VI.] SPECIES AND TIME. 153 



the oldest forms liavinc^ been found as well furnished with 

 organs of sight as the very last of the gxoup which has left 

 its remains accessible to observation. 



Such instances, however, as well as the abrupt way in 

 which marked and special forms (as the Pterodactyles, &c., 

 before referred to) appear and disappear from the geological 

 record, are of course explicable on the Darwinian theory, 

 provided a sufhciently enormous amount of past time be 

 allowed. The alleged extreme, and probably great, impei- 

 fection of that record may indeed be pleaded in excuse for 

 the absence of transitional forms. But it is an excuse.^ 

 Nor is it possible to deny the a p7'iori probability of the 

 preservation of at least a few minutely transitional forms 

 in some instances, if every species without exception has 

 arisen exclusively by such minute and gradual transitions. 



It remains now to turn to the other considerations with 

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

 total amount of time which other sciences show to be 

 allowable for organic evolution ; and (2) the proportion 

 existing, on Darwinian principles, between the time ante- 

 rior to the earlier fossils, and the time since ; as evidenced 

 by the proportion between the amount of evolutionary 

 change during the latter epoch and that which must have 

 occurred anteriorly. 



Sir William Thomson has lately - "advanced arguments 

 from three distinct lines of inquiry, agreeing in one approxi- 

 mate result. The three lines of inquiry are — 1, The action 

 of the tides upon the earth's rotation ; 2, The probable 

 length of time during which the sun has illuminated this 



^ As Professor Huxley, Avitli his characteristic candour, fully admitted 

 in his lecture ou the Dinosauria before referred to. 



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



