COLOKATION AFFECTED BY THE BNVIEONMENT. 81 



American species by Mr. Edwards, Professor Weismann has 

 contributed, in an appendix to the English edition of his 

 " Studies in the Theory of Descent," some further remarks 

 upon the subject. Mr. Edwards experimented upon several 

 American butterflies, including Grapta interrogationis, a 

 species allied to our " Comma." The summer form of this 

 butterfly is known as G. umbrosa, the winter form is G. 

 Fabricii ; but there are intermediate generations which are 

 mixed, that is, they consist of both forms in varying propor- 

 tions. Some eggs laid by a female umbrosa of the second 

 brood were hatched under natural conditions, confined by a 

 muslin bag upon a branch of the food plant. As the pupte 

 formed, they were removed at intervals to an ice box, in 

 order to find out if it were essential that exposure to cold 

 should take place immediately after pupation for an efi'ect 

 to be produced. If left to nature, it would be expected 

 that the resulting butterflies, belonging as they do to an 

 intermediate brood, would be of both forms. The result was 

 that every specimen belonged to the form umbrosa, but they 

 were darker-coloured in parts. Professor Weismann points 

 out that as two out of the four annual generations are mixed — 

 that is, consist of summer and winter forms — " the conclusion 

 is inevitable that these forms were not produced by the 

 gradual action of heat and cold." "When the same conditions 

 produce two different results, neither can be said to be due 

 to those conditions. So says Professor Weismann ; but is 

 this conclusion certain ? 



It seems quite conceivable that the same condition may 

 produce different results upon different individuals, if we bear 

 in mind that one result is merely a maintenance of the status 

 quo, while the other is a change : individuals must, indeed do, 

 vary in susceptibility to external influence, as I point out 



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