1 6 EVOLUTIONS OF ORGANIZATION. 



taneous generation had, at least at a former period 

 ^of the earth's history, existed, were the same as 

 influenced Lamarck ; but his hypothesis of the 

 mode in which variety and complexity have been 

 reached is different, namely, " that the several 

 series of animated beings, from the simplest and 

 oldest, up to the highest and most recent, are, 

 under the providence of God, the results, first, of 

 an impulse which has been imparted to the forms 

 of life, advancing them in definite times, by gener- 

 ation through grades of organization terminating 

 in the highest dicotyledons and vertebrata, these 

 grades being few in number, and generally marked 

 by intervals of organic character, which we find to 

 be a practical difficulty in ascertaining affinities ; 

 second, of another impulse connected with the vital 

 forces, tending in the course of generations to 

 modify organic structures in accordance with ex- 

 ternal circumstances, as food, the nature of the 

 habitat and the meteoric agencies, these being the 

 'adaptations' of the natural theologian." 1 Thus, 

 while he saw the possibility of a modifying power 

 being exerted by external circumstances, he appre- 

 ciated clearly the necessity of recognizing an 

 element on which it was exerted ; and the necessity 

 for that "impulse," as he calls it, ought to have 



1 Vestiges of the Nat. Hist, of Creation, loth edition, p. 155. 



