PRESIDENTIAL ADDRESS. 389 
cient to justify us in believing that every successive modification must have been 
due to a response on the part of the organism to some environmental change. 
Even if the external conditions remained practically identical throughout long 
periods of time, we must remember that the internal conditions would be 
different in each generation, because each generation starts with a slightly 
increased capital and carries on its development a little further under internal 
conditions modified accordingly. 
At this point it may be asked, Is the response to environmental stimuli a purely 
mechanical one, and, if so, how can we account for the fact that at every stage 
in its evolution the organism is adapted to its environment? We shall have to 
return to this question later on, but it may be useful to point out once more 
that there is good reason to believe—especially from the experimental work of 
Jennings—that the response of even a unicellular organism to stimuli is to a 
large extent purposive; that the organism learns by experience, by a kind of 
process of trial and error, how to make the response most favourable to itself 
under any given change of conditions; in other words, that the organism selects 
those modes of response that are most conducive to its own well-being. Under the 
term response to stimuli we must of course include those responses of the living 
protoplasm which result in modifications of bodily structure, and hence the 
evolution of bodily structure will, on the whole, be of an adaptive character 
and will follow definite lines. There is good reason for believing, however, that 
many minor modifications in structure may arise and persist, incidentally as it 
were, that have no significance as adaptations. 
One of the most remarkable and distinctive features of the lower vertebrates 
is the presence of gill-slits as accessory organs of respiration. These gill-slits 
are clearly an adaptation to aquatic life. When the ancestors of the higher 
vertebrates left the water and took to life on land the gills disappeared and 
were replaced by lungs, adapted for air-breathing. The change must, of course, 
have been an extremely gradual one, and we get a very clear indication of how 
it took place in the surviving dipnoids, which have remained in this respect in 
an intermediate condition between the fishes and the amphibia, possessing and 
using both gills and lungs. 
We also know that even the most highly specialised air-breathing vertebrates, 
which never live in water and never require gills or gill-slits at all, nevertheless 
possess very distinct gill-slits during a certain period of their development. This 
is one of the most familiar illustrations of the law of recapitulation, and my 
only excuse for bringing it forward now is that I wish, before going further, to 
consider a difficulty—perhaps more apparent than real—that arises in connection 
with such cases. 
It might be argued that if gill-slits arose in response to the stimuli of aquatic 
life, and if these stimuli are no longer operative in the case of air-breathing 
vertebrates, then gill-slits ought not to be developed at any stage of their 
existence. This argument is, I think, fully met by the following considerations. 
_ At any given moment of ontogenetic development the condition of any organ 
is merely the last term of a series of morphogenetic stages, while its environ- 
ment at the same moment—which of course includes its relation to all the other 
organs of the body—is likewise merely the last term of a series of environmental 
stages. We have thus two parallel series of events to take into consideration in 
endeavouring to account for the condition of any part of an organism—or of the 
organism as a whole—at any period of its existence :— 
EH, E, E; ; : : A 5 . En environmental stages. 
M, M, M; - - f : : - Mn morphogenetic stages. 
Ontogeny is absolutely conditioned by the proper correlation of the stages of 
these two series at every point, and hence it is that any sudden change of 
environment is usually attended by disastrous consequences. Thus, after the 
fish-like ancestors of air-breathing vertebrates had left the water and become 
amphibians, they doubtless still had to go back to the water to lay their eggs, 
in order that the eggs might have the proper conditions for their development. 
Obviously the environment can only be altered with extreme slowness, and 
one of the first duties of the parent is to provide for the developing offspring 
conditions as nearly as possible identical with those under which its own develop- 
