The entire history of anything, Spencer tells us, must include its appearance out of the imperceptible, and its disappearance again into the imperceptible.  Be it a single object, or the whole universe, an account which begins with it in a concrete form, or leaves off with its concrete form, is incomplete.  He uses a familiar instance, that of a cloud appearing when vapour drifts over a cold mountain top, and again disappearing when it emerges into warmer air.  The cloud emerges from the imperceptible as heat is dissipated.  It is dissolved again as heat is absorbed and the watery particles evaporate.  Spencer esteems this an analogue of the appearance of the universe itself, according to the nebular hypothesis.  Yet assuredly, as the cloud presupposes vapours which had previously condensed, and the vapour clouds that had previously evaporated, and as clouds dissolve in one place even at the moment that they are forming in another, so we are told of nebulae which are in every phase of advance or of decline.  To ask which was first, solid masses or nebulous haze, is much like recurring to the riddle of the hen and the egg.  Still, we are told, we have but to extend our thought beyond this emergence and subsidence of sidereal systems, of continents, nations, men, to find a permanent totality made up of transient individuals in every stage of change.  The physical assumption with which Spencer sets out is that the mass of the universe and its energy are fixed in quantity.  All the phenomena of evolution are included in the conservation of this matter and force.

Besides the criticism which was offered above, that the mere law of the persistence of force does not initiate our series, there is a further objection.  Even within the series, once it has been started, this law of the persistence of force is solely a quantitative law.  When energy is transformed there is an equivalence between the new form and the old.  Of the reasons for the direction evolution takes, for the permanence of that direction once it has been taken, so that the sequence of forms is a progression, the explication of a latent nature—­of all this, the mere law of the persistence of force gives us no explanation whatever.  The change at random from one form of manifestation to another might be a striking illustration of the law of the persistence of force, but it would be the contradiction of evolution.  The very notion of evolution is that of the sequence of forms, so that something is expressed or achieved.  That achievement implies more than the mere force.  Or rather, it involves a quality of the force with which the language of mechanism does not reckon.  It assumes the idea which gives direction to the force, an ideal quality of the force.