PRESIDENTIAL ADDRESS. 397 
have very little bearing upon the problem of evolution. If we like to call the 
new groups of individuals that originate thus ‘ species,’ well and good, but it 
only means that we give that name, as a matter of convenience, to any group 
of closely related individuals which are distinguished by recognisable characters 
from the individuals of all other groups, and which hand on those characters to 
their descendants so long as the conditions remain the same. This, perhaps, is 
what we should do, and just as we have learnt to regard individuals as the 
temporary offspring of a continuous stream of germ-plasm, so we must regard 
species as the somewhat more permanent but nevertheless temporary offshoots 
of a continuous line of progressive evolution. Individuals are to species what 
the germ-plasm is to individuals. One species does not arise from another 
species, but from certain individuals in that species, and when all the individuals 
become so specialised as to lose their power of adaptation, then changes in the 
environment may result in the extinction of that line of descent. 
It is hardly necessary to point out that no explanation that we are able to 
give regarding the causes of either phylogenetic or ontogenetic evolution can be 
complete and exhaustive. Science can never hope to get to the bottom of things 
in any department of knowledge; there is always something remaining beyond 
our reach. If we are asked why an organism chooses the most appropriate 
response to any particular stimulus, we may suggest that this is the response 
that relieves it from further stimulation, but we cannot say how it learns to 
choose that response at once in preference to all others. If we are asked to 
account for some particular mutation, we may say that it is due to some modifi- 
cation in the constitution or distribution of the chromosomes in the germ-cells, 
but even if we knew exactly what that modification was, and could express it 
in chemical terms, we could not really say why it produces its particular result 
and no other, any more than the chemist can-say why the combination of two 
gases that he calls oxygen and hydrogen gives rise to a liquid that he calls water. 
There is one group of ontogenetic phenomena in particular that seem to defy 
all attempts at mechanistic interpretation. I refer to the phenomena of restitu- 
tion, the power which an organism possesses of restoring the normal condition 
of the body after it has been violently disturbed by some external agent. The 
fact that a,newt is able to regenerate its limbs over and over again after they 
have been removed, or that an echinoderm blastula may be cut in half and 
each half give rise to a perfect larva, is one of the most surprising things in the 
domain of biological science. We cannot, at present at any rate, give any satis- 
factory mechanistic explanation of these facts, and to attribute them to the 
action of some hypothetical Entelechy, after the manner of Professor Hans 
Driesch, is simply an admission of our inability to do so. Wecan only say that 
in the course of its evolution each organism acquires an individuality or whole- 
ness of its own, and that one of the fundamental properties of living organisms 
is to maintain that individuality. They are able to do this in a variety of 
ways, and can sometimes even replace a lost organ out of material quite different 
from that from which the organ in question is normally developed, as in the 
case of the regeneration of the lens of the eye from the iris in the newt. That 
there must be some mechanism involved in such cases is, of course, self-evident, 
and we know that that mechanism may sometimes go wrong and _ produce 
monstrous and unworkable results; but it is, I think, equally evident that the 
organism must possess some power of directing the course of events, so as 
generally to secure the appropriate result; and it is just this power of directing 
chemical and physical processes, and thus employing them in its own interests, 
that distinguishes a living organism from an inanimate object. 
In conclusion I ought, perhaps, to apologise for the somewhat dogmatic tone 
of my remarks. I must ask you to believe, however, that this does not arise 
from any desire on my part to dogmatise, but merely from the necessity of 
compressing what I wished to say into a totally inadequate space. Many years of 
patient work are still needed before we can hope to solve, even approximately, 
the problem of organic evolution, but it seemed to me permissible, on the 
present occasion, to indulge in a general survey of the situation, and see how 
far it might be possible to reconcile conflicting views and bring together a 
number of ideas derived from many sources in one consistent theory. 
