152 The Scottish Naturalist. 



different types of living beings in the earliest times. Modifica- 

 tion of structure goes on rapidly and far in the process of 

 naturalisation. And With all else that the primary replenish- 

 ment of the earth offers in favour of a maximum multiplication 

 of genera, it was like a constant process of naturalisation pro- 

 secuted for a long season and on a vast scale — every inch of 

 every area, as it successively grew fit for life putting in its 

 demand for occupation, and hastening the diversification of all 

 the types of life in order to suit itself with its own, and get the 

 right form for the right place. This goes to account for 

 Polarity so far as the early extreme of the geological scale is 

 concerned. 



2. We find in dimatal changes another class of circumstances 

 that affect powerfully the conditions of life and favour the 

 action of natural selection. The greater the changes of 

 temperature, the more, so far forth, is this influence exerted on. 

 the conditions of, life and on the acceleration of structural 

 diversification. And the more frequently these vicissitudes 

 occur the greater accumulation will there be of the diversifying 

 effects. Climate has no doubt always through all formations 

 varied with greater or lesser extremes, and quicker or slower 

 vicissitudes, and over larger or smaller areas. Subterranean 

 action elevating or depressing areas, change of ocean currents 

 to heat or cool them, and other constantly occurring geographical 

 changes, not to speak of the place of the earth's orbit in the 

 heavens, would make climate come and go to some extent 

 continually, and exert a subtle and an efficacious power on the 

 conditions of life, and on the vital forms depending thereon. So 

 far as volcanic action is an indication of such more general 

 geological action as affects such changes, it is to be noted that 

 there is a dearth of such action from the Permian to the Chalk 

 age. We have evidence of great vicissitudes of temperature 

 during Tertiary times, from beginning to end. Lately indications 

 of glaciation have been detected in some older formations. But 

 there is no evidence of any such alternations of temperature 

 as are known to have characterised the latest ages. It is these 

 that have enjoyed a, maximum degree of this means of altering 

 organic types ; and so far again there is an account rendered of 

 the assertion of Polarity in reference to these ages — that there 

 was at this later extreme of the geological scale, a second 



