To demonstrate that the Earth’s Flora and Fauna, regarded either as wholes or in their separate species, have progressed in definiteness, is of course no more possible than it was to demonstrate that they have progressed in heterogeneity: lack of facts being an obstacle to the one conclusion as to the other. If, however, we allow ourselves to reason from the hypothesis, now daily rendered more probable, that every species of organic form up to the most complex, has arisen out of the simplest through the accumulation of modifications upon modifications, just as every individual organic form arises; we shall see that in such case there must have been a progress from the indeterminate to the determinate, both in the particular forms and in the groups of forms. We may set out with the significant fact that many of the lowest living organisms (which are analogous in structure to the germs of all higher ones) are so indefinite in character that it is difficult, if not impossible, to decide whether they are plants or animals. Respecting sundry of them there are unsettled disputes between zoologists and botanists; and it has even been proposed to group them into a separate kingdom, forming a common basis to the animal and vegetal kingdoms. Note next that among the _Protozoa_, extreme indefiniteness of shape is very general. In the shell-less Rhizopods and their allies, not only is the form so irregular as to admit of no description, but it is neither alike in any two individuals nor in the same individual at successive moments. By the aggregation of such creatures, are produced, among other indefinite bodies, the sponges—bodies that are indefinite in size, in contour, in internal arrangement, and in the absence of an external limiting membrane. As further showing the relatively indeterminate character of the simplest organisms, it may be mentioned that their structures vary very greatly with surrounding conditions: so much so that, among the _Protozoa_ and _Protophyta_, many forms which were once classed as distinct species, and even as distinct genera, are found to be merely varieties of one species. If now we call to mind how precise in their attributes are the highest organisms—how sharply cut their outlines, how invariable their proportions, and how comparatively constant their structures under changed conditions, we cannot deny that greater definiteness is one of their characteristics; and that if they have been evolved out of lower organisms, an increase of definiteness has been an accompaniment of their evolution. That in course of time, species have become more sharply marked off from other species, genera from genera, and orders from orders, is a conclusion not admitting of a more positive establishment than the foregoing; and must, indeed, stand or fall with it. If, however, species and genera and orders have resulted from the process of “natural selection,” then, as Mr.