used
by the greatest natural philosophers. The undulatory theory of light has
thus been arrived at; and the belief in the revolution of the earth on
its own axis was until lately supported by hardly any direct evidence.
It is no valid objection that science as yet throws no light on the far
higher problem of the essence or origin of life. Who can explain what
is the essence of the attraction of gravity? No one now objects
to following out the results consequent on this unknown element of
attraction; notwithstanding that Leibnitz formerly accused Newton of
introducing "occult qualities and miracles into philosophy."

I see no good reasons why the views given in this volume should shock
the religious feelings of any one. It is satisfactory, as showing how
transient such impressions are, to remember that the greatest discovery
ever made by man, namely, the law of the attraction of gravity, was also
attacked by Leibnitz, "as subversive of natural, and inferentially of
revealed, religion." A celebrated author and divine has written to
me that "he has gradually learned to see that it is just as noble a
conception of the Deity to believe that He created a few original forms
capable of self-development into other and needful forms, as to believe
that He required a fresh act of creation to supply the voids caused by
the action of His laws."

Why, it may be asked, until recently did nearly all the most eminent
living naturalists and geologists disbelieve in the mutability of
species? It cannot be asserted that organic beings in a state of nature
are subject to no variation; it cannot be proved that the amount of
variation in the course of long ages is a limited quantity; no clear
distinction has been, or can be, drawn between species and well-marked
varieties. It cannot be maintained that species when intercrossed are
invariably sterile and varieties invariably fertile; or that sterility
is a special endowment and sign of creation. The belief that species
were immutable productions was almost unavoidable as long as the history
of the world was thought to be of short duration; and now that we have
acquired some idea of the lapse of time, we are too apt to assume,
without proof, that the geological record is so perfect that it would
have afforded us plain evidence of the mutation of species, if they had
undergone mutation.

But the chief cause of our natural unwillingness to admit that one
species has given birth to other and d