Fairhurst points out the difficulties in which the evolutionist becomes involved through the fixity of species. He writes: "It is well known that as a rule distinct species will not cross, and that if they do cross the offspring are not fertile. On the other hand, it is true that all _varieties_ of a species readily cross, producing fertile offspring. This has commonly been regarded as a well-defined distinction between varieties and species. If the varieties of pigeons which are so different from each other did not freely cross, and if the mongrel offspring were not fertile, Darwin's argument as to the production of new _species_ under domestication would be complete. The fact is, we do not know of the origin of any two species of animals that do not cross and whose offspring are not fertile; in other words, we do not know of the origin of _species,_ but only of _varieties_. The origin of species that will not cross and produce fertile offspring is _assumed_ from the origin of varieties that do cross and produce fertile offspring. This leaves the evolutionists to account for one of the most difficult things in connection with this theory, namely, how did varieties of animals of the same species become cross-sterile?* [[*So that they were unable to interbreed. Only if such cross-sterility exists, could they exist thereafter as independent new species.--G.]] Several things must occur simultaneously before cross-sterility between parent and offspring could occur and become effective, namely, a number of individuals must be born at the same time possessing the same variation, the variation must be useful, these individuals must be fertile with each other, they must be cross-sterile with the parent form," as, otherwise, the offspring would revert to type, "and, finally, the few, if any, individuals thus produced and being widely scattered through the species, must find each other before they could propagate. I regard it impossible that these things could all occur simultaneously." (_"Organic Evolution,"_ p. 333.) Mr. Huxley is forced to this admission: "After much consideration, and with assuredly no bias against Mr. Darwin's views, it is our clear conviction that, as the evidence stands, it is not absolutely proven that a group of animals, having all the characters exhibited by species in nature, has ever been originated by selection, whether artificial or natural." And again. "Our acceptance of the Darwinian hypothesis must be provisional so long as one link in the chain of evidence is wanting; and so long as all the animals and plants certainly produced by selective breeding from a common stock are fertile with one another, that link will be wanting."