EVOLUTION 365 



transferred to a locality where S. rubra was the only willow 

 available. The female sawflies began to lay eggs on this 

 form, and in the course of a few years this habit became 

 so well-established that when access to S. andersoniana 

 was again made possible none of the flies showed any 

 disposition to return to the '' ancestral " plant. The 

 " acquired habit of oviposition " on the new species had 

 become " germinally fixed," and the sawfly race was 

 " being forced along an evolutionary path away from the 

 parent species." 



To sum up this brief discussion on the Lamarckian 

 factor in evolution, it must be admitted that belief in it is 

 encouraged on account of the simple manner in which it 

 explains — if it be a true cause — many observed facts of life. 

 How can, for example, the winglessness of parasitic insects 

 or the blindness of cave-insects be more simply explained 

 than by invoking in the one case cessation of flight and in 

 the other a uniformly dark environment ? But it must 

 be admitted that the amount of positive evidence, observa- 

 tional or experimental, to support belief in use-inheritance 

 is as yet small. The prudent attitude would seem 

 therefore to be one of open-minded scepticism, for the work 

 of Tower, Harrison, and others has made the dogmatic 

 scepticism which has prevailed during the past forty years 

 about the Lamarckian factor appear unreasonable. It is 

 certain that germinal modifications, which can be inherited, 

 may be induced through nutritional or environmental 

 change, but we have as yet no ground for asserting that 

 such modifications are so strongly established in the race 

 that they become powerful factors in the working out of 

 evolutionary progress. 



Fifty years after the publication of de Lamarck's theory, 

 the great work of Charles DarVvdn (1859), to which reference 

 has already (p. 325) been made, was published. The title 

 of this book was noteworthy because it conveys in a few 

 words the essence of Darwin's theory. He called it : '' The 

 Origin of Species by means of Natural Selection or the 

 Preservation of Favoured Races in the Struggle for Life." 



