than to any other group of crustaceans, the natural inference is that they have descended from the hermits. But while they have taken on a crablike form and mode of life, the loss of symmetry and appendages due to life in the coiled shells of gastropods could never be regained. Teleological explanations of the peculiarities in question are inapplicable; the only rational interpretation is furnished by the theory of descent.

In addition to structural resemblances we may note here another index of genetic relationship which is furnished by blood

tests. The blood of animals responds to the introduction of foreign proteins, such as are contained in the blood serum of an alien species, by the production of antibodies. If we make a number of injections of human blood serum into the veins of a rabbit at intervals of one or two days, and then withdraw some of the rabbit's blood and obtain the clear serum, it will be found that the rabbit's serum can be employed as a delicate test for the presence of human blood. When a small bit of human blood is added to the sensitized rabbit serum, it produces a white precipitate. If blood from another kind of animal is introduced, no precipitate, or only a very slight one, is formed. The rabbit's blood has been modified so as to react to human proteins, and

the test of human blood thus afforded is one which has frequently proven of value in the detection of crime.

It has been shown by the extensive and valuable researches of Nuttall and Graham Smith that these blood reactions may

FIG. 197-Photograph of a chimpanzee intent on threading a needle. (After Gregory.)

be used as a test of genetic relationship. While sensitized serum reacts most strongly to the blood of the animal used in producing the sensitized condition, it also reacts more or less to the blood of related species. Even very slight and slow reactions are given to the blood of more distantly related groups, the degree of reaction being roughly proportional to the degree of structural similarity. Blood tests, therefore, afford a means of indicating

affinities. Human blood reacts more strongly with the blood of the anthropoid apes than with that of the other Old World monkeys, and more strongly with the blood of the latter than with that of the New World monkeys, and least of all with the blood of the lemurs. These blood tests are indications of chemical similarity, and chemical similarity parallels structural simi

larity. Organisms are probably similar in structure because they are similar in chemical composition. Blood relationship is not merely a metaphorical expression; it represents a chemical fact, and one which affords important additional evidence of the common descent of the members of related groups.

E. THE EVIDENCE FROM

EMBRYOLOGY

Organisms resemble each other in the way in which they develop as they resemble each other in their adult structure. It frequently happens that embryos reveal similarities not exhibited by the fully developed organisms. Karl Ernst von Baer, one of the great pioneers in the study of embryology, laid down the principle, since known as von Baer's law, that embryos of different organisms of the same phylum most closely resemble one another in their earliest stages and that the more nearly alike the organisms are, the longer they follow the same path in their development. A mammal and a fish, for instance,

pursue a common course only for a short way, but two kinds of mammals have a closely similar development for a much longer period; and if they belong to allied species they begin to diverge only at a relatively late stage.

In the hands of several evolutionists, the conclusions of von Baer have been employed in the support of a related generalization known as the doctrine of recapitulation, or the biogenetic law. This doctrine, as expressed by Professor Ernst Haeckel, its most prominent exponent, is that "the rapid and brief development of the individual (ontogeny) is a condensed synopsis of the long and slow history of the stem (phylogeny).” Admittedly the resemblances between the two types of development cannot be exact, but, however we may interpret the facts, there is a similarity between the series of forms met with in going from the simplest to the more highly developed animals, and the series of stages passed through in the development of the embryo. Some writers have contended that the embryos of higher animals resemble, not the adults of forms lower in the scale, but rather the embryos of these forms. So far as the evidence for evolution is concerned, this is not an essential point, the important fact being that the embryonic development of different organisms presents points of resemblance that can be rationally accounted for only by the hypothesis of a common descent. Let us note some of these similarities.

In the development of all vertebrate animals there appear on the sides of the neck region a series of slits which in the fishes and many larval amphibians break through into corresponding slits which push out from the walls of the pharynx. Openings are thus created through which water, taken into the mouth, may pass to the outside as it does in the respiration of these animals. Although the gill slits begin in the embryos of reptiles, birds, and mammals, much as they do in the fishes, their further development is soon checked, and as a rule they do not form an open communication with the throat. One of these slits is modified in the higher vertebrates to form the Eustachian tube which leads from the pharynx to the middle ear. Parts of the epithelial

lining of the gill slits develop into the thymus and some other small glandular bodies of uncertain function. But, for the most part, all traces of gill slits disappear in the adult animal.

The gill apparatus in the fishes is supported by a series of cartilaginous or bony arches lying between the slits. The hyoid arch, which supports the tongue, is the homologue of the gill

FIG. 199-Human embryos: A, right side; B, longitudinal section through the middle; C, front view; a, arches of the aorta; b, brain; e, ear vesicle; gs, gill slits; h, heart; uc, umbilical cord by which the embryo is attached to the uterus of the mother. (After His.)

arches lying behind it, but it does not bear gill filaments except in some of the more primitive cartilaginous fishes. In the mammals the hyoid arch is no longer a complete bony structure. Its lower end persists as a part of the hyoid bone; the upper end in many mammals (including man) forms the bony styloid process which fuses with the base of the skull, while the stylo-hyoid ligament, which connects the two parts, makes the hyoid arch complete. In the birds and mammals there is little left of the