230 GEOGRAPHICAL DISTRIBUTION [pt. ii 



no breaks in the continuity of the figures, and that not only on 

 the totals, but in individual families and genera. We have also 

 seen that there is no need for rapid dispersal, when the time 

 available is considered (cf. p. 33). 



It is thus fairly clear that the existing distribution of species 

 and genera, in probably the great majority of cases, represents 

 only the dispersal possible in the time that has elapsed since their 

 evolution. If one could return to the world after ten thousand 

 years, one might find an appreciable extension of their area l^y 

 existing species, but to expect it in a short time is more sanguine 

 than reasonable. 



The fact that the composition and distribution of the floras of 

 the outlying islands of New Zealand can to a large extent be 

 predicted from a knowledge of the distribution in New Zealand 

 of the New Zealand flora (pp. 66-75) is a very strong argument 

 indeed in favour of the view that dispersal depends chiefly upon 

 age, i.e. that it is determined by various factors which when one 

 deals with long periods are found to act at a more or less uniform 

 speed, and that consequently the existing dispersal of species 

 does not represent the end of the chapter, but only the point 

 which has so far been reached. 



If one accept the two suppositions under discussion, it is quite 

 impossible to explain numerous facts in distribution which are 

 easily explained by aid of Age and Area, for instance, the fact 

 that the Auckland Islands have -i5 per cent, of their flora mono- 

 cotyledonous, the Chathams 31 per cent., and the Kermadecs 

 only 21 per cent.; or that the plants of the floras of these out- 

 l}'ing islands (p. 67) are unusually widespread in New Zealand, 

 and those of the Chathams much more so than those of the 

 Aucklands and the Kermadecs. It is impossible with these sup- 

 positions to do any prediction about distribution at ah, whereas 

 nearly a hundred predictions have already been successfully 

 made with the assistance of Age and Area, and have added con- 

 siderably to our knowledge of the distribution of plants in the 

 New Zealand area. 



In regard to the third hypothesis (p. 229), the supposition that 

 species and genera occupy just those places to which they are 

 suited has usually been taken for granted, and a vast amount of 

 energy has been devoted to the problem of finding out why they 

 are suited. But, as has just been pointed out, we can no longer 

 safely draw this conclusion. If a species is not suited to its loca- 

 tion, it will probably die out, as is apparently happening with 



