THEORIES OP LAMARCK AND DARWIN 381 



and an opinion to which he held unwaveringly to the close 

 of his life. It would be of great interest to determine when 

 Lamarck changed his views, and upon what this radical rever- 

 sal of opinion was based; but we have no sure record to 

 depend upon. Since his theory is developed chiefly upon 

 considerations of animal life, it is reasonable to assume that 

 his evolutionary ideas took form in his mind after he began 

 the serious study of animals. Doubtless, his mind having 

 been prepared and his insight sharpened by his earlier studies, 

 his observations in a new field supplied the data which led 

 him directly to the conviction that species are unstable. 

 As Packard, one of his recent biographers, points out, the 

 first expression of his new views of which we have any record 

 occurred in the spring of 1800, on the occasion of his opening 

 lecture to his course on the invertebrates. This avowal of 

 belief in the extensive alteration of species was published in 

 1 801 as the preface to his Systeme des Animaux sans 

 Vertebres. Here also he foreshadowed his theory of evo- 

 lution, saying that nature, having formed the simplest 

 organisms, "then with the aid of much time and favor- 

 able circumstances . . . formed all the others." It has 

 been generally believed that Lamarck's first public ex- 

 pression of his views on evolution was published in 1802 

 in his Recherches sur V Organisation des Corps Vivans, 

 but the researches of Packard and others have established 

 the earlier date. 



Lamarck continued for several years to modify and am- 

 plify the expression of his views. It is not necessary, how- 

 ever, to follow the molding of his ideas on evolution as 

 expressed in the opening lectures to his course in the years 

 1800, 1802, 1803, and 1806, since we find them fully elab- 

 orated in his Philosophie Zoologique, published in 1809, 

 and this may be accepted as the standard source for the 

 study of his theory. In this work he states two propositions 



