HOW IS ORGANIC EVOLUTION CAUSED? 405 



lives, all animals undergo perpetual transformations ; which 

 are in part produced by their own exertions, in consequence 

 of their desires and aversions, of their pleasures and their 

 pains, or of irritations, or of associations ; and many of these 

 acquired forms or properties are transmitted to their pos- 

 terity." While it embodies a belief for which a great deal 

 is to be said, this passage involves the assumption that 

 desires and aversions, existing before experiences of the ac- 

 tions to which they are related, were the originators of the 

 actions, and therefore of the structural modifications caused 

 by them. In his PMlosophie Zoologiqne, Lamarck 



much more specifically asserts " le sentiment ■ interieur" 

 to be in all creatures that have developed nervous sys- 

 tems, an independent cause of those changes of form which 

 are due to the exercise of organs : distinguishing it from 

 that simple irritability possessed by inferior animals, which 

 cannot produce what we call a desire or emotion ; and 

 holding that these last, along with all "qui manquent 

 de systeme nerveux, ne vivent qu'a 1'aide des excitations 

 qu'ils recoivent de l'exterieur." Afterwards he says — "je 

 reconnus que la nature, obligee d'abord d'ernprunter des 

 milieux environnans la puissance excitatrice des mouvemens 

 vitaux et des actions des animaux imparfaits, sut, en com- 

 posant de plus en plus 1' organisation animale, transporter cette 

 puissance dans l'iiiterieur meme de ces etres, et qu'a la fin, 

 elle parvint a mettre cette meme puissance a la disposition 

 de Tmdividu." And still more definitely he contends that 

 if one considers " la progression qui se montre dans la com- 

 position de 1' organisation," * * * " alors on eut pu aperce- 

 voir comment les besoins, d'abord reduits a nullite, et dont 

 le nombre ensuite s'est accru graduellement, ont amene le 

 penchant aux actions propres a y satisfaire ; comment les 

 actions de venues habituelles et energiques, ont occasionne* le 

 developpement des organes qui les executent." 



ISbw though this conception of Lamarck is more precisely 

 stated, and worked out with much greater elaboration and 



