382 THE LAW OF EVOLUTION CONTINUED. 



so irregular as to admit of no description; and it is neither 

 alike in any two individuals nor in the same individual at 

 successive moments. By aggregation of such creatures, are 

 produced, among other indefinite bodies, the Sponges — 

 bodies that are indefinite in size, in contour, in internal 

 arrangement. As further showing how relatively indeter- 

 minate are the simplest organisms, it may be mentioned 

 that their structures vary greatly with surrounding con- 

 ditions : so much so that, among the Protozoa and Pro- 

 tophyta, many forms which were once classed as distinct 

 species, and even as distinct genera, are found to be merely 

 varieties of one species. If now we call to mind 



how precise in their attributes are the highest organisms — 

 how sharply cut their outlines, how invariable their pro- 

 portions, and how comparatively constant their structures 

 under changed conditions; we cannot deny that greater 

 definiteness is one of their characteristics. "We must admit 

 that if they have been evolved out of lower organisms, an 

 increase of definiteness has been an accompaniment of their 

 evolution. 



That, in course of time, species have become more sharp- 

 ly marked off from other species, genera from genera, and 

 orders from orders, is a conclusion not admitting of a more 

 positive establishment than the foregoing; and must, 

 indeed, stand or fall with it. If, however, species and 

 genera and orders have arisen by " natural selection," then, 

 as Mr. Darwin shows, there must have been a tendency to 

 divergence, causing the contrasts between groups to be- 

 come greater. Disappearance of intermediate forms, less 

 fitted for special spheres of existence than the extreme forms 

 they connected, must have made the differences between 

 the extreme forms decided; and so, from indistinct and 

 unstable varieties, must slowly have been produced distinct 

 and stable species — an inference which is in harmony with 

 what we know respecting races of men and races of domestic 

 animals. 



