34 GENETICS AND EUGENICS 



walnut leaves and presented the paler coloration; the second 

 and third generations were fed on oak leaves but retained the 

 modified coloration. In the third generation, however, the 

 female showed partial return to normal coloration. 



Pictet observed some cases in which moths became so 

 completely accustomed to the diet of walnut leaves that their 

 coloration became normal. Delage regards this as greatly- 

 weakening the case for inherited modification. He interprets 

 the case thus. Walnut leaves are in general a poor diet for 

 gipsy-moth larvae. They weaken the animal. This weakness 

 persists through one or more generations, doubtless because 

 of impaired constitution of the egg, but is not certainly trans- 

 mitted as an acquired character. Indeed the race may re- 

 cover from the weakening produced by the changed diet. 



6. Temperature experiments. Many experiments have been 

 performed with moths and butterflies in which the pupae 

 were subjected to abnormally low or abnormally high tem- 

 peratures. The effects of both extremes are in many cases 

 similar. In general extremely low or extremely high tem- 

 peratures produce darker adults. Fischer reared adults from 

 pupae of Arctia caja exposed to a very low temperature, 8° C. 

 Abnormally dark adults were obtained in this way. Some of 

 the darkest of these, produced under normal conditions un- 

 usually dark offspring. Fischer considers that the induced 

 modifications were transmitted. But this is far from certain 

 for (1) the moths vary in darkness of coloration under normal 

 conditions. It is not established that the supposedly induced 

 variations lie outside the range of normal variation. (2) 

 Fischer's treatment served to show what animals were nat- 

 urally inclined to become dark, for these under treatment 

 would become darkest, and from such Fischer bred. The 

 supposed transmission of an acquired characteristic may be 

 regarded in this case as nothing but the transmission of a 

 natural or inborn characteristic, the treatment serving as a 

 guide to selection. 



Weismann, however, influenced by studies of his own upon 

 variation in color of butterflies in northern and in southern 



