The evidence brought forward by Mr. Darwin in support of his hypothesis is of three kinds.  First, he endeavours to prove that species may be originated by selection; secondly, he attempts to show that natural causes are competent to exert selection; and thirdly, he tries to prove that the most remarkable and apparently anomalous phaenomena exhibited by the distribution, development, and mutual relations of species, can be shown to be deducible from the general doctrine of their origin, which he propounds, combined with the known facts of geological change; and that, even if all these phaenomena are not at present explicable by it, none are necessarily inconsistent with it.

There cannot be a doubt that the method of inquiry which Mr. Darwin has adopted is not only rigorously in accordance with the canons of scientific logic, but that it is the only adequate method.  Critics exclusively trained in classics or in mathematics, who have never determined a scientific fact in their lives by induction from experiment or observation, prate learnedly about Mr. Darwin’s method, which is not inductive enough, not Baconian enough, forsooth, for them.  But even if practical acquaintance with the process of scientific investigation is denied them, they may learn, by the perusal of Mr. Mill’s admirable chapter “On the Deductive Method,” that there are multitudes of scientific inquiries, in which the method of pure induction helps the investigator but a very little way.

“The mode of investigation,” says Mr. Mill, “which, from the proved inapplicability of direct methods of observation and experiment, remains to us as the main source of the knowledge we possess, or can acquire, respecting the conditions and laws of recurrence of the more complex phaenomena, is called, in its most general expression, the deductive method, and consists of three operations:  the first, one of direct induction; the second, of ratiocination; and the third, of verification.”

Now, the conditions which have determined the existence of species are not only exceedingly complex, but, so far as the great majority of them are concerned, are necessarily beyond our cognizance.  But what Mr. Darwin has attempted to do is in exact accordance with the rule laid down by Mr. Mill; he has endeavoured to determine certain great facts inductively, by observation and experiment; he has then reasoned from the data thus furnished; and lastly, he has tested the validity of his ratiocination by comparing his deductions with the observed facts of Nature.  Inductively, Mr. Darwin endeavours to prove that species arise in a given way.  Deductively, he desires to show that, if they arise in that way, the facts of distribution, development, classification, &c., may be accounted for, i.e. may be deduced from their mode of origin, combined with admitted changes in physical geography and climate, during an indefinite period.  And this explanation, or coincidence of observed with deduced facts, is, so far as it extends, a verification of the Darwinian view.