The Scottish Naturalist. . . 103 



evolution through natural selection, which explains so much 

 else in the field of natural science, explain also why genera 

 were produced in the varying manner asserted by Forbes ? It 

 was the impression that this idea may not be altogether ground- 

 less that induced me to trouble the Society by now calling to its 

 remembrance such a forgotten and hopeless-like subject as 

 " Polarity in the distribution of genera.''"' 



Polarity is a law of observation — a generalised expression of 

 the observed arrangement of the facts concerned. It is a law 

 of the same class as was in Kepler's hands, the law of the 

 Ellipticity of the planetary orbits. Kepler, with his law of the 

 observed motions, had to wait for Newton with his causal 

 theory of these motions. In like manner, if Forbes were right 

 in that law of palseontological observation, which he called 

 Polarity, he waits for his Newton with some causal theory 

 showing what *makes the facts so. And the question I am 

 putting is this, Has not recent science furnished the cause 

 wanted ? Is there not something in the hypothesis of natural 

 selection as the vera causa of the indefinite modification and 

 transmutations of organic forms — in its conditions, its methods 

 of operation, and its results, that would determine the facts to be 

 somewhat just as* polarity describes them ? On a considera- 

 tion of the conditions under which natural selection works 

 favourably and otherwise, and of the conditions under which, 

 by geological evidence it has actually operated from first to last, 

 it would indeed seem" as if an explanation of this polar distribu- 

 tion of genera might be given largely in terms of the Darwinian 

 theory. Mr. Wallace, though he argues against the possibility 

 of establishing any such generalisation of the facts himself, 

 strangely argues a theory to account for their being as that 

 generalisation asserts\they are. His explanation is shortly this, 

 geological activity cogues diminished production, geological 

 repose increased production, of the specific and generic forms : 

 so that if a maximum number of genera be concentrated in the 

 earliest and latest formations, and a minimum in Permian and 

 Triassic formations, we have only to suppose that there was 

 geological repose in the earliest ages of life, with a fit maxi- 

 mum of generic forms corresponding ; that intensity of geologi- 



* This paper was read to the Perthshire Society of Natural Science. 



