are selected.
The predominant survival of (a) entails the survival of the adaptive
variations which are inherited. The contributory acquisitions (+M) are
not inherited; but they are none the less factors in determining the
survival of the coincident variations. It is surely abundantly clear
that this is Darwinism and has no tincture of Lamarck's essential
principle, the inheritance of acquired characters.

Whether Darwin himself would have accepted this interpretation of some
at least of the evidence put forward by Lamarckians is unfortunately
a matter of conjecture. The fact remains that in his interpretation
of instinct and in allied questions he accepted the inheritance of
individually acquired modifications of behaviour and structure.

Darwin was chiefly concerned with instinct from the biological rather
than from the psychological point of view. Indeed it must be confessed
that, from the latter standpoint, his conception of instinct as a
"mental faculty" which "impels" an animal to the performance of certain
actions, scarcely affords a satisfactory basis for genetic treatment. To
carry out the spirit of Darwin's teaching it is necessary to link more
closely biological and psychological evolution. The first step towards
this is to interpret the phenomena of instinctive behaviour in terms
of stimulation and response. It may be well to take a particular case.
Swimming on the part of a duckling is, from the biological point
of view, a typical example of instinctive behaviour. Gently lower a
recently hatched bird into water: coordinated movements of the limbs
follow in rhythmical sequence. The behaviour is new to the individual
though it is no doubt closely related to that of walking, which is
no less instinctive. There is a group of stimuli afforded by the
"presentation" which results from partial immersion: upon this
there follows as a complex response an application of the functional
activities in swimming; the sequence of adaptive application on the
appropriate presentation is determined by racial preparation. We know,
it is true, but little of the physiological details of what takes place
in the central nervous system; but in broad outline the nature of the
organic mechanism and the manner of its functioning may at least
be provisionally conjectured in the present state of physiological
knowledge. Similarly in the case of the pecking of newly-hatched chicks;
there is a visual presentation, there is probably a coo