But let us suppose the case for evolution to be ideally favorable, and assume that in every instance we possessed a perfect gradation of forms between two extremes, such, for example, as occurs in the Ammonite series, even then we would be far from having a true demonstration of the point at issue. Bateson has called our attention to the danger of confounding sterile and instable _hybrids_ with intergradent species. “Examine,” he says, “any two thoroughly distinct species which meet each other in their distribution, as for instance, _Lychnis diurna_ and _vespertina_ do. In areas of overlap are many intermediate forms. These used to be taken to be transitional steps, and the specific distinctness of _vespertina_ and _diurna_ was on that account questioned. Once it is known that these supposed intergrades are merely mongrels between the two species the transition from one to the other is practically beyond our powers of imagination to conceive. If both these can survive, why has their common parent perished? Why, when they cross, do they not reconstruct it instead of producing partially sterile hybrids? I take this example to show how entirely the facts were formerly misrepresented.” (_Heredity_, Smithson. Inst. Rpt. for 1915, p. 369.) Similarly, T. H. Morgan has shown, with reference to _mutants_, the fallacy of inferring common descent from the phenomenon of intergradence, and what holds true for a series of intergradent mutants would presumably also hold true of a series of intergradent species, could such a series be found and critically distinguished from hybrid and mutational intermediates. In short, the Darwinian deduction of common origin from the existence of intergradence must now be regarded as a thoroughly discredited argument. “Because we can often arrange the series of structures in a line extending from the very simple to the more complex, we are apt to become unduly impressed by this fact and conclude that if we found the complete series we should find all the intermediate steps and that they have arisen in the order of their complexity. This conclusion is not necessarily correct.” (“A Critique of the Theory of Evolution,” p. 9.) Having cited such a series of gradational mutations ranging between the long-winged, and completely wingless condition, in the case of the Vinegar Fly (_Drosophila melanogaster_), as well as two similar graded series based on pigmentation and eye color, he concludes: “These types, with the fluctuations that occur within each type, furnish a complete series of gradations; yet historically they have arisen independently of each other. Many changes in eye color have appeared. As many as thirty or more races differing in eye color are now maintained in our cultures. Some of them are so similar that they can scarcely be separated from each other. It is easily possible beginning with the darkest eye color, sepia, which is a deep brown, to pick out a perfectly graded series ending with pure white eyes.