16 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, jirs¢, 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.’* 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 
1Vestiges of the Nat. Hist. of Creation, roth edition, p. 155. 
