i88o.] The Evolution of Scientific Knowledge. 417 
For just as the varying sound of the voice from feeble treble 
to resonant bass testifies to the gradual development of the 
vocal organs, so does the evolution ol scientific knowledge 
testify to the evolution of the brain power and brain com- 
plexity of which that knowledge is one of the products. . 
Before we proceed to the consideration of some special 
instances of this evolution as exemplified by the sciences of 
astronomy, geology, and chemistry, it will be well to devote 
a short space to the broader question, From what has bcience 
in general been evolved ? To this question there can, I 
think, be but one answer. The more or less organised 
knowledge which we call Science has sprung from the unor- 
ganised general knowledge of our uncultured ancestors, in 
an Essay on the “ Genesis of Science ” Mr. Herbert Spencer 
has treated this subject with masterly clearness. The first 
step towards knowledge of any kind is classification ; and 
classification is based on the recognition of likeness and un- 
likeness. We only know an object when we recognise that 
it is like something we have before met with. If it be like 
nothing that we have before seen, or heard of, we. say that 
we do not know what it is. And just as classification is the 
grouping together of like things, so is reasoning the grouping 
together of like relations among things. Now it is by the 
extension of these processes of grouping together like things 
and like relations among things that Science arises. But so 
far the science is only qualitative . It is only when the 
recognition of likeness grows into the recognition of equality 
that science becomes quantitative ; for equality-— equality 
between things and equality between relations is the funda- 
mental conception 'which underlies all mathematics. Out 
of this conception of equality therefore springs exact science , 
endowed with the power of quantitative prevision. We 
may, in fadt, from one point of view, divide Science into 
three stages — that of merely qualitative prevision, that o 
vaguely quantitative prevision, and that of exadt quantitative 
prevision. Let us take an example of each. From a study 
of the appearance of the sky, and from observations of the 
barometer and hygrometer, I may be able to foretell a stoim ; 
but my predidtion cannot be in any sense quantitative, for 
the storm may last a couple of hours or continue for as 
many days. After careful diagnosis the skilful physician 
knows that the administration of a certain drug will have 
a certain effedt upon his patient ; but his prevision is only 
vaguely, not exadtly, quantitative. On the other hand, after 
taking into consideration certain perturbations of the pla- 
netary system, M. Leverrier, in 1846, inferred the presence 
