I apprehend that the foundation of the theory of natural selection is the fact that living bodies tend incessantly to vary.  This variation is neither indefinite, nor fortuitous, nor does it take place in all directions, in the strict sense of these words.

Accurately speaking, it is not indefinite, nor does it take place in all directions, because it is limited by the general characters of the type to which the organism exhibiting the variation belongs.  A whale does not tend to vary in the direction of producing feathers, nor a bird in the direction of developing whalebone.  In popular language there is no harm in saying that the waves which break upon the sea-shore are indefinite, fortuitous, and break in all directions.  In scientific language, on the contrary, such a statement would be a gross error, inasmuch as every particle of foam is the result of perfectly definite forces, operating according to no less definite laws.  In like manner, every variation of a living form, however minute, however apparently accidental, is inconceivable except as the expression of the operation of molecular forces or “powers” resident within the organism.  And, as these forces certainly operate according to definite laws, their general result is, doubtless, in accordance with some general law which subsumes them all.  And there appears to be no objection to call this an “evolutionary law.”  But nobody is the wiser for doing so, or has thereby contributed, in the least degree, to the advance of the doctrine of evolution, the great need of which is a theory of variation.

When Mr. Mivart tells us that his “aim has been to support the doctrine that these species have been evolved by ordinary natural laws (for the most part unknown), aided by the subordinate action of ‘natural selection’” (pp. 332-3), he seems to be of opinion that his enterprise has the merit of novelty.  All I can say is that I have never had the slightest notion that Mr. Darwin’s aim is in any way different from this.  If I affirm that “species have been evolved by variation[1] (a natural process, the laws of which are for the most part unknown), aided by the subordinate action of natural selection,” it seems to me that I enunciate a proposition which constitutes the very pith and marrow of the first edition of the “Origin of Species.”  And what the evolutionist stands in need of just now, is not an iteration of the fundamental principle of Darwinism, but some light upon the questions, What are the limits of variation? and, If a variety has arisen, can that variety be perpetuated, or even intensified, when selective conditions are indifferent, or perhaps unfavourable, to its existence?  I cannot find that Mr. Darwin has ever been very dogmatic in answering these questions.  Formerly, he seems to have inclined to reply to them in the negative, while now his inclination is the other way.  Leaving aside those broad questions of theology, philosophy, and ethics, by the discussion of which neither the Quarterly Reviewer nor Mr. Mivart can be said to have damaged Darwinism—­whatever else they have injured—­this is what their criticisms come to.  They confound a struggle for some rifle-pits with an assault on the fortress.