' 2 i8 PROBLEMS OF GENETICS 



This distinction arose as soon as the species was taken into 

 the hothouse, but when the modified individuals were put back 

 into the original conditions, a lower temperature and scanty 

 food-supply, the next generation returned to their original form. 

 After being cultivated for two years and about 40 generations in 

 the more favourable conditions, when similarly put back into 

 the lower temperature with scanty food the first generation born 

 in these conditions was helmeted like the modified parents. 

 Woltereck is of opinion that the ova were still unformed at the 

 time the parents were put back, and the influence of the favour- 

 able conditions upon the unformed ova he speaks of as a "prae- 

 induction." The effect never extended beyond the one gener- 

 ation, after which the strain returned to its original state. 



The fact that the influence on the offspring was not mani- 

 fested at first led Woltereck to expect that by more prolonged 

 cultivation in the favourable conditions a further extension of 

 this influence would be produced, but this expectation was never 

 fulfilled, though the attempt was made again and again. 



Similar experiments were made with Hyalodaphnia cucullata, 

 which is far more sensitive to cultural influences, and in nature 

 manifests a considerable elongation of the helmet as a seasonal 

 modification, but the results were essentially the same as in the 

 preceding case, no modification extending beyond the first 

 generation born after the restoration to normal conditions? 



The only criticism of these extremely interesting results which 

 suggests itself is that perhaps the original appearance of the 

 modification was not in reality due to an accumulated effect of 

 the conditions, but to some change in the conditions themselves 

 which was not noticed. It is difficult to see how length of 

 time or even the lapse of several generations could have so specific 

 an effect on the race. It is no doubt often vaguely supposed 

 by many that a long period of time may be necessary for the 

 effect of climate or of other environmental conditions to be 

 produced in an organism which does not thus respond at first. 

 I have never been able to see any reason for this opinion nor how 



5 See Woltereck, Verh. d. Deut. ZooL Ges., 1909, p. no; and 1911, p. 142. This 

 is a subject which can only be properly appreciated on reference to the original 

 papers. Several complications are involved to which I have not here alluded. 



