THEORY OF EVOLUTION 35 



fact, an unfolding of what pre-existed in the 

 egg, and the term still carries with it some- 

 thing of its original significance. 



Nageli's speculation written several years 

 after Darwin's "Origin of Species" may be 

 taken as a typical case. Nageli thought that 

 there exists in living material an innate power 

 to grow and expand. He vehemently pro- 

 tested that he meant only a mechanical prin- 

 ciple but as he failed to refer such a principle 

 to any properties of matter known to physicists 

 and chemists his view seems still a mysterious 

 affirmation, as difficult to understand as the 

 facts themselves which it purports to explain. 



Nageli compared the process of evolution 

 to the growth of a tree, whose ultimate twigs 

 represent the living world of species. Natural 

 selection plays only the role of the gardener 

 who prunes the tree into this or that shape but 

 who has himself produced nothing. As an 

 imaginative figure of speech Nageli's compari- 

 son of the tree might even today seem to hold 

 if we substituted "mutations" for "growth", 

 but although we know so little about what 

 causes mutations there is no reason for suppos- 



