7 2 THE FACTORS OF ORGANIC EVOLUTION. 



In all cases the homogeneous tends to pass into the hetero 

 geneous, and the less heterogeneous into the more hetero 

 geneous. In the primordial units of protoplasm, then, the 

 step with which evolution commenced must have been the 

 passage from a state of complete likeness throughout the 

 mass to a state in which there existed some unlikeness. 

 Further, the cause of this step in one of these portions of 

 organic matter, as in any portion of inorganic matter, must 

 have been the different exposure of its parts to incident 

 forces. What incident forces ? Those of its medium or 

 environment. &quot;Which were the parts thus differently 

 exposed ? Necessarily the outside and the inside. In 

 evitably, then, alike in the organic aggregate and the 

 inorganic aggregate (supposing it to have coherence enough 

 to maintain constant relative positions among its parts), the 

 first fall from homogeneity to heterogeneity must always 

 have been the differentiation of the external surface from 

 the internal contents. No matter whether the modifica 

 tion was physical or chemical, one of composition or of 

 decomposition, it comes within the same generalization. 

 The direct action of the medium was the primordial factor 

 of organic evolution. 



And now, finally, let us look at the factors in their 

 ensemble, and consider the respective parts they play: 

 observing, especially, the ways in which, at successive 

 stages, they severally give place one to another in degree of 

 importance. 



Acting alone, the primordial factor must have initiated 

 the primary differentiation in all units of protoplasm alike. 

 I say alike, but I must forthwith qualify the word. For 

 since surrounding influences, physical and chemical, could 

 not be absolutely the same in all places, especially when 

 the first rudiments of living things had spread over a 

 considerable area, there necessarily arose small contrasts 

 between the degrees and kinds of superficial differentiation 



