180 H. C. GUPPY, M.B., F.R.S.E., ON PLANT-DISTPvIBUTION 



having 32 and Sparganiacea^ only 15 ; while Alismaceif and 

 Potameo} each possess about 70. On the otlier hand the 

 species of many land genera amount to hiuidreds, and tliose of 

 the families not infrequently mount to thousands. The vast 

 family of the Eubiacere contains genera like Psychotria that 

 comprise between 600 and 700 described species. Those of the- 

 Compositiv number about 10,000 ; whilst those of the 

 Leguminosa^; Labiata?, and Graniineie run also into thousands. 

 However, taking the small families witli the large, we should 

 be well within the mark if we assumed that for every aquatic 

 species ten land species have been developed, or, in other 

 words, that the dift'erentiation has been ten times as rapid 

 among land plants. From a rough computation I should 

 imagine that the average number of species in a land family 

 would not be less than 600 and in an aquatic family not more 

 than 40, so that the assumption errs on the safe side. 



Although the family type represented in tlie aquatic genus 

 Naias must be very ancient, it can scarcely be said to have 

 advanced beyond the first stage of differentiation.* The tardy 

 differentiation of aquatic plants is primarily due to the slow 

 response of their conditions of existence to the secular 

 differentiation of the earth's climate : but this retarding 

 influence has been intensified by their freedom of dispersal 

 through the agency of waterfowl. This brings me to the second 

 cause of the unequal rate of differentiation over the world. 



(2) The lack of }iniformit}f in tlie operations of the (lisiKTsinrj 

 agencies. — Although this cause is the least important of the 

 two, it is necessary to discuss it because the standpoint 

 adopted may not be familiar to all. It is well known that 

 isolation favours differentiation, and, since all means of 

 dispersal tend to retard this process, it follo\Cs that the agents 

 which; like the winds and currents, have been most uniform in 

 their operation in space and time will, as a rule, have been most 

 effective in retarding change, wiiilst those which have been 

 irregular in their action, as in the case of birds, will have been 

 less effective in checking the process. This difference, however, 

 is one of degree and not of kind, since all nature has responded 

 to the secular differentiation of climate and to the diversifica- 

 tion of the surface conditions, the winds, the currents, and 

 birds alike, but organized beings most of all. 



* " Naias forms a distinct and apparently pi iniitive t_v])e of Mono- 

 cotyledon " (Piendk' in the nK)nograph on the Naiadacea', Ars Pflanzcnreicli, 

 iv/l2, 1901). 



