The Scottish Naturalist. 153 



maximum of generic productions. As to climate in the 

 earliest ages, these ages possess an element of vicissitude 

 peculiar and special to them, for something surely is to be 

 attributed to the effects of the originally incandescent state of 

 the earth. We have not more reason to believe that there was 

 an incandescent state at all, then we have to believe that as the 

 globe cooled and the incandescence, once universal, receded 

 from the circumference towards the centre, it would leave 

 "behind it, when it had shrank so far as to make life possible at 

 all, the whole terrestrial surface subjected to more than the torrid 

 temperature of a palm house, to lessen by degrees through all 

 the grades of heat or cold endurable by life. If the post- 

 cambrian earth be all the earth that was ever seen, and if this 

 cooling from incandescence be part of the physical history of 

 Siluria, it makes a notable addition for the time to the other 

 climatal sources of change in vital conditions, and thus it is 

 brought about that the earliest as well as the latest epochs — the 

 time of the early maximum of generic production as well as that 

 of the late, claims a preponderance of this second source of 

 organic transformation. 



3d. The grade in the scale of organisation which the forms of 



life of any period had reached, gave the conditions of life more 

 or less effect in modifying type through natural selection. The 

 higher the organisation of any being, it enters into more com- 

 plex relations with the organic and inorganic conditions of life; 

 and hence probably the fact that higher organisms change more 

 rapidly than those that are lower. Whencesoever it may be, 

 there is some reason, says Darwin, to believe that organisms 

 high in the scale change more quickly through natural selection 

 than those that are low. So far then as this is true, it will have 

 an influence in determining a greater profusion of genera to- 

 wards those periods when the scale of life had risen high, and 

 especially in those formations that supplied most largely the 

 conditions for the development of the higher forms on an ex- 

 tensive scale. That is to say, given favourable conditions for 

 the occupation of wide spaces by the vertebrate forms of life, 

 we have, so far, provision made for the favourable action of 

 natural selection and the rapid multiplication of generic types. 

 Now this range of conditions falls on tertiary and present times. 

 Mammals were few in earlier times. Birds not many more. 

 The maximum development of both classes marks the close of 



