HOW IS ORGANIC EVOLUTION CAUSED ? 405 



li-ves, 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 pce- 

 terity." '\Vhile 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 Philosophie Zoologique, 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 : distinguishiag 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 I'aide des excitations 

 tja'ils recoivent de I'esterieur." Afterwards he says — "je 

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

 milieux environnans la piiissmice excitatrice des mouvemens 

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

 posant de plus en plus I'organisation animale, transporter cetto 

 puissance dans I'interieur memo de ces etres, et qu'd la fin, 

 eUe parvint k mettre cette memo puissance k la disposition 

 de I'iadividu." And still more definitely he contends that 

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

 position de I'organisation," * * * " alors on eut jdu aperce- 

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

 le nombre ensuite s'est accru graduellement, ont amen^ le 

 penchant aux actions propres a y satisfaire ; coiament les 

 actions devenues habituelles et energiques, ont occasionn^ le 

 d^veloppement des organes qui les executont." 



Now though this conception of Lamarck is more precisely 

 etatedj and worked out with much greater elaboration and 



