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 eiforts from within. 



" From the foregoing these two general laws may be 

 deduced : — 



" Firstly. That m every ammal wMeh has not passed 

 its limit of development, the more frequent and sustadned 

 employment of any organ develops and aggrandizes it, 

 giving it a power proportionate to the dwation of its 

 employment, while the same organ in default of constant 

 use heoomes insensibly weakened and deteriorated, de- 

 creasing imp&reeptibly in power until it finally dis- 

 appears.\ 



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

 velopment, du£ to use or disuse, are transmitted to offspring, 

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

 animals from which the offspring have descended." % 



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. % Page 236. 



