This gastrula theory, now generally accepted, is one of Haeckel's two great fundamental contributions to the evolution philosophy with the history of which his life work is so intimately linked. The other contribution is the theory, even more famous and now equally undisputed, that every individual organism, in its em-bryological development, rehearses in slurred but unmistakable epitome the steps of evolution by which the ancestors of that individual came into racial being. That is to say, every mammal, for example, originating in an egg stage, when it is comparable to a protozoon, passes through successive stages when it is virtually in succession a gastrula, a fish, and an amphibian before it attains the mammalian status, because its direct ancestors were in succession, through the long geological ages, protozoons, gastrulae, fishes, amphibians before the true mammal was evolved. This theory cast a flood of light into many dark places of the Darwinian philosophy. It was propounded in 1866 in Professor Haeckel's great work on morphology, and it has ever since been a guiding principle in his important philosophical studies. It was through this same work on morphology that Haeckel first came to be universally recognized as the great continental champion of Darwinism--the Huxley of Germany. Like Huxley, Haeckel had at once made the logical application of the Darwinian theory to man himself, and he sought now to trace the exact lineage of the human family as no one had hitherto attempted to fathom it. Utilizing his wide range of zoological and anatomical knowledge, he constructed a hypothetical tree of descent--or, if you prefer, ascent--from the root in a protozoon to the topmost twig or most recent offshoot, man. From that day till this Haeckel's persistent labors have been directed towards the perfection of that genealogical tree. This work on morphology was much too technical to reach the general public, but in 1868 Haeckel prepared, at the instigation of his friend and confrère Gagenbaur, what was practically a popular abridgment of the technical work, which was published under the title of _The Natural History of Creation_. This work created a furor at once. It has been translated into a dozen languages, and has passed through nine editions in the original German. Through it the name of Haeckel became almost a household word the world over, and subject for mingled applause and opprobrium--applause from the unprejudiced for its great merit; opprobrium from the bigoted because of the unprecedented candor with which it followed the Darwinian hypothesis to its logical goal.