146 
SPECIFIC AREAS 
certain kinds of trees ; climbers with a few exceptions can 
only go where there are erect plants or other supports; 
many plants depend upon insects to a greater or less extent, 
and the distribution of the latter affects that of the plants 
(see p. 94), and so on. 
Discontinuous Distribution ; Geological Influ- 
ences. A species having arisen upon any area will tend to 
spread over that area, but whether it ultimately covers it all 
or not will depend upon the various agents enumerated 
above. Many species have an enormous range, e.g. from 
Britain to Kamtschatka, others only a restricted one. A 
species becomes rarer and rarer towards the limit of its area, 
if this be determined by general climatic conditions, but 
may be as numerous at the limit as within the area if the 
limit be a natural barrier. As it spreads over a large area a 
species will often give rise to new varieties or to new species, 
and thus a genus will be formed, occupying a larger area 
than any one of its species, but nevertheless a continuous 
area. The same reasoning applies to a group of genera, 
or a natural order. How then does it come about that so 
many species, genera, and orders occupy disjointed areas at 
the present time, e.g. the Canellaceae, Hamamelidaceae, 
Styracaceae, Angelica, Bystropogon, Cedronella, Cedrus, 
Chiogenes, Desmanthus, Epigaea, Eriocaulon, Neotinea, 
Pachysandra, Shortia, &c. (see Part II.)? Again, how does 
it happen that many species are common to the various 
ranges of mountains in each temperate zone, yet do not 
occur in the intervening lowlands? and how is it that certain 
forms occur in temperate regions of both Europe and North 
America, though the bulk of the flora is so unlike in the 
two cases, and there is no possibility of an 4 accidental' 
transport over such a distance ? Accidental transport may 
account for a few cases in which the species-area is not very 
much disjointed, but will not explain the occurrence of 
many Japanese genera in atlantic America, or other such 
cases. It is possible that in some cases the group or genus 
under consideration is polyphyletic (p. 29), the different 
phyla having arisen in different regions from species already 
separated in space 1 . Again, as a species or family passes 
its zenith and begins to dwindle, the area occupied by it will 
1 Willis, Podostemaceae , Ann. Perad. I, 1902, p. 4 j.8. 
