72 The Scottish Naturalist. 



of genera, at any time, is no test of the numbers of either 

 specific or individual life at that time extant. And the general- 

 isation is relative solely to generic numbers. " The number of 

 species in a group at any given epoch," says Forbes, " is to be 

 excluded, not being an element in the discussion, though apt to 

 be introduced through mistake of the generalisation attempted 

 to be attained." The criticisms of Pictet and Wallace, for one 

 thing, do not keep generic diversity exclusively in view, and are 

 therefore thus far irrelevant. 



From the grounds recently proposed and widely accepted as 

 to the origination of the forms of living beings, it might be 

 thought that this theory would have very unsafe footing, or in- 

 deed be an impossible one, inasmuch as its author held to the 

 independent creation and permanent distinction of species. 

 Doubtless such was his view. A species was to him a group 

 of individual beings not only morphologically identical, but 

 connected genealogically, being of one blood, and that blood 

 such as had never run, and never should run, in the veins of 

 another species. A genus correspondingly was a group of 

 species allied by certain structural affinities. It was an idea 

 which the Creator realised or expressed and embodied variously 

 in the various species which are classed under it. Whether 

 those views would have held their ground with Forbes in the 

 revolutionary times to which a longer life would have intro- 

 duced him is hard to say. But polarity does not stand or fall 

 with them. They are not essential to it, though they formed 

 with it part of the scientific furniture of the same mind, and 

 were therefore readily wrought up with it, whether necessary 

 or not. It can be expressed equally well in terms of the 

 Darwinian hypothesis, — in which hypothesis moreover we 

 shall bye-and-bye try to find its physical explanation. Either 

 Forbes or Darwin could use the following language, and it 

 describes the law of polarity perfectly, the force that operates 

 to the production of organic types differing to the degree of 

 generic distinctness (be that force natural selection with the 

 the one theorist, or the Creator's immediate power with the 

 other) manifested itself in maximum energy in the earliest and 

 latest ages of geological time. What may be the genus-pro- 

 ducing power is plainly indifferent to this generalisation, which 

 deals solely with the results of its work. And this leads to 



