90 Proceedings of the Koyal Society of Edinburgh. [Sess. 
types of structure. In each case the structure of the individuals bears 
a relation of adaptation to certain physical conditions, also limited in 
number, which constitute their environment. In a general way this 
has been fully recognised by morphologists, but the essentially physio- 
logical element involved in adaptation is not always so clearly appre- 
hended. It frequently happens that animals of different structural type 
are subjected to one and the same change of environment. Each has 
now the same physiological problem (or the same group of problems) to 
solve. The solution may or may not involve gross change in structure. 
As an example I shall take a particular change in environment, which 
from its frequent occurrence has been and still is of first-class importance, 
viz. change from an aquatic to a land life. We shall restrict our attention 
to the effect upon the body-covering. The surface even of aquatic animals 
tends to become specialised into two regions — (1) respiratory and (2) 
general integumentary. If land adaptation, which involves risk of 
evaporation from the body-surface, is to occur, the general integument 
becomes absolutely or relatively impermeable to water (cf. Amphibia 
with Sauropsida and Mammalia). Even histologically such a change 
might be considered trivial. In order to permit the passage of gases 
the respiratory surface on the other hand has to remain moist.* We 
have only to consider the various groups of arthropods or of vertebrates 
to realise what profound structural changes may eventually be traced to 
the necessity of complying with this physiological principle. 
While incidentally mentioning that this mode of viewing the 
respiratory organs might have set at rest Cuvier’s difficulty with regard 
to them from the point of view of analogy (see Lucas’s paper), I wish 
particularly to press home the fact that the whole process of structural 
or phylogenetic development is itself subordinate to the operation 
of external physical conditions, constituting the environment. The 
experimental analysis of these conditions, in so far as they affect the 
organism, has always been the business of physiology. It must also be 
the business of physiology (or at least of physiological technique) to 
investigate the relation between the unfolding structure and this 
controlling environment. 
On every hand there are indications that the operation of the 
environment in modifying structure may be more direct than is assumed 
in the Darwinian principle of adaptation by elimination of unsuitable 
variations. The serial homoplasies, cases of reversion of all kinds (eyes, 
* This assertion, partly a post hoc one, is worthy of treatment by the general 
physiologists. 
