Taking, for example, the earliest Protozoa--the Foraminifera and Radiolaria--we find two lines of being that in endless varieties, but with little material change, extend from the earliest periods to the present time. In successive ages they are represented by families, genera, and species, which are regarded as distinct, and known by different names. But these humble animals are very variable, and what seem to us to be new types may be merely varieties of ancestral forms. We might even affirm that, for all we know, these two great groups, as they exist in the present ocean, are lineal descendants of those that flourished in the Eozoic. We could not prove this, unless we were to find somewhere a continuous succession of deep-sea deposits that would show the gradual changes that had occurred. On the other hand, it is hard to believe that one individual life, so to speak, could have continued unimpaired to animate successive and increasing masses of matter in all the vast time extending from the Eozoic to the modern. It is also at least equally possible that the causes and conditions, whatever they were, that produced the earliest Protozoa may have acted again and again in later times, originating new lines of descent with renewed vitality. Still, the tracing of these almost incredibly long lines of descent, if they are such, is a proper, though difficult, subject of scientific research, whatever may be the result. Something has been attempted in this direction over limited portions of time; but a vast amount of patient labour is required before certainty can be attained even in this department of investigation.