If the garden worm is cut in two, the head-part will reproduce the tail-portion. If the little fresh-water polyp, Hydra, be cut into a number of pieces, each segment will reproduce a perfect animal. Many lizards, after losing their tails by violence, manufacture new tails through the agency of the latent hereditary units contained in the body cells of the stump left. If the tentacle or “horn” of a snail, which contains an eye with a perfect lens and retina, be cut off, the animal can reproduce another one with a perfect eye, and this can be repeated a number of times. Often newts, when fighting with one another, or lobsters when fighting, lose a leg or a claw. These highly organized animals have the power of creating new limbs, making bones, ligaments, muscles, nerves, cuticle, and so on. All of this is done _through the hereditary masses in the nuclei of the body cells_ at the site of the injury. A study of the phenomena of polymorphism in hydroids and insects will beautifully and most interestingly illustrate latent hereditary units. Much that is speculative and fanciful is included under the subject of atavism, and the safest plan for pathologists and biologists, in considering any abnormality, is to remember a golden rule of Gegenbaur, that only those structures are reversional which are taxonomically not far distant or phylogenetically not very old. Embryology is also a very important check in considering such subjects. In mankind supernumerary limbs and digits, microcephalia and micrencephalia, have been looked upon as reversions to the simian type. Lombroso, in contrasting the criminal with normal man, looks upon his _homo delinquens_ as an illustration of atavism, contrasting with _homo sapiens_. But, as Ziegler, in his _Pathology_, well observes, many writers have gone too far in this respect, and have characterized as atavistic formations various acquired pathological formations and fresh variations of germ cells. I think one can safely say that supernumerary ribs and those supernumerary nipples and mammary glands along the line of the deep epigastric and internal mammary arteries are truly atavistic structures; also certain muscles normally belonging to those mammalia which come near to man in the scale of relationship, and which appear in man as muscular variations, are reversional. Children are often born with pigmented hairy patches on their bodies known as moles; sometimes these hairy moles are only of the size of a split pea, in other cases they are several square inches in area, while in rare cases almost all of the trunk may be thus covered. Although many similar pathological cases are often but marked variations called sports, yet the illustrations mentioned are undoubtedly reversional. Of the multitudinous illustrations of atavism that could be mentioned I wish to refer to but one more case.