LAMARCK'S TBEORY OF DESCENT 



303 



the effects of its inner feelings ; which I shall constantly 

 prove by known facts. 



" Thus, to arrive at a knowledge of the true 

 causes of so many different forms and so many dif- 

 ferent habits of which the known animals offer us 

 examples, it is necessary to consider that circum- 

 stances infinitely diversified, but all slowly changing, 

 into which the animals of each race are successively 

 thrown, have caused, for each of them, new wants 

 and necessarily changes in their habits. Moreover, 

 this truth, which cannot be denied, being once rec- 

 ognized, it will be easy to see how the new needs 

 have been able to be satisfied, and the new habits 

 formed, if any attention be given to the two follow- 

 ing laws of nature, which observation always confirms : 



"First Law. 



" In every animal which has not exceeded the term 

 of its development, the more frequent and sustained 

 use of any organ gradually strengthens this organ, de- 

 velops and enlarges it, and gives it a strength propor- 

 tioned to the length of time of such use ; while the 

 constant lack of use of such an organ imperceptibly 

 weakens it, causes it to become reduced, progressively 

 diminishes its faculties, and ends in its disappearance. 



" Second Law. 



" Everything which nature has caused individ- 

 uals to acquire or lose by the influence of the 

 circumstances to which their race may be for a long 

 time exposed, and consequently by the influence of 

 the predominant use of such an organ, or by that of 

 the constant lack of use of such part, it preserves by 

 heredity {gdn^ratioti) and passes on to the new indi- 

 viduals which descend from it, provided that the 

 changes thus acquired are common to both sexes, or 



