Apart from experimental investigation, the results of comparative anatomy, even if they are amplified by those of comparative embryology, and even if they include fossil as well as living organisms, do no more than suggest the occurrence of an evolutionary process. It is in vain that we attempt a demonstration of transmutation of forms of life by showing that a similarity of structure is to be observed in all animals belonging to the same group. We may show successfully that the skeleton of the limbs and limb-girdles of vertebrate animals is anatomically the same series of parts, whether it be the arms and legs of man, or the wings and legs of birds, or the pectoral and pelvic fins of fishes: such homologies as these were indeed suggested by the mediæval comparative anatomists apart altogether from any notions as to an evolutionary process. We may show that the simplicity of the skeleton of the head of man is apparent only, and that in it are to be traced most of the anatomical elements that enter into the skull and visceral arches of the fish; and that fusions and losses and translocations of parts have occurred and can be made to account for the observed differences of form. All this might just as easily be explained by assuming a process of special creation, or the gradual development of a plan or design. Just as God made Eve from a superfluous rib taken from the body of her husband, so He may have formed the auditory ossicles of the higher vertebrate from those parts of the visceral arches of the lower forms which had become superfluous in the construction of the more highly organised creature. However much the language of evolution may force itself on biology, it does no more than symbolise the results of anatomy and embryology, and provide a convenient framework on which these may be arranged. But if, as all modern experimental work shows, the form of the organism is, in the long run, the result of its interaction with the environment; if, as indeed we see, this form is not an immutable one, but a stage in a flux; and if deviations from it may occur with all the appearance of spontaneity, then it would appear that the observed facts of comparative anatomy and embryology are capable of only one explanation. They represent the results of an evolutionary process, and the relationships that morphological studies indicate are no longer merely logical, but really material ones. We can now endeavour to utilise these results in the attempt to trace the directions taken by the process of evolution.