Naturae leges generationis in hybridis. Hominis leges ex observatis a posteriori. I am not clear as to the parts assigned in the first sentence respectively to the "_Medulla_" and the "_Cortex_," beyond that Linnaeus conceived that multiformity was first brought about by diversity in the "_Cortex_." The passage is rendered still more obscure if read in connection with the essay on "_Generatio Ambigena_," where he expresses the conviction that the _Medulla_ is contributed by the mother, and the _Cortex_ by the father, both in plants and animals.[5] But however that may be, he regards this original diversity as resulting in the constitution of the Natural Orders, each represented by one individual. In the second aphorism the Omnipotent is represented as creating the genera by intermixing the individual _plantae classicae_, or prototypes of the Natural Orders. The third statement is the most remarkable, for in it he declares that Species were formed by the act of Nature, who by inter-mixing the genera produced _Species congeneres_, namely species inside each genus, to the number which now exist. Lastly, Chance or Accident, intermixing the species, produced as many varieties as there are about us. Linnaeus thus evidently regarded the intermixing of an originally limited number of types as the sufficient cause of all subsequent diversity, and it is clear that he draws an antithesis between _Creator_, _Natura_, and _Casus_, assigning to each a special part in the operations. The acts resulting in the formation of genera are obviously regarded as completed within the days of the Creation, but the words do not definitely show that the parts played by Nature and Chance were so limited. Recently also E. L. Greene[6] has called attention to some curious utterances buried in the _Species Plantarum_, in which Linnaeus refers to intermediate and transitional species, using language that even suggests evolutionary proclivities of a modern kind, and it is not easy to interpret them otherwise. Whatever Linnaeus himself believed to be the truth, the effect of his writings was to induce a conviction that the species of animals and plants were immutably fixed. Linnaeus had reduced the whole mass of names to order and the old fantastical transformations with the growth of knowledge had lapsed into discredit; the fixity of species was taken for granted, but not till the overt proclamation of evolutionary doctrine by Lamarck do we find the strenuous and passionate assertions of immutability characteristic of the first half of the nineteenth century. It is not to be supposed that the champions of fixity were unacquainted with varietal differences and with the problem thus created, but in their view these difficulties were apparent merely, and by sufficiently careful observation they supposed that the critical and permanent distinctions of the true species could be discovered, and the impermanent variations detected and set aside.