SUMMARY OF'PHILOSOPHIE ZOOLOGIQUE: 299 



illusion. We establish, therefore, the three following 

 propositions : 



" 1. That every considerable and sustained change 

 in the surroundings of any animal involves a real change 

 in its needs. 



" 2. That such change of needs involves the necessity 

 of changed action in order to satisfy these needs, and, 

 in consequence, of new habits.* 



" 3. It follows that such and such parts, formerly 

 less used, are now more frequently employed, and in 

 consequence become more highly developed ; new parts 

 also become insensibly evolved in the creature by its 

 own efforts from within. 



" From the foregoing these two general laws may be 

 deduced : 



" Firstly. That in every animal which has not passed 

 its limit of development, the more frequent and sustained 

 employment of any organ develops and aggrandizes it, 

 giving it a power proportionate to the duration of its 

 employment, while the same organ in default of constant 

 use lecomes insensibly weakened and deteriorated, de- 

 creasing imperceptibly in power until it finally dis- 

 appears.^ 



" Secondly. That these gains or losses of organic de- 

 velopment, due to use or disuse, are transmitted to offspring, 

 provided they have been common to loth sexes, or to the 

 animals from which the offspring have descended." J 



Lamarck now sets himself to establish the fact that 

 animals have developed modifications which have been 

 transmitted to their offspring. 



* ' Phil. Zool.,' torn. i. p 234. t Page 235. J Page 236. 



