r 9 4 PROBLEMS OF GENETICS 



{219, took place on fragilis, and there were 127 on the tomentose 

 bush, which we are told was six times as large as the fragilis. 

 The larvae from fragilis were next put on the tomentose species 

 and reared on it. When they became imagines they were simi- 

 larly given their choice, with the result that there were 104 

 ovipositions on the tomentose species and only 83 on fragilis. 

 In the next generations there were 48 ovipositions on the tomen- 

 tose and 11 on fragilis. Finally the fourth generation made 

 15 ovipositions on the tomentose and none on fragilis. 



The difficulty about such experiments is obviously that one 

 has no assurance that the change of instinct, in so far as there 

 is any, may not be a mere consequence of the captivity. It 

 must, besides, be extremely difficult to arrange the experiment so 

 that there is really an equal choice between the two bushes, when 

 one stands beside the other. Przibram, in quoting this case, 

 considers that as the tomentose bush was about six times as 

 large as the fragilis, some indication of the relative attractiveness 

 of the two may be obtained by dividing the ovipositions on the 

 larger bush by six, but I imagine the matter must be much more 

 complex. 



Schroder's second example is not more convincing, in my 

 opinion, though Semon regards it as one of the most important 

 pieces of evidence. It concerns a leaf-rolling moth, Gracilaria 

 stigmatella, the larva of which is said normally to make its house 

 by bending over the tips of the sallow leaves on which it feeds. 

 Schroder placed larvae on leaves from which the tips had been 

 cut, and these larvae made their houses by rolling over the sides 

 of the leaves. Their offspring were again fed on leaves without 

 tips, and as before, they rolled in the leaf-margins either on one 

 side or both. The offspring of this second generation were then 

 fed on entire leaves. There were 19 houses made by these (?I9) 

 larvae, and of them 15 were normal, made by folding down the 

 tips of the leaves, while 4 were abnormal, made by rolling in the 

 leaf-margins. Schroder says that in nature he has only twice 

 seen abnormal houses ; but it is clearly essential not only that the 

 frequency of such variability in nature should be thoroughly 

 examined, but also that we should know whether when the species 



