294 MONTGOMERY. [Vor. XII. 
may take, for example, a species inhabiting a small island, 
which is comparatively uniform in character throughout its 
extent (in vegetation, etc.), and at all seasons of the year 
(climate, etc.). Such insular species are very numerous among 
land-inhabiting animals of tropical distribution. The environ- 
ment influencing the insular species being then so uniform and 
unchanging in its action upon it, the species could more easily 
and quickly adapt itself to it, than if the environment were 
changeable. Taking then an insular species, which, we have 
reasons to suppose, has inhabited a particular island for a com- 
paratively long period, —and there are usually certain criteria 
whereby we may judge whether its residence there has been of 
long duration, — we must suppose that it had time to become 
adapted to its environment; and if we have equally good 
reasons for supposing that the character of the environment 
itself has not changed, it must seem probable that its adaptation 
to the environment is comparatively perfect. Granting sucha 
species, accordingly, to be closely adapted to its environment, 
let us consider what changes, if any, would occur in the 
organism, if the environment should change. And it may be 
remarked just here, that marked changes have been recorded 
by geologists and others in different districts, as is well known, 
such as a surface rising or sinkage, changes in vegetation due 
to a prolonged drought, the invasions of organisms strange to 
the region, destruction of life caused by epidemics, etc. 
And in fact, in many districts where the environment appears 
to the human sense to be practically unchanging, a slow and 
gradual change may nevertheless be taking place. 
Now such an organism is adapted to its environment, at the 
same time that its various organs must necessarily be cor- 
related to one another. By correlation of the organs is meant, 
as was explained in the introductory part of this paper, their 
mutual dependence upon, and concerted physiological action 
with, each other. Thus, in the case under question, we must 
treat together the two facts: (1) the adaptation of the organism 
to its environment, and (2) the physiological and morphological 
correlation of its organs. When a change of environment 
occurs, z.e. when consequently a new and different environment 
