manifested that the whole series may be compared to a chain having for one of its terminal links a unicellular organism, and for the other, man himself. But yet, all these intermediate links have made an independent existence, without any direct connection with each other, except on the lines of their genetic descent, through which alone their common attributes have been acquired. Another remarkable fact is, that the present-day species are never exactly similar to their fossil ancestors. Hence it follows that some modification in the somatic structure of organisms takes place in the course of time; and it is significant that this modification does not proceed at a uniform rate, nor is it dependent on the time that has elapsed since the latter were in life. For there are some species which have so persistently resisted all evolutionary movements that they continue to flourish through several geological periods without having undergone any material change. For example, the Lamp-shells, King-Crabs, Pearly Nautilus, and many others, have come down from palæaeozoic times to the present day with comparatively little modification through these millions of years. Indeed, so unprogressive were some of them that they preferred to die out rather than change their antiquated ways. Such animals Professor Cleland very appropriately designates as "terminal forms of life" (Journal of Anat. and Phys.) vol. viii., p. 350).
The same remarks apply to many of the Protozoa of the present day, some of which would be undistinguishable from their predecessors in all ages had we the means of making a comparison between them. This idea is so far borne out by the organisms recently dredged up from the bed of the Atlantic—Foraminifera, Sponges, Corals, etc.—many of which have been declared to be similar to analogous forms in chalk. On the other hand, at a much later period than the Cambrian, many genera and species had arisen, flourished for a comparatively short time, and then vanished for ever. Such were the Ichthyosaurus, Plesiosaurus, Pterodactyle, and many other strange forms of swimming and flying reptiles, which became extinct towards the end of the Cretaceous period. The study of the life-history of these extinct animals is most fascinating to all lovers of the marvellous, and most instructive to the
