THEORIES OF EVOLUTION 145 



theme but much less rigorously than Lamarck. The 

 earlier theories were either too vague and discursive 

 or too inadequate to serve as foundations. La- 

 marck's theory was so much more thoroughly thought 

 out that it completely superseded all earlier attempts 

 and marks the beginning of evolutionary thought in 

 its modern sense. 



Lamarck (Fig. 29) first gave expression to his 

 evolutionary ideas in 1800, in an introductory lecture 

 to a course of instruction regarding the invertebrates. 

 This was published in 1801. The theory was some- 

 what elaborated in the years 1802, 1803, and 1806. 

 Finally, it was fully expounded in Philosophic 

 Zoologique, in 1809, and that year marks the first 

 distinct epoch in the rise of evolutionary thought. 



In this book he sets forth the basis of his con- 

 clusions. He ascribes to the effects of use and disuse 

 the various modifications of organic structures use 

 tending to increase, and disuse to decrease the size 

 and efficiency of organs. The changes thus pro- 

 duced, by organisms adapting themselves to the con- 

 ditions under which they live, were supposed by 

 Lamarck to be directly inherited and improved (or 

 further decreased) in succeeding generations. 



It appears, he says: "that time and favorable con- 



