show this relationship as conclusively as the vertebrate relationship of birds and mammals is shown by the presence of a backbone in each of them. It will be seen that the facts in this imaginary case belong to the same category with the facts of homology, but that they
furnish a much more complete index of relationship, since they cover the whole life of the animal, instead of its adult form alone.
As a matter of fact we do find in nature something like this hypothetical case, and it is universally recognized that an acquaintance with all the stages in the growth of an animal is the greatest aid to the discovery of its true affinities; as is well shown by the case of entoconcha, and by the barnacles which were classed among the mollusca until a knowledge of their development showed that they are Crustacea. When descriptive embryology was in its infancy, it so frequently happened that a knowledge of younger stages threw a flood of light upon the affinities of doubtful forms, that naturalists felt a growing hope that here was the true key to the natural system of classification, and that all they needed for reading the riddle was a thorough knowledge of the whole course of development of each form of life. If the embryology of each animal were a fixed quantity, this purely descriptive knowledge would undoubtedly furnish such a basis for phylogenetic generalizations; but the great advances which have been made in this field within the last twenty years show conclusively that this is not the case, but that the early stages in the life of an animal may undergo modifications which are quite independent of any which may meanwhile take place in the adult, so that, while the