This theory is now the central axis of all biological investigation in all its branches, from ethics to histology, from anthropology to bacteriology. In the light of this theory every peculiarity of structure, every character or quality of individual or species, has a meaning and a cause. It is the work of the investigator to find this meaning as well as to record the fact. "One of the noblest lessons left to the world" by Darwin is this, Mr. Frank Cramer tells us,--"this, which to him amounted to a profound, almost religious, conviction, that every fact in nature, no matter how insignificant, every stripe of color, every tint of flowers, the length of an orchid's nectary, unusual height in a plant, all the infinite variety of apparently insignificant things, is full of significance. For him it was an historical record, the revelation of a cause, the lurking-place of a principle." According to the theory of evolution every structure of to-day finds its meaning in some condition of the past. The inside of an animal tells what it really is, for it bears the record of heredity. The outside of an animal tells where its ancestors have been, for it bears record of concessions to environment. Similarity in essential structure is known as _homology_. By the theory of evolution homology, wherever it is found, is proof of blood-relationship. The theory of organic evolution through natural law was first placed on a stable footing by the observations and inductions of Darwin. It has therefore been long known as Darwinism, although that term has been usually associated with the recognition of natural selection as the great motive power in organic change. Darwinism was at first regarded as a "working hypothesis." It is now an integral part of biological science, because all opposing hypotheses have long since ceased to work. It is as well attested as the theory of gravitation, and its elements are open to less doubt. All investigations in biology must assume it, as without it most such investigations would be impossible. Naturalists could no more go back to the old notion of special creation for each species and its organs than astronomers could go back to the old notion of guiding angels as directors of planetary motion. Without the theory of organic development through natural selection, the biological science of to-day would be impossible.