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Sinnott . — The * Age and Area ' Hypothesis 
emphasize the complexity of the whole problem and which throw doubt on 
the universal applicability of some of Dr. Willis’s conclusions. 
In the first place there seem to be many factors other than age which 
are important in determining the extent of territory over which a species is 
dispersed. No region of any considerable area can be said to be without 
c well-marked barriers ’ of many sorts, such as differences in temperature, 
moisture, and soil composition, and the presence of competing or parasitic 
types, all of which are recognized as powerful factors in limiting the area 
occupied by a species. Ecologists find that even plants growing side by 
side are frequently living under very different conditions, and that a region 
which is superficially uniform may actually present considerable environ- 
mental complexity and may possess many barriers to certain plant types. 
A species limited on all sides by effective barriers will not be able to extend 
its limits, no matter how long it may exist ; and with types which have thus 
reached the boundary of their possible ranges, area of dispersal will obviously 
afford no clue as to antiquity. A highly specialized form, occupying 
a relatively narrow ‘ ecological niche ’, may in reality be much older than 
one which from its greater adaptability under diverse environments is able to 
thrive over a wider area. 
Very many species, however, have apparently not yet attained by any 
means the extreme possible limits of their ranges, and are still expanding. 
It is among such types that the relation between age and area may be 
looked for ; but even here, there are such decided differences between 
plants in the rapidity with which they are able to extend their boundaries 
that no hard and fast rule can safely be laid down. A species with means 
for rapid dispersal will evidently overrun a wider area in a given length of 
time than will a more slowly moving type. 
Another factor of decided importance in determining the area occupied, 
and one which is perhaps worthy of special emphasis because it has usually 
been overlooked, is the growth habit of a species. A. de Candolle noticed 
many years ago that trees have narrow ranges and herbs wide ones. His 
list of 1 17 species which are found over at least half of the land area of the 
globe includes nothing but herbaceous types. This is probably due in part 
to the fact that most herbs are able to produce seed in a very short time and 
in very great abundance, and in part to the fact that their short vegetative 
period enables them to take advantage of temporarily favourable conditions 
and to thrive in many places where they would not be able to maintain 
a permanent existence above ground. It is of interest to note the distribution 
of the three important growth forms — trees, shrubs, and herbs — represented 
in Trimen’s ‘ Flora of Ceylon the data from which furnished the basis for 
the hypothesis under discussion. Willis divides the species of the island 
into three groups on the basis of the extent of their range: (1) those which 
are endemic or limited to Ceylon ; (2) those which are of somewhat wider 
