Finally, it is worth noticing, that, though natural selection is scientifically explicable, variation is not.  Thus far the cause of variation, or the reason why the offspring is sometimes unlike the parents, is just as mysterious as the reason why it is generally like the parents.  It is now as inexplicable as any other origination; and if ever explained, the explanation will only carry up the sequence of secondary causes one step farther, and bring us in face of a somewhat different problem, which will have the same element of mystery that the problem of variation has now.  Circumstances may preserve or may destroy the variations; man may use or direct them; but selection, whether artificial or natural, no more originates them than man originates the power which turns a wheel, when he dams a stream and lets the water fall upon it.  The origination of this power is a question about efficient cause.  The tendency of science in respect to this obviously is not towards the omnipotence of matter, as some suppose, but towards the omnipotence of spirit.

So the real question we come to is as to the way in which we are to conceive intelligent and efficient cause to be exerted, and upon what exerted.  Are we bound to suppose efficient cause in all cases exerted upon nothing to evoke something into existence,—­and this thousands of times repeated, when a slight change in the details would make all the difference between successive species?  Why may not the new species, or some of them, be designed diversifications of the old?

There are, perhaps, only three views of efficient cause which may claim to be both philosophical and theistic.

1.  The view of its exertion at the beginning of time, endowing matter and created things with forces which do the work and produce the phenomena.

2.  This same view, with the theory of insulated interpositions, or occasional direct action, engrafted upon it,—­the view that events and operations in general go on in virtue simply of forces communicated at the first, but that now and then, and only now and then, the Deity puts his hand directly to the work.

3.  The theory of the immediate, orderly, and constant, however infinitely diversified, action of the intelligent efficient Cause.

It must be allowed, that, while the third is preeminently the Christian view, all three are philosophically compatible with design in Nature.  The second is probably the popular conception.  Perhaps most thoughtful people oscillate from the middle view towards the first or the third,—­adopting the first on some occasions, the third on others.  Those philosophers who like and expect to settle all mooted questions will take one or the other extreme.  The “Examiner” inclines towards, the “North American” reviewer fully adopts, the third view, to the logical extent of maintaining that “the origin of an individual, as well as the origin of a species or a genus, can be explained only by the direct action of an intelligent creative cause.”  This is the line for Mr. Darwin to take; for it at once and completely relieves his scientific theory from every theological objection which his reviewers have urged against it.