328 LAMARCK, HIS LIFE AND WORK 
Nature has gradually created the different organs of 
animals, varying the structure and situation of these 
organs according to circumstances, and has progres- 
sively improved their powers. She has begun by 
borrowing from without, so to speak—from the en- 
vironment—the productive force, both of organic 
movements and those of the external parts. ‘She 
has thus transported this force [the result of heat, 
electricity, and perhaps others (p. 307)] into the ani- 
mal itself, and, finally, in the most perfect animals 
she has placed a great part of this force at their dis- 
posal, as I will soon show.” 
This force incessantly introduced into the lowest 
animals sets in motion the visible fluids of the body 
and excites the irritability of their contained parts, 
giving rise to different contractile movements which 
we observe; hence the appearance of an irresistible 
propensity (penchant) which constrains them to ex- 
ecute those movements which by their continuity or 
their repetition give rise to habits. 
The most imperfect animals, such as the /zfusorza, 
especially the monads, are nourished by absorption 
and by “an internal inhibition of absorbed matters.” 
“They have,” he says, “no power of seeking their 
food, they have not even the power of recognizing 
it, but they absorb it because it comes in contact 
with every side of them (avec tous les points de leur 
individu), and because the water in which they live 
furnishes it to them in sufficient abundance.” 
“ These frail animals, in which the subtile fluids of 
the environing wz/zeux constitute the stimulating 
cause of the orgasm, of irritability and of organic 
