io6 THE POPULAR SCIENCE MONTHLY. 



proper to industrial life ; and that, so long as these two modes of life, 

 utterly alien in their natures, have to be jointly carried on, there will con- 

 tinue this jumbling of the regulative systems they respectively require. 

 The second of the objections above noted, as needing to be other- 

 wise dealt with than by further explanation of the formula of evolu- 

 tion, concerns the increase of likeness among developing systems of 

 civil law ; in proof of which increase of likeness Mr. Leslie quotes 

 Sir Henry Maine to the effect that " all laws, however dissimilar in 

 their infancy, tend to resemble each other in their maturity" : the 

 implication to which Mr. Leslie draws attention being, that in respect 

 of their laws societies become not more heterogeneous but more homo- 

 geneous. Now, though in their details systems of law will, I think, 

 be found to acquire, as they evolve, an increasing number of differ- 

 ences from one another, yet in their cardinal traits it is probably true 

 that they usually approximate. How far this militates against the 

 formula of evolution we shall best see by first considering the analogy 

 furnished by animal organisms. Low down in the animal kingdom 

 there are simple mollusks with but rudimentary nervous systems a 

 ganglion or two and a few fibers. Diverging from this low type we 

 have the great sub-kingdom constituted by the higher mollusca and 

 the still greater sub-kingdom constituted by the vertebrata. As these 

 two types evolve, their nervous systems develop ; and though in the 

 highest members of the two they remain otherwise unlike, yet they ajj- 

 proximate in so far that each acquires great nervous centers : the large 

 cephalopods have clustered ganglia which simulate brains. Compare, 

 again, the mollusca and the articulata in respect of their vascular sys- 

 tems. Fundamentally unlike as these are originally, and remaining 

 unlike as they do throughout many successive stages of ascent in these 

 two sub-kingdoms, they nevertheless are made similar in the highest 

 forms of both by each having a central propelling organ a heart. 

 Now, in these and in some cases which the external organs furnish, 

 such as the remarkable resemblance evolution has produced between 

 the eyes of the highest mollusca and those of the vertebrata, it may 

 be said that there is implied a change toward homogeneity. No zo- 

 ologist, however, would admit that these facts really conflict with the 

 general law of organic evolution. As already explained, the tendency 

 to progress from homogeneity to heterogeneity is not intrinsic but ex- 

 trinsic. Structures become unlike in consequence of unlike exposures 

 to incident forces. This is so with organisms as wholes, which, as they 

 multiply and spread, are ever falling into new sets of conditions ; and 

 it is so with the parts of each organism. These pass from primitive 

 likeness into unlikeness as fast as the mode of life places them in dif- 

 ferent relations to actions primarily external and secondarily inter- 

 nal ; and with each successive change in mode of life new unlikenesses 

 are superposed. One of the implications is that, if in organisms other- 

 wise different there arise like sets of conditions to which certain parts 



