Darwin was not the author of the theory of evolution, but he made the theory alive and real to the imagination.  He showed us what a master key it is for unlocking the riddle of the life of the globe.  He launched biological science upon a new career and made it worthy of its place in the great trilogy of sciences, astronomy, geology, and biology, of which Tennyson, in his poem “Parnassus,” recognized only the first two.  Had Tennyson written his poem in our day he would undoubtedly have included biology among his “terrible Muses” that tower above all others, eclipsing the glory of the great poets.  Or is it true that we find it easier to accept the theory of the evolution of the worlds and suns from nebulous matter than to accept the theory of the evolution of man from the maze of the lower animal forms?  It is less personal to us.  The astronomer has the advantage of the biologist in one important respect:  he can show us in the heavens now the process of the evolution of worlds actually going on, but the biologist cannot show us the transformation of one species into another taking place to-day.  We can sound the abysses of astronomic space easier than we can sound the abysses of geologic time.  The stars and the nebulae we have always with us, but where are the myriad earlier forms that were the antecedents of the present animal life of the globe?  True, the palaeontologist finds a more or less disjointed record of them in the stratified rocks and sees in a measure the course evolution has taken, but he does not actually see it at work as does the astronomer.  More than that, the forces the astronomer deals with are mechanical and chemical, but the biologist deals with a new force called life that often reverses or defies mechanical and chemical forces, but which is yet so identified and blended with them that we cannot conceive it apart from them.  The stomach does not digest itself, nor gravity hold the blood in the lower extremities.  The tree lifts up its weight of fluids and solids and holds aloft its fruit and foliage in spite of gravity; its growing roots split and lift the rocks; mosses and lichens disintegrate granite; vital energy triumphs over chemical and mechanical energy.

Biological laws are much more subtle and difficult to trace and formulate than chemical and mechanical laws.  Hence the student of organic evolution can rarely arrive at the demonstrable certainties in this field that he can in the sphere of chemistry and mechanics.  It is very doubtful if life can ever be explained in terms of these things.  Life works through chemical combinations and affinities, and yet is it not more than chemistry?  It works with and through mechanical principles and forces, and yet it is evidently more than mechanics.  It is manifested through matter, and yet no analysis of matter can reveal its secret.  It comes and goes while matter stays; we destroy life, but cannot destroy matter.  It is as fugitive as the wind which fills all sails one minute and is gone the next.