We have a third objection, one, fortunately, which has nothing to do with geology.  We can only state it here in brief terms.  The chapter on hybridism is most ingenious, able, and instructive.  If sterility of crosses is a special original arrangement to prevent the confusion of species by mingling, as is generally assumed, then, since varieties cross readily and their offspring is fertile inter se, there is a fundamental distinction between varieties and species.  Mr. Darwin therefore labors to show that it is not a special endowment, but an incidental acquirement.  He does show that the sterility of crosses is of all degrees; upon which we have only to say, Natura non facit saltum, here any more than elsewhere.  But, upon his theory he is bound to show how sterility might be acquired, through natural selection or through something else.  And the difficulty is, that, whereas individuals of the very same blood tend to be sterile, and somewhat remoter unions diminish this tendency, and when they have diverged into two varieties the cross-breeds between the two are more fertile than either pure stock—­yet when they have diverged only one degree more the whole tendency is reversed, and the mongrel is sterile, either absolutely or relatively.  He who explains the genesis of species through purely natural agencies should assign a natural cause for this remarkable result; and this Mr. Darwin has not done.  Whether original or derived, however, this arrangement to keep apart those forms which have, or have acquired (as the case may be), a certain moderate amount of difference, looks to us as much designed for the purpose, as does a rachet to prevent reverse motion in a wheel.  If species have originated by divergence, this keeps them apart.

Here let us suggest a possibly attainable test of the theory of derivation, a kind of instance which Mr. Darwin may be fairly asked to produce—­viz., an instance of two varieties, or what may be assumed as such, which have diverged enough to reverse the movement, to bring out some sterility in the crosses.  The best marked human races might offer the most likely case.  If mulattoes are sterile or tend to sterility, as some naturalists confidently assert, they afford Mr. Darwin a case in point.  If, as others think, no such tendency is made out, the required evidence is wanting.

A fourth and the most formidable difficulty is that of the production and specialization of organs.

It is well said that all organic beings have been formed on two great laws:  unity of type, and adaptation to the conditions of existence.[I-12] The special teleologists, such as Paley, occupy themselves with the latter only; they refer particular facts to special design, but leave an overwhelming array of the widest facts inexplicable.  The morphologists build on unity of type, or that fundamental agreement in the structure of each great class of beings which is quite independent of their habits or conditions