more or less warm—that in the absence of a certain molecular vibration, the re-arrangement of parts does not go on. We see, again, that in hybernating animals, loss of heat carried to a particular point, results in extreme retardation of the organic changes. Yet further, we see that in animals which do not hybernate, as in man, prolonged exposure to extreme cold, produces an irresistible tendency to sleep (which implies a lowering of the functional activity); and then, if the abstraction of heat continues, this sleep ends in death, or arrest of functional activity. Lastly, we see that when the temperature is lowered till the contained water solidifies, there is a stoppage not only of those molecular re-arrangements which constitute life and development, but also of those molecular re-arrangements which constitute decomposition. Evidently then, both sensible and insensible agitations among the components of an aggregate, facilitate any re-distributions to which there may be a tendency. When that rhythmic change in the relative positions of the units which constitutes vibration, is considerable, the relative positions of the units more readily undergo permanent changes through the action of incident forces. * * * * * § 104. These special conditions to Evolution, are clearly but different forms of one general condition. The abstract proposition, that a permanent re-arrangement of units is possible only when they have neither absolute immobility nor absolute mobility with respect to each other, we saw to be practically equivalent to the proposition, that extreme cohesion and extreme want of cohesion among the units are unfavourable to Evolution. Be this cohesion or want of cohesion that which physically characterizes the matter as we ordinarily know it; be it that cohesion or want of cohesion distinguished as chemical; or be it that cohesion or want of cohesion consequent on the degree of molecular vibration; matters not, in so far as the general conclusion is concerned. Inductively as well as deductively, we find that the genesis of such permanent changes in the relative positions of parts, as can be effected without destroying the continuity of the aggregate, implies a medium stability in the relative positions of the parts: be this stability physical, chemical, or that which varies with the state of agitation. And as might be anticipated _à priori_, it is proved _à posteriori_, that this re-arrangement of parts goes on most actively in those aggregates whose units are moderately influenced by all these forces which affect their mobility.