As applied to life, then, the evolutionary idea is briefly this—­that plants and animals have all a natural origin from a single primitive living creature, which itself was the product of light and heat acting on the special chemical constituents of an ancient ocean.  Starting from that single early form, they have gone on developing ever since, from the homogeneous to the heterogeneous, assuming ever more varied shapes, till at last they have reached their present enormous variety of tree, and shrub, and herb, and seaweed, of beast, and bird, and fish, and creeping insect.  Evolution throughout has been one and continuous, from nebula to sun, from gas-cloud to planet, from early jelly-speck to man or elephant.  So at least evolutionists say—­and of course they ought to know most about it.

But evolution, according to the evolutionists, does not even stop here.  Psychology as well as biology has also its evolutionary explanation:  mind is concerned as truly as matter.  If the bodies of animals are evolved, their minds must be evolved likewise.  Herbert Spencer and his followers have been mainly instrumental in elucidating this aspect of the case.  They have shown, or they have tried to show (for I don’t want to dogmatise on the subject), how mind is gradually built up from the simplest raw elements of sense and feeling; how emotions and intellect slowly arise; how the action of the environment on the organism begets a nervous system of ever greater and greater complexity, culminating at last in the brain of a Newton, a Shakespeare, or a Mendelssohn.  Step by step, nerves have built themselves up out of the soft tissues as channels of communication between part and part.  Sense-organs of extreme simplicity have first been formed on the outside of the body, where it comes most into contact with external nature.  Use and wont have fashioned them through long ages into organs of taste and smell and touch; pigment spots, sensitive to light or shade, have grown by infinite gradations into the human eye or into the myriad facets of bee and beetle; tremulous nerve-ends, responsive sympathetically to waves of sound, have tuned themselves at last into a perfect gamut in the developed ear of men and mammals.  Meanwhile corresponding percipient centres have grown up in the brain, so that the coloured picture flashed by an external scene upon the eye is telegraphed from the sensitive mirror of the retina, through the many-stranded cable of the optic nerve, straight up to the appropriate headquarters in the thinking brain.  Stage by stage the continuous process has gone on unceasingly, from the jelly-fish with its tiny black specks of eyes, through infinite steps of progression, induced by ever-widening intercourse with the outer world, to the final outcome in the senses and the emotions, the intellect and the will, of civilised man.  Mind begins as a vague consciousness of touch or pressure on the part of some primitive, shapeless, soft creature:  it ends as an organised and co-ordinated reflection of the entire physical and psychical universe on the part of a great cosmical philosopher.