The Scottish Naturalist. 1 5 1 



fluencing one another. They might be grouped as geological, 

 geographical, climatal, and organic. I shall select the following — 



1. One class concerns areas of occupation. The origination 

 of new areas, making a fresh demand on such forms of life as 

 suit the circumstances thus emergent, the alteration of areas 

 already occupied — alteration in extent, position, exposure, degree 

 of isolation ; or again, the number of areas causing a multiplica- 

 tion of changed conditions ; all this variety affecting areas, 

 furnishes one class of circumstances that tell very powerfully on 

 the conditions of life, and on the intensification and acceleration 

 of the process of selection and the modification of structure. 

 To appreciate the influence thus exercised, two important 

 principles must be borne in mind : — First; That every area tends 

 to be peopled with all the life which it can possibly support- 

 The increase of life and the eagerness with which organic 

 beings lie in wait to seize every inch of unoccupied ground, 

 whether in a new or an old, but only partially, occupied field, 

 secures that every area shall ever make progress towards full 

 occupation. Second, and what is of main consequence, it is 

 only through means of a stock of greatly diversified organisms 

 that any area is occupied by all the life which it is capable of 

 supporting. 



Now the question is, when did these two principles operate 

 in the peopling of areas of occupation that were most favourably 

 ordered for the action of natural selection, and through natural 

 selection accelerate at a maximum rate the diversification of 

 structure and the production of generic types ? Certainly if 

 ever, it must have been in the earliest ages, when the earth was 

 but growing towards the consummation of its capacities as a life- 

 bearing world. If many new fields must be occupied with life 

 when life is a new thing, and being devoid as yet of a great 

 wealth of forms ready provided to furnish the occupants, must 

 provide them as it were on the instant ; if only great diversity 

 of structure can fit such living inhabitants as there are for the 

 full occupation of the areas that are waiting till these inhabitants 

 shall increase and multiply and replenish them ; and if the 

 conditions of life thus raised and multiplied suit themselves 

 through natural selection with the forms they are fitted to 

 maintain — all this seems but a concentration of reasons why 

 we should expect to find a great profusion of generically 



