634 TRANSACTIONS OF SECTION I, 
a machine played upon by environmental events and reacting thereto in a way 
determined by its chemical and physical structure. 
Can we not learn something of value in our common life by adopting this 
objective point of view and regarding man as the latest result of a continuous 
process of evolution which, begun in far-off ages, has formed, proved, and rejected 
myriads of types before man himself appeared on the surface of the globe ? 
Adaptation. 
In his study of living beings the physiologist has one guiding principle which 
plays but little part in the sciences of the chemist and physicist, namely, the 
principle of adaptation. Adaptation or purposiveness is the leading charac- 
teristic of every one of the functions to which we devote in our text-books the 
chapters dealing with assimilation, respiration, movement, growth, reproduc- 
tion, and even death itself. Spencer has defined life as ‘ the continuous adjust- 
ment of internal relations to external relations.’ Every phase of activity in a 
living being is a sequence of some antecedent change in its environment, and is 
so adapted to this change as to tend to its neutralisation and so to the sur- 
vival of the organism. This is what is meant by adaptation. It will be seen 
that not only does it involve the teleological conception that every normal activity 
must be for the good of the organism, but also that it must apply to all the 
relations of living beings. It must therefore be the guiding principle, not 
only in physiology, with its special preoccupation with the internal relations of 
the parts of the organism, but also in the other branches of biology, which treat 
of the relations of the living animal to its environment and of the factors which 
determine its survival in the struggle for existence. Adaptation therefore must 
be the deciding factor in the origin of species and in the succession of the 
different forms of life upon this earth. 
Origin of Life. 
A living organism may be regarded as a highly unstable chemical system 
which tends to increase itself continuously under the average conditions to 
which it is subject, but undergoes disintegration as a result of any variation 
from this average. The essential condition for the survival of the organism 
is that any such disintegration shall result in so modifying the relation of the 
system to the environment that it is once more restored to the average in which 
assimilation can be resumed. 
We may imagine that the first step in the evolution of life was taken when, 
during the chaotic chemical interchanges which accompanied the cooling down 
of the molten surface of the earth, some compound was formed, probably with 
absorption of heat, endowed with the property of polymerisation and of growth 
at the expense of surrounding material. Such a substance could continue to 
grow only at the expense of energy derived from the surrounding medium, 
and would undergo destruction with any stormy change in its environment. 
Out of the many such compounds which might have come into being, only such 
would survive in which the process of exothermic disintegration tended towards 
a condition of greater stability, so that the process might come to an end spon- 
taneously and the organism or compound be enabled to await the more favour- 
able conditions necessary for the continuance of its growth. With the continued 
cooling of the earth, the new production of endothermic compounds would 
probably become rarer and rarer. The beginning of life, as we know it, was 
possibly the formation of some complex, analogous to the present chlorophyll 
corpuscles, with the power of absorbing the newly penetrating sun’s rays and 
of utilising these rays for the endothermic formation of further unstable com- 
pounds. Once given an unstable system such as we have imagined, with two 
phases, viz. (1) a condition of assimilation or growth by the endothermic forma- 
tion of new material ; (2) a condition of ‘ exhaustion,’ in which the exothermic 
destructive changes excited by unfavourable external conditions came to an 
end spontaneously—the great principle of natural selection or survival of the 
fittest would suffice to account for the evolution of the ever-increasing complexity 
of living beings which has occurred in the later history of this globe, The 
