6o6 THE POPULAR SCIENCE MONTHLY. 



By another law, flowing from the same primary principle, the parts 

 of a whole diverge from one another in proportion to their diversity, 

 and group themselves together in proportion to their resemblances. 

 Motions that are alike in direction or intensity, acting on these parts, 

 drive them in the same direction, and with the same velocity, whence 

 results an integration of these parts, while those driven by motions 

 unlike in direction or intensity go in different directions Avith differ- 

 ent velocities, separate from one another, are disintegrated. This is 

 the law of segregation, the application of which brings into promi- 

 nence the heterogeneous character of the products of change, by giv- 

 ing to their heterogeneity a clearer and more definite nature. 



Finally, we note another consequence of the persistence of force. 

 Every change in an aggi'egation of sensible parts is conditioned by 

 opposing forces, the one representing action, the other reaction ; the 

 one the tendency to change, the other resistance ; their antagonism 

 can end only when equilibrium has been established, by the dissipa- 

 tion of the excess of the one force over the other. A body subject to 

 any disturbance whatever, owing to a modification of its circum- 

 stances, tends toward equilibrium with its new circumstances ; and, as 

 the different forces acting on it have not the same intensity, those 

 which are weaker soon find tlieir equilibrium, while those which are 

 stronger continue to give motion to the body, and then the latter pre- 

 sents the spectacle of an aggregate whose parts are in an invariable 

 ratio to each other, while the total aggregate is ever changing its rela- 

 tions to external objects. This is equilibrium mobile, unstable equi- 

 librium, and it serves as a transition to a more perfect equilibrium, 

 or else to a renewal of the internal movements which have already 

 found equilibrium. 



The action of these laws of change of objects and their parts leads 

 to two contrary results, according to the mode of distribution of the 

 forces in action. We have evolution, i. e., change with integration 

 of matter, dissipation of internal motion, increase of the number and 

 diversity of the parts, whenever the external forces are not such as to 

 break the bond which unites them ; we have dissolution, continuous or 

 discontinuous, i. e., a change with disaggregation of matter; absorp- 

 tion of motion (which, becoming internal, drives the constituent units 

 with greater velocity) and diminution both of the numbers and of 

 the diversity of the parts, whenever the external forces are sufficiently 

 intense to destroy the cohesion of the aggregate and to restore to its 

 parts their original inde{)endence. 



The work of Mr. Spencer in his " Biology " consists in referring to 

 these general laws the generalizations obtained in the various parts 

 of the domain of biology, and in discerning those which possess the 

 character of necessity. This course has the twofold advantage of 

 giving to these generalizations greater authority, and of introducing 

 into a coordinated system of philosophy the science whose general- 



