188 
THE PRINCIPLES OF BIOLOGY. 
We find that change of structure is always due to change 
of incident forces, and that for organic evolution there is 
sufficient cause in the continual changes in environments; 
and further that, conforming to the law of “ the instability 
of the homogeneous,” there has been continual differentiation 
among individuals and aggregates of individuals, of which 
those only whose internal actions have been but slightly 
incongruous with external actions have survived. But survival 
through changes of conditions implies adjustment to the new 
conditions, and this is inductively verified, for adaptation is 
expressible in mechanical terms as direct equilibration, and 
natural selection is similarly expressible as indirect equili¬ 
bration. 
In the earliest times, changes in external inorganic forces 
were the only causes of the successive modifications of 
organisms ; but whilst these must always continue to operate, 
the actions of organisms on each other have become ever 
increasing sources of such modifications, until in man they 
are the chief factors, 
As regards the internal processes of change entailed by 
these external causes, there always has been and will be a 
survival of the fittest; but whilst natural selection (indirect 
equilibration) has been and remains the only process among 
lower organisms, there grows with the evolution of organisms 
—having some activity—a direct equilibration or adaptation 
which plays an increasingly important part, until finally, 
among civilised human races, it becomes the main factor, 
social arrangements even tending to prevent the survival of 
the fittest. -- 
Chapter XIV., “ The Convergence of the Evidences.” 
For the doctrine of “ The Evolution of Life,” three classes 
of evidence have been assigned, of which the a priori were 
partly negative and partly positive. 
The Special-Creation-Hypothesis was found to be worthless, 
but the more the Evolution-Hypothesis was considered the 
clearer its truth became, and this grew even more indubitable 
when the two hypotheses were confronted with the general 
truths established by naturalists. These inductive evidences 
occupied four chapters, embracing the arguments from Classi¬ 
fication, Embryology, Morphology, and Distribution (in space 
and time), each of which groups contained arguments pointing 
to the same conclusion, and, moreover, the conclusion to be 
inferred from the arguments of any one group is the same as 
that to be inferred from the arguments of all the other groups, 
thus giving to the induction a very high degree of probability. 
