D.— ZOOLOGY. 79 
But all environmental stimuli are not external to the organism. 
Just as the various steps in the metabolic process are dependent on 
those which preceded them, so when an organism becomes differentiated 
into parts, when the main process becomes sub-divided into subsidiary 
ones, these react on each other. What is internal to the whole becomes 
external to the part. An external stimulus may set up an internal 
metabolic change, giving rise to a response whose extent and nature 
depend on the structure of the mechanism and its state when stimulated, 
that is to say, on the effect of previous responses. Such a response may 
act as an internal stimulus giving rise to a further response, which may 
modify the first, and so on. Parts thus become marvellously fitted to 
set going, inhibit, or regulate each other's action; and thus arises that 
power of individual adaptation, or self-regulation, so characteristic of 
living organisms. The processes of temperature regulation, of respira- 
tion, of excretion are examples of such delicate self-regulating 
mechanisms in ourselves. But one of the great advantages thereby 
gained by organisms is that they can regulate their own growth and 
ensure their own ‘right ’ development. Whereas the simplest plants 
and animals are to a great extent, so to speak, at the mercy of their 
external environment, except in so far as they can move from unfayour- 
able to more favourable surroundings ; whereas their characters appear 
in response to external stimuli which may or may not be present, and 
eyer which they have little or no control—the higher organisms (more 
especially the higher animals), as it were, gradually substitute internal 
for external stimuli. Food material is provided.im. the ovum, and the 
size, structure and time of appearance of various characters. are regu- 
lated to a great extent by use and by the secretions of various endocrinal 
glands, the action of which has been so successfully studied, among 
others, by Sir Sharpey Schafer in this University. Thus, as is well 
shown in man, the higher animals acquire considerable independence, 
and are little affected in their development by minor changes of 
environment. Inheritance is thus made secure by ensuring that the 
necessary conditions are always present. 
We may seem to have wandered far from our original question ; but 
the answer now appears to be that only those characters can be regu- 
larly inherited which depend for their appearance on conditions always 
fulfilled in the normal environment (external or internal); and those 
characters will not be regularly inherited which depend on stimuli that 
may or may not be present. Thus, while the offspring of a dark- 
skinned race will be dark in whatever climate they are born, those of a 
fair-skinned race will be born fair, but may be darkened by sun-burn, 
if they spend their holiday in the open. 
Now it will be said, and not without some truth, that all this is 
mere commonplace admitted by all; but, if so, it is, I think, often 
ignored or misunderstood in discussions on heredity, more especially 
in semi-popular writings, and sometimes even in’ scientific works. 
However, I quite willingly admit that the real problems Darwin left to 
‘be solved by the evolutionist are the nature of the germinal factors 
themselves, and more especially the origin of the differences between 
them. the origin of those changes which give rise to mutations. 
