The Problems of Ecology. 193 
fact that meets us is its diversity. This diversity, however, is an 
ordered diversity. It is due in the first place, of course, to the 
enormous number of different species of plants that exist, but 
these species are not mixed haphazard upon the surface of the 
earth—they are associated in aggregates which are the result of 
perfectly definite and ascertainable, though often immensely 
complex, causes. Some of these causes may be called phylogenetic. 
Certain stocks or families of plants are confined to certain regions 
of the earth because their ancestors were evolved in those regions 
and have not succeeded in spreading beyond them. Thus many 
great families are exclusively tropical and subtropical e.g. the 
Myrtaceee, the Melastomacece, the Scitaminece. Others are mainly or 
exclusively temperate, e.g. Ranunculacece, Pafaveracece, Cniciferce, &c. 
There is no reason to assume that there is anything inherent in 
the constitution of the members of many of these families which 
prevents their spreading into those regions of the earth where they 
are not in fact found. We must rather suppose that having first 
appeared and obtained a footing in a certain region they have 
either not as yet had time to spread out of those regions, or (and 
this is probably the explanation in the majority of cases) they have 
failed to widen the limits of their distribution owing to the compe¬ 
tition of the occupants of the other regions of the earth. In some 
cases, however, of highly specialised families, of which we may take 
the Podostemacece and the Balanoflioracece as extreme instances, the 
members of the family have become so highly specialised to 
particular circumstances of life as to preclude the possibility of 
their spreading beyond the area of prevalence of these particular 
environmental conditions. 
On the other hand many families are cosmopolitan. Among 
the lower forms of plant life, as for instance the Algae and Fungi, 
not only is this true of the immense majority of the families, it is 
true also of many genera and species. The generic and even the 
specific identity of the algae inhabiting the freshwaters of the 
Tropics with those of an English pond or stream is a remarkable 
fact, in striking contrast with the extreme rarity of generic identity 
between the members of the Phanerogamic floras of temperate and 
tropical regions. Here again we may distinguish two types of 
cause. In the case of some of the lower plants it may be that the 
immense time which has elapsed since they appeared on the 
surface of the earth has enabled them to spread over practically 
the whole globe, but the determining cause in the majority of cases 
is in all probability the ease with which they are distributed and 
