I do not propose to argue the case for evolution, which I regard as being quite as well proven as most other historical facts, but to discuss its pos… - J.B.S. Haldane

We must... carefully distinguish between two quite different doctrines which Darwin popularised, the doctrine of evolution, and that of natural selection. It is quite possible to hold the first and not the second. Similarly with regard to the doctrines of Darwin's great contemporary Marx, it is possible to adopt socialism but not historical materialism.

The change from one stable equilibrium to the other may take place as the result of the isolation of a small unrepresentative group of the population, a temporary change in the environment which alters the relative viability of different types, or in several other ways...

In scientific thought we adopt the simplest theory which will explain all the facts under consideration and enable us to predict new facts of the same kind. The catch in this criterion lies in the word "simplest." It is really an aesthetic canon such as we find implicit in our criticisms of poetry or painting. The layman finds such a law as <math> {\mathrm{\partial }x \over \mathrm{\partial } t} = \Kappa { \, \mathrm{\partial}^2x \over \mathrm{\partial }y^2}</math> much less simple than "it oozes," of which it is the mathematical statement. The physicist reverses this judgment, and his statement is certainly the more fruitful of the two, so far as prediction is concerned. It is, however, a statement about something very unfamiliar to the plain man, namely, the rate of change of a rate of change.