It is a common opinion that only the reproductive cells of man and animals are immortal, and that the other elements of the body are mortal, the latter, if they escape violence, dying a natural death. A contrast has been drawn between the mortal cells in which is resident the life of the body and the immortal cells on which the species depends. However, when non-reproductive cells possess the power of regeneration, it is impossible to deny their immortality. When a polyp or a worm reproduces by division, a large number of cells go to form the new individual, and these cells are immortal in the fashion of the infusoria. Immortal animals occur only among the lower invertebrates. The power of regeneration fades away in the higher ranks of the scale of life. Whilst worms may be divided in several pieces, each piece being capable of regeneration so as to form a new worm, when molluscs are cut they display only a limited capacity for regeneration. If the antennæ of a snail be amputated they will be renewed, but if the whole creature be cut in pieces death follows. Some of the lower vertebrates, such as newts and salamanders, can renew the tail and the limbs, but they cannot reproduce by division. Birds and mammals, the higher vertebrates, have very little power of regeneration, and tail and limbs are never reformed in their cases. It seems to be the case that the advance in the general organisation of animals has involved a loss in the reproductive capacity of the cells and tissues. Even in the highest animals, some organs, such as the liver, still possess regenerative capacity; but, on the other hand, many cells have lost the power of regeneration completely. The nervous cells, in particular, which are the highest and most perfectly organised elements of the body, cannot reproduce themselves. After their initial appearance in the course of embryonic development, they pass their lives without regenerating or reproducing. In acquiring the highest qualities, that is to say, their psychical activity, they have lost completely the power of reproduction, the distinctive feature of immortal cells. If cells doomed to natural death really exist, it is in the nervous tissues that we must look for them. [Illustration: FIG. 18.—Ephemerids. ]