FKOM AX OLD STANDPOINT. 



179 



results of the differentiating change ! AVe can scarcely doubt 

 that the primitive aquatic family type of Xaias, durini^^ its 

 differentiation into 32 species now spread over the world, has 

 witnessed the development from the primitive type of the 

 Composita? of its 13 tribes, of its 730 genera, and of its 

 10,000* and odd species that now^ between them occupy the 

 land-surface of the globe. Other primitive family types have, 

 however, during this period disappeared in the differentiating 

 process, being only recognisable now in the common characters 

 of a group of closely related families that occupy between them 

 the area of the original family. Amongst such buried primitive 

 families we have mentioned the Scitaminese. In other cases, 

 again, even the connections between the secondary families 

 have disappeared, and we get solitary families restricted to 

 particular regions and standing aloof from nearly all their kind. 

 Of such families, that of the Pandanaceie has already been 

 cited as an instance. 



What, we may now ask, is the explanation of this unequal 

 rate of the differentiating process in the plant- world ? The 

 determining causes are to be found primarily in the lack of 

 u.niformity in the differentiation of the life-conditions, and 

 secondarily in the lack of uniformity in the operations of the 

 dispersing agencies. 



(1) The lack of uniformity in the differentiation of the life- 

 coiulitions. — Since that ancient period when similar conditions 

 of existence occurred over most of the earth and swamps 

 prevailed, the primitive life-conditions have been broken up to 

 a much greater extent for some kinds of plants than for others. 

 Thus, whilst the aquatic habit comes nearest to the primeval 

 condition, the terrestrial habit has differentiated in a thousand 

 ways on account of the great diversification of modes of life 

 and the resulting large number of possible combinations of all 

 that goes to determine the conditions for plant-life on the land. 

 Aquatic plants might therefore be expected to have clianged 

 much less rapidly than land plants ; and the more complete the 

 submergence, the slower would have been the change. Very 

 few, if any, of the families containing only aquatic or 

 subaquatic plants possess as many as 100 described species. 

 In most cases the number falls far short of this, Xaiadaceae 



* A generation (33 year:s) has passed away since the publication of 

 Mr. Bentham's monograph on this family, when the described species 

 were placed at about 9,400. The number must have been considerably 

 increased since that date. 



