150 The Scottish Naturalist. 



selection has operated with varying intensity at different times, 

 and specially that it operated with augmented force at the 

 extremes of the geological series, and with a force lessening from 

 each extreme towards the centre? Does it appear that natural 

 selection was operating at a maximum when the production of 

 genera is found to have been at a maximum ? and when this 

 had its minimum, is there reason to think that that had its 

 minimum coincidently ? or, to put the question in a form that 

 leads directly to a mode of settling it, did the conditions that 

 affect the operation of natural selection concur most favourably 

 when genera are said to have been produced most abundantly, 

 and least favourably when genera were least abundant ? Natural 

 selection operates with a force and a rapidity proportionate to 

 the favourable character of certain conditions. Now, if it could 

 be shown that these conditions preponderated in the earliest 

 and latest times, we should then have a cause existent for the 

 preponderant number of genera in these ages produced ; and if 

 these conditions could be shown to have grown deficient in an 

 increasing degree for these returns till they failed most of all in 

 Permian and Triassic times, we should then have the other 

 affirmation of Polarity established and accounted for — the 

 paucity of genera originating in those times. Now my conten- 

 tion is, that there is a variety of considerations which look 

 favourably on the attempt to convert each of these hypotheticals 

 into a categorical. 



What are the conditions favourable to the action of natural 

 selection ? There is one formula that covers them all, viz., 

 changes in the external conditions of life. This, looking away 

 of course from the tendency to variation innate to organisms, 

 is the one element of force in the operation of natural selection 

 that lies nearer than any other to the effect of modifying one 

 type into another. Expose an organism to some change of con- 

 ditions, from whatever source, and you lay a foundation for 

 natural selection taking the advantage of favourable variations 

 and accumulating them to the extent pf transmuting the type. 

 Let us look then at the sources of change in the conditions of 

 life, with an eye to the inquiry, what geological formations may 

 have been favoured most in respect of them, and what least. The 

 survey must be rapid. These sources of change are numerous, 

 varied, mutually complicated, and producing or variously in- 



