Evolution as a philosophical doctrine applicable to all phenomena, whether physical or mental, whether manifested by material atoms or by men in society, has been dealt with systematically in the ’Synthetic Philosophy’ of Mr. Herbert Spencer.  Comment on that great undertaking would not be in place here.  I mention it because, so far as I know, it is the first attempt to deal, on scientific principles, with modern scientific facts and speculations.  For the ‘Philosophic positive’ of M. Comte, with which Mr. Spencer’s system of philosophy is sometimes compared, though it professes a similar object, is unfortunately permeated by a thoroughly unscientific spirit, and its author had no adequate acquaintance with the physical sciences even of his own time.

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The doctrine of evolution, so far as the present physical cosmos is concerned, postulates the fixity of the rules of operation of the causes of motion in the material universe.  If all kinds of matter are modifications of one kind, and if all modes of motion are derived from the same energy, the orderly evolution of physical nature out of one substratum and one energy implies that the rules of action of that energy should be fixed and definite.  In the past history of the universe, back to that point, there can be no room for chance or disorder.  But it is possible to raise the question whether this universe of simplest matter and definitely operating energy, which forms our hypothetical starting point, may not itself be a product of evolution from a universe of such matter, in which the manifestations of energy were not definite—­in which, for example, our laws of motion held good for some units and not for others, or for the same units at one time and not at another—­and which would therefore be a real epicurean chance-world?

For myself, I must confess that I find the air of this region of speculation too rarefied for my constitution, and I am disposed to take refuge in ‘ignoramus et ignorabimus.’

[Sidenote:  Other achievements in physical science.]

The execution of my further task, the indication of the most important achievements in the several branches of physical science during the last fifty years, is embarrassed by the abundance of the objects of choice; and by the difficulty which everyone, but a specialist in each department, must find in drawing a due distinction between discoveries which strike the imagination by their novelty, or by their practical influence, and those unobtrusive but pregnant observations and experiments in which the germs of the great things of the future really lie.  Moreover, my limits restrict me to little more than a bare chronicle of the events which I have to notice.

[Sidenote:  Physics and chemistry.]