16 J.D. Hooker, Introductory Essay to the Flora of Tasmania. ; 
14, The most prominent feature in distribution is that cireum- 
scription of the area of species, which so forcibly suggests the — 
hypothesis that all the individuals of each species have sprung — 
from a common parent, and have spread in various directions — 
from it. It is true that the area of some (especially cryptogamic — 
and aquatic plants) is so great that we cannot indicate any appa- — 
rent centre of diffusion, and that others are so sporadic that they — 
appear to have had many such centres; but these species, though — 
more numerous than is usually supposed, are few in comparison 
with those that have a definite or circumscribed area. a 
With respect to this limitation in area,* species do not essen: — 
tially differ from varieties on the one hand, or from genera and 
higher groups on the other; and indeed, in respect of distribu- 
tion, they hold an exactly intermediate position between them, 
varieties being more restricted in locality than species, and these 
again more than genera. 
The universality of this feature (of groups having defined 
areas) affords to my mind all but conclusive evidence in favor 
of the hypothesis of similar forms having had but one parent, 
or pair of parents. And further, this circumscription of spécies 
and other groups in area, harmonizes well with that principle of 
divergence of form, which is opposed to the view that the same 
exists in the well-known fact that plants do not necessarily in- 
habit those areas in which they are constitutionally best fitted 
to thrive and to propagate; 
would most like to, but. wh 
enemies. We have seen (13) that most plants are at warfare 
_ * It is a remarkable fact that there are some striking anomalies in the distribu- 
tion of plants into provinces, as com i i i i 
as, should lead us to anticipate that plant types are, geologi ki 
ng, 
types are, and so] believe 
ace , and I would extend the doctrine even to plants of highly complex 
structure, 
