150 THE GENESIS OF SPECIES. [Chap. 



the a priori probability of the preservation of at least a 

 few mlnutelt/ transitional forms in some instances if ever)/ 

 species without exception has arisen exclusively by sucli 

 minute and gradual transitions. 



It remains, then, to turn tto the other considerations 

 with regard to the relation of species to time: namely (1), 

 as to the total amount of time allowable by other sciences 

 for organic evolution ; and (2) the proportion existing, on 

 Darwinian j)rinci})les, between the time anterior to the ear- 

 lier fossils, and the time since ; as evidenced by the pro- 

 portion between the amount of evolutionary change during 

 the latter epoch and that which must have occurred ante- 

 riorly. 



Sir William Thomson has lately ' advanced arguments 

 from three distinct lines of inquiry, and agreeing in one ap- 

 proximate result. The three lines of incpjiry were — 1. The 

 action of the tides upon the earth's rotation. 2. The prob- 

 able length of time during which the sun has illuminated 

 this planet ; and 3. Tiie temperature of the interior of the 

 earth. The result arrived at by these investigations is a 

 conclusion that the existing state of things on the eartli, 

 life on the earth, all geological history showing continuity 

 of life, must be limited within some such pt;riod of })ast 

 time as one hundred million years. Tiie ilrst question 

 which suggests itself, supposing Sir W. Thomson's views 

 to be correct, is. Is this period any thing like enough for 

 the evolution of all organic forms by "Natural Selection ?" 

 The second is. Is this i)eriod any thing like enough for tlie 

 disposition of the strata whicli nuist have been deposited if 

 all organic forms have been evolved by minute steps, ac- 

 cording to the Darwinian theory ? 



In the first place, as to Sir William Thomsom's views, the 

 author of this book cannot j^resume to advance any opin- 

 ion ; but the fact that they have not been refuted, pleads 



' " Transactions of the Geological Society of Glasgow," vol. iii. 



