If [he writes] these generic types [of plants] first came before us in such clearly defined forms, when and where did the transitional states make their appearance? The extreme evolutionists constantly affirm of those who believe in special creation that they "habitually suppose the origination to occur in some region remote from human observation," and that "the conception survives only in connexion with imagined places where the order of organic phenomena is unknown." It is legitimate to retort upon them that they as habitually resort to "strata now covered by the sea"--to rocks "from which all traces of such fossils as they probably included have been obliterated by igneous action," and to mysterious "migrations from pre-existing continents to continents that were step by step emerging from the ocean." Unfortunately, so far as the vegetable kingdom is concerned, we have as yet failed to discover any traces of these mysterious strata or hypothetical continents in which the transitions from one plant-type to another were being brought about. The believers in special creations are not the only reasoners who have made free use of hypothetical possibilities. He presently adds: We have no evidence that unaided Nature has produced a single new type during the historic period. We can only conclude that the wonderful outburst of genetic activity which characterized the Tertiary age was due to some unknown factor, which then operated with an energy to which the earth was a stranger, both previously and subsequently. The knowledge of this factor is what we need in order to perfect our philosophy; and until we obtain that knowledge, many things must remain unaccounted for, so far as primeval vegetation is concerned. And elsewhere Professor Williamson reiterates the same idea:[264] I contend stoutly [he says] that, however numerous may be the facts that sustain the doctrine of evolution (and I am prepared to admit that there are many that do so in a remarkable manner), this unexplained outburst of new life demands the recognition of some factor not hitherto admitted into the calculations of the evolutionist school.