TJie Scottish Naturalist. 69 



two extremes of the geological series. The energy of the genus- 

 producing power, whatever that was, manifested itself, not with 

 an- altogether irregular intensity throughout the geological for- 

 mations, nor with an intensity ever regularly increasing or de- 

 creasing from the beginning onwards, but with such a sort of 

 regulated variation that there is a maximum intensity at the 

 epochs farthest removed from each other, the Silurian and the 

 Tertiary, whilst from these two poles of time it diminishes 

 towards the Permian and Triassic formations in which at last 

 it dwindles to a minimum. Such is the simple sense of the some- 

 what abstruse looking formula, Polarity in the Geological dis- 

 tribution of Genera. 



That some very observable variation in the numbers of types 

 produced at different times should occur, would be, on Dar- 

 winian principles for instance, not surprising, but rather matter 

 of anticipation. If diversification of structure and of type at all 

 depend on natural causes originating anyhow in the conditions 

 of life, it clearly follows that, unless the conditions of life have 

 throughout the successive ages operated with a rigid uniformity 

 of preponderance, there must have been some amount and 

 order of difference in the comparative numbers of generically 

 different organisms produced in the periods. The facts might 

 furnish an argument in either of two directions. If physical 

 geology should establish that the conditions operating to the 

 transmutation of type had a varying preponderance of influence 

 throughout the different periods, the consequence would be in- 

 ferrible that there would be a corresponding difference in the 

 production of generic forms. Similarly, in the reverse order 

 of inference, if an inequality were observed in the proportionate 

 numbers of genera, such as for instance a maximum production 

 at one time, and a minimum at another, we would be entitled 

 to seek the cause of this in a corresponding variation in the 

 influencing conditions. Now this theory of Polarity, so called, 

 affirms that an augmented production of generically different 

 types has marked the origin and the close of the history of 

 organic life. The questions that arise are of course these two, 

 — First, Is the fact so ? And, second, Can it be accounted for 

 on Darwinian or any other principles ? 



The designation employed is borrowed from the physical 

 sciences. It is used here in a sense only analogous to that 

 which it bears there. 



