CAUSES OF GENETIC VARIATION 219 



it is to be translated into terms of physiological fact, and I 

 imagine that in those cases in which the lapse of time is really 

 required for the production of an effect, the influence of the 

 prolongation is rather on the conditions than on the organisms. 

 The response of the organisms thus probably indicates not that 

 the creature is at length feeling the effects because of their 

 accumulated action on itself, but that the conditions have at 

 length ripened. 



As this sheet is passing through the press Agar has published 6 

 an abstract of evidence as to another comparable case in a par- 

 thenogenetic strain in the daphnid, Simocephalus vetulus. When 

 fed on certain abnormal foods the shape of the body is changed, 

 the edges of the carapace being rolled backwards so as to expose 

 the appendages. The offspring of animals thus modified showed 

 similar modification in the first, and to a very slight degree, in 

 the second generation, though the original mothers were removed 

 to normal conditions before their eggs were laid. In the third 

 generation there was "a very pronounced reaction in the opposite 

 direction." Agar suggests that the change may be due to some 

 toxin-like substances, carried on passively by the egg into the 

 next generation, against which the protoplasm eventually pro- 

 duces an anti-body. 



The experiments which have been in recent years regarded 

 by evolutionary writers as the most conclusive proof that direct 

 environmental action may produce germinal variation are those 

 of Professor W. L. Tower, of Chicago, on Leptinotarsa, the 

 potato beetles. This work has attained considerable celebrity 

 and has been generally accepted as making a definite extension 

 of knowledge. After frequently reading Tower's papers and 

 after having been privileged to see some of the experiments in 

 progress (in 1907) I am still in doubt as to the weight which 

 should be assigned to this contribution. 



The work is described in tw r o chief publications, the first of 

 which appeared in I9o6. 7 This treatise contains a vast amount 

 of information about numerous species and varieties of these 



*Proc. Roy. Soc., B, Vol. 86, 1913, p. 113. 



7 An Investigation of Evolution in Chrysomelid Beetles of the Genus Leptinotarsa, 

 Carnegie Publications, 1906, No. 48. 



