234 GEOGRAPHICAL DISTRIBUTION [pt. ii 



Area with its implication that all, or nearly all, species are in 

 process of enlarging their area of dispersal, not some enlarging 

 and some contracting it. There is no need to quote the evidence 

 a second time (cf. Chapter xii, and pp. 132, 16-i, 165, 174, 178, 

 188, 190, 197). 



But if these new views be accepted, it is clear that a good 

 many changes must take place in our mode of viewing the 

 problems of distribution, which it must not be forgotten ha\e 

 hitherto been regarded as insoluble. One of the chief among 

 these is the problem of Invasions of plants from other countries. 

 If it be supposed that the dispersal of a species depends simply 

 upon its age (representing the average effect of the active factors) 

 and the barriers that it meets, and that when once it is estab- 

 lished in any place it will rarely die out there except as the result 

 of rather sudden or violent changes of conditions^, and further 

 that only when these changes attack it at the margin of its area 

 will they cause any diminution of total area "occupied," then 

 it is clear that the problem of invasions can be studied with some 

 hope of obtaining results. This has been illustrated in Chapter 

 VIII, which deals with the invasions of New Zealand. It was there 

 shown that by taking the places at which the maxima of species, 

 endemic and wide, occur, one may get a cl\ie to the different 

 invasions that have reached the country, and the directions from 

 which they came. But in a coimtry Avithout any endemics at all. 

 the same princi])lcs may be applied to its " wides." This has lately 

 been done for Britain by Mr J. R. Matthew, whose work (74) 

 gives great promise for the future (and cf. p. 114). Careful account 

 must be taken of the conclusions of geology, but if we get rid 

 of the ideas that (proportionately) mamj species are necessarily 

 dying out, and that most have reached their possible limits of 

 dispersal, we can study invasion and spread with some hope of 

 arriving at definite results, a proceeding wliich has been im- 

 possible under the older views of these matters. 



If genera give rise to others in a casual way, and at more or 

 less casual spots (as the way in which the endemic genera in any 

 country occur at scattered points would seem to indicate), then 

 it is clear that in any part of the world one must expect to find 

 a casual mixture of genera of different sizes, made up in much 

 the same way as is the entire flora of the world, or one of the 



1 E.g. the oncoming of excessive cold, heat, dryness, dampness ; clearance, 

 fire, submergence, etc. 



