Ill] CAUSES OF VARIATION 27 



But as mentioned above, it is possible that the 

 same factor acting on body and germ-cells may pro- 

 duce different results in the two cases, so that the 

 individual on which the influences have acted may 

 show one modification and its offspring another. It 

 is also possible that not aU the germ-cells will be 

 affected alike, and so among the progeny some will 

 show modification and others not, or some may be 

 differently affected from others ; for the conditions of 

 stability of different germ-cells may conceivably be 

 different. Certain experiments on insects give reason 

 for supposing that this is so. The results obtained by 

 Fischer, Standfuss and others from exposing pupae 

 of butterflies and moths to abnormal temperatures, 

 while not entirely concordant among themselves, on 

 the whole indicate that moderate degrees of heat and 

 cold tend to alter in the same way the whole batch of 

 insects treate<l, often in the direction of varieties of 

 the species naturally occurring in warmer or colder 

 climates. But excessive heat or cold causes extreme 

 variations among only a small proportion of the 

 insects treated, and among the offspring of these ab- 

 normal specimens only a small fraction are abnormal, 

 and some of these have not the same abnormality 

 as the parents. These observations, together with 

 the fact that the variations produced by heat, cold, 

 and other disturbing factors, may all be similar, 

 suggest that extreme conditions may upset the 

 stability of the type, causing abnormalities to appear. 



