their lower degree of 

 specialisation. It must also be remembered that the more pri- 

 mitive Angiosperms, which entered the water at an earlv period, 

 had merely to take possession of a field undisputed by other seed 

 plants, whereas species embarking on an aquatic life at the 

 present day are exposed to acute competition from the numerous 

 well-established hydrophytes with which the fresh waters of the 

 world are already so fully stocked ^. 



1 Hutchinson, J. (1916). 



2 Since this chapter was written, I have learned that some of mv 

 conclusions were anticipated bv Boulger, G. S. (igccj. 



A. w. p. . 21 



