Ill] CAUSES OF VARIATION 25 



there is no evidence that they are inherited. For 

 example, many butterflies have two generations in 

 the year, one of which lives through its whole life- 

 history in the summer and the other passes the winter 

 as a pupa (chrysalis). In some cases the two genera- 

 tions are strikingly different, and it has been shown 

 that by freezing the pupae of the summer brood at 

 the right stage, specimens like the spring brood can 

 be obtained. The difference between the two genera- 

 tions is thus due to the action of cold on the pupa. 

 But the two forms regularly alternate in nature and 

 the effects of cold are not inherited. In plants, some 

 species produce quite different leaves according to 

 whether they are grown in water or in dry soil, but 

 the conditions act on the individual, and do not affect 

 its progeny. In such a case, what is inherited is 

 the faculty of making a certain definite response to 

 definite conditions, and this faculty is present whether 

 the conditions operate or not. In man such diseases 

 as tuberculosis are commonly called hereditary ; this 

 however does not mean that the child has the disease 

 because his parent had it, but that the parent had 

 a constitution liable to that disease, and the child 

 inherits a similar constitutional liability. If the 

 parent had never been exposed to infection the 

 child would still inherit the liability, for what is 

 transmitted is not the disease or its effects, but the 

 faculty of acquiring it if exposed. It will be found 



