Darwin, then, himself and his most enthusiastic adherents must admit that at some given moment these _races_ became suddenly incapable of crossing with their predecessors. Whence then arises the _sterility_ which separates _species_? When, and at what moment will the physiological _bond_ be broken, which unites the original species with its modified descendants, even when this modification is carried as far as the ordinary ox and the niata? What will be the determining cause of this great fact which obtains through the whole economy of the organic kingdom? In his work upon the _Variation of Animals and Plants_, Darwin replies: “Since species do not owe their mutual sterility to the accumulative action of natural selection, and a great number of considerations show us that they do not owe it to a creative act, we ought to admit that it has been produced incidentally during their gradual formation, and is connected with some unknown modification of their organisation.” We have seen that, in the last editions of the _Origin of Species_, he refuses to admit that fertility among mongrels is general, taking his stand upon our _ignorance_ on the subject of crossings between wild _varieties_ (_races_). Thus, in order to admit the physiological transmutation of race into species, a fact which is contrary to all positive facts, Darwin and his followers reject the secular results of experience and observation, and substitute in their place a _possible accident_, and the _unknown_. The Darwinian theory relies entirely upon the possibility of this transmutation. We see upon what data the hypothesis of this possibility rests. Now, in a _truly liberal spirit_, I ask every _unprejudiced_ man, however little he may be conversant with science, the question, is it upon such foundations that a general theory in physics or chemistry would be founded? V. Moreover, the argument, of which we have just seen an example, may be found in every page of Darwinian writings. Whether a fundamental question, such as we have just been examining, or a minor problem, as the transmutation of the tomtit into the nuthatch, is under discussion, _possibility_, _chance_, and _personal conviction_ are invariably adduced as convincing reasons. Is modern science established upon such foundations?