CH. xiii] D. GEOGRAPHICAL DISTRIBUTION 145 



time. These old forms, being simpler, would show less adaptation 

 to any particular conditions, but would probably show greater 

 adaptability. This conception agrees much better with the facts, 

 which go to show, as was pointed out by Darwin, that the organi- 

 sation of the widely dispersed species is definitely simple rather 

 than complex, when allies only are considered, as must always 

 be the case in general comparisons with regard to age (cf. p. 29) or 

 dispersal. 



All the facts that are known go to show that in the majority of 

 cases an individual plant arises in a place at no great distance 

 from that where its parent is to be found. If it survive, and grow 

 to the reproductive stage, one may conclude not only that chance 

 has favoured it, but also that it has probably passed through the 

 sieve of natural selection, and may be said to be more or less 

 suited to that locality. If the seed, however, be carried to a 

 greater distance than usual, say to more than 250 or 500 metres, 

 whether it prove so suited to its new locality as to survive and 

 reproduce there will depend upon a number of things. It may 

 find a good deal of difference in the soil, though not perhaps in 

 the climate, and if it has been carried beyond the range of the 

 particular association of plants in which it has been growing, 

 there may be considerable biological differences, which again may 

 be accompanied by soil changes and the like. It will then be a 

 matter of chance whether it prove suited to the new locality — to 

 talk of adaptation in a seed only newly arrived, though it may 

 prove suited to the place, would be going too far. If it survive to 

 the reproductive stage, it will probably have begun by that time 

 to adapt itself to its new surroundings. In each successive genera- 

 tion this adaptation will continue, until, after a time which is 

 probably different in each case, it has again become fully adapted 

 to local conditions. This process may continue until, after a very 

 long period, the species may cover, as does Hydrocotyle asiatica 

 (p. 58), a very large area of the surface of the globe. If we 

 abandon the notion that adaptation is shown by the structural 

 characters of plants, but that it is much more the physiological or 

 functional adaptation that must go on in any plant that moves 

 about and comes continually into new conditions, the supposi- 

 tion that we have just given explains, with the aid of age and 

 area, why species are arranged over the world in "wheels within 

 wheels", why the largest numbers are found upon the smallest 

 areas, and those that occupy larger areas decrease in a "hollow 



curve". 



WED 



