232 GEOGRAPHICAL DISTRIBUTION [pt. ii 



species and genera to come under this head, one must be satisfied 

 with a small number; to the great bulk the contention is not 

 applicable. We have seen, and seen it so strikingly in numerous 

 instances that there can be no doubt that it is a general rule, 

 that the species in a gi^en countr}^ endemic or not, are grouped 

 there (according to the areas that they occupy) in a perfectly 

 definite manner, Avhich is always the same. The wides are found 

 (when there are also endemics) with many in the class of largest 

 area, and numbers decreasing downwards, the endemics arranged 

 in the reverse direction. This regular arrangement is completely 

 opposed to the idea of relic nature, for how could there be many 

 at the last stage of relicdom, fewer at the last but one, still fewer 

 at the last but two, and so on? It is equally opposed to the idea 

 of local adaptation, it may be Avorth while to point out, for why 

 should there be many adapted to the smallest areas, with num- 

 bers steadily decreasing upwards. Still more difficult is it to 

 explain, upon either of these suppositions, why the wides (if 

 endemics occur also) should be arranged in the reverse direction^. 

 If there be special local adaptation, then the wides must be 

 much better suited to the country than the locally evolved 

 forms ! 



Inasmuch as all families and genera, of reasonable size, agree 

 in arrangement, some mechanical explanation is needed to ac- 

 count for the mechanical regularity, and the only reasonable one 

 suggested is age (for youth cf. pp. 89, 92). Age in itself, as already 

 explained, does nothing, but it allows time for the active factors 

 in distribution to produce their effect. To accept age as a mechani- 

 cal explanation simply means that we regard these factors as 

 producing a resultant or total effect which goes on at an average 

 speed, so that age becomes a measure of dispersal. The dispersal 

 is of course stopped sooner or later by barriers, physical or eco- 

 logical, including the barrier imposed by the fact that a species 

 has reached the extreme of temperature, dryness, etc., that it 

 can -vAithstand. The real difference between the old view of dis- 

 persal and that given by Age and Area is that imder the latter 

 we regard almost all species as in process of extending their areas 

 of dispersal, not some as extending their areas and as many or 

 more contracting theirs (cf. footnote on p. 174). The exceptions 

 to this — the real relics — are comparati\'ely few and far between, 



'^ When, as in Britain, there are no endemics, the wides diminish upwards, 

 but show considerable mmibers in the most widely dispersed classes, owing 

 to accumulation there of species that could not rise higher. 



