A crowd, for instance, is of this fugitive character: it assembles and it disperses, its existence as a crowd is over, but its constituent elements persist; and the same can be said of a planet or a sun. Yet for some "soul" or underlying reality even in these temporary accretions there is permanence of a sort:—Tyndall's "streak of morning cloud," though it may have "melted into infinite azure," has not thereby become non-existent, although as a visible object it has disappeared from our ken and become a memory only. It is true that it was a mere aggregate or accidental agglomeration—it had developed no self-consciousness, nothing that could be called personality or identity characterised it,—and so no individual persistence is to be expected for it; yet even it—low down in the scale of being as it is—even it has rejoined the general body of aqueous vapour whence, through the incarnating influence of night, it arose. The thing that _is_, both _was_ and _shall be_, and whatever does not satisfy this condition must be an accidental or fugitive or essentially temporary conglomeration or assemblage, and not one of the fundamental entities of the universe. It is interesting to remember that this was one of the opinions strongly held by the late Professor Tait, who considered that persistence or conservation was the test or criterion of real existence. The question, How many fundamental entities in this sense there are, and what they are, is a difficult one. Many people, including such opposite thinkers as Tait and Haeckel, would say "matter" and "energy"; though Haeckel chooses, on his own account, to add that these two are one. (Perhaps Professor Ostwald would agree with him there; though to me the meaning is vague.) Physical science, pushed to the last resort, would probably reply that, within its sphere of knowledge at the present stage, the fundamental entities are _ether_ and _motion_; and that of other things at present it knows next to nothing. If physical science is interrogated as to the probable persistence, _i.e._, the fundamental existence, of "life" or of "mind," it ought to reply that it does not know; if asked about "personality," or "souls," or "God,"—about all of which Professor Haeckel has fully-fledged opinions—it would have to ask for a definition of the terms, and would speak either not at all or with bated breath concerning them.