J8 EXPERIMENTAL EVOLUTION LECT. 



that many know not whether he is to be accounted 

 as a friend or as a foe. The truth is that Buffon's 

 views on the unity of species lacked unity themselves. 

 From 1753 to 1756, it is quite clear, as Geoffroy Saint 

 Hilaire has shown, that he believed in their immut- 

 ability. "Species in animals," he says, "are all 

 separated from each other by an interval which Nature 

 cannot cross." Later on, his writings show a different 

 turn of mind, and from 1761 to 1766, more particularly 

 give evidence thereof. " One is surprised at the prompt- 

 ness with which the species vary, and at the ease with 

 which they become altered and assume new forms." 



This theory of the mutability of species he con- 

 sidered, some years later, to be rather exaggerated, 

 and he returned to more moderate views, though not 

 abandoning the theory of variability and mutability, 

 which, in his opinion, are due to the direct influence 

 of environment. 



After Buffon comes Lamarck, a friend and pupil of 

 the former. Lamarck was the first to state distinctly, 

 in any developed form, the theory of the variability 

 and transmutation of species, which many had before 

 him briefly proposed or supposed, and he tried to dis- 

 cover the cause of this variability. 1 The facts of varia- 



1 See his Philosophic ZoologicjJie, 1809 ; Introduction a FHistoire 

 naturelle des Animaux sans Verlebres, 1815 ; and Systeme de Con- 

 naissances positives ', 1820. 



