But this fundamental criticism aside, Spencer's handling of biological problems is nothing short of masterful. In his chapters on growth, development, function, adaptation, generation ("genesis"), heredity, variation, etc., although not a specialist in any branch of biology, he marshals an immense body of facts in support of fundamental principles, many of which had never before been discovered. In dealing with heredity he postulates the existence of "physiological units," later changed to "constitutional units." The "Principles of Biology" was published in 1864, and therefore Spencer could have known nothing of Darwin's "pangenesis," treated in his "Variations of Animals and Plants under Domestication," which appeared in 1868. But Spencer's "physiological units," as he points out,[1] are not at all the same as Darwin's "gemmules." They are still less similar to Weismann's "biophores."[2] They are nothing but "compound molecules (as much above those of albumen in complexity as those of albumen are above the simplest compounds," and are for the same organism "substantially of one kind." Why he did not admit that they are merely forms of protoplasm we do not know, but certain it is that biologists are now coming to believe that no hereditary units in the sense of independent bodies exist, and that all the phenomena of heredity, obscure and recondite as they are, can be as easily conceived to result from the action of protoplasm in various ways not yet fully understood, as from any imaginary bearers of hereditary "Anlagen."[3]
It is in Part III. on the "Evolution of Life" that the philosopher comes forth in his full power. After disposing of the special creation hypothesis, he attacks the cosmic principles underlying the organic world. Many of those enumerated in "First Principles" are shown to be in full force on the biotic plane. The process from homogeneity to heterogeneity finds its clearest exemplifications here, and the two great principles of differentiation and integration are formulated and illustrated with wonderful force. We can not here even enumerate all the biological principles set forth in this work, but the application of the principle of equilibration to the organic world can not be passed over in silence. The Lamarckian principle of increase by use and atrophy from disuse, called somewhere by Spencer "use-inheritance," and early recognized by him as "the inheritance of functionally acquired modifications," now becomes, in the new terminology of biology, "direct equilibration," while natural selection, which Spencer, along with many others mentioned by Darwin in the later editions of the "Origin of Species," had foreshadowed before that work appeared,
