NATURAL SELECTION; ETC. 123 



Here it would not be amiss to consider the general 

 principle of gradation throughout organic Nature a 

 principle which answers in a general way to the Law of 

 Continuity in the inorganic world, or rather is so anal- 

 ogous to it that both may fairly be expressed by the 

 Leibnitzian axiom, Natura non agit saltatim. As an 

 axiom or philosophical principle, used to test modal 

 laws or hypotheses, this in strictness belongs only to 

 physics. In the investigation of Nature at large, at 

 least in the organic world, nobody would undertake to 

 apply this principle as a test of the validity of any 

 theory or supposed law. But naturalists of enlarged 

 views will not fail to infer the principle from the phe- 

 nomena they investigate to perceive that the rule 

 holds, under due qualifications and altered forms, 

 throughout the realm of Nature ; although we do not 

 suppose that Nature in the organic world makes no 

 distinct steps, but only short and serial steps not in- 

 finitely fine gradations, but no long leaps, or few of 

 them. 



To glance at a few illustrations out of many that 

 present themselves. It would be thought that the dis- 

 tinction between the two organic kingdoms was broad 

 and absolute. Plants and animals belong to two very 

 different categories, fulfill opposite offices, and, as to 

 the mass of them, are so unlike that the difficulty of 

 the ordinary observer would be to find points of com- 

 parison. Without entering into details, which would 

 fill an article, we may safely say that the difficulty with 

 the naturalist is all the other way that all these 

 broad differences vanish one by one as we approach the 

 lower confines of the two kingdoms, and that no also- 



