The Making of Species 



The formulae indicate a very different arrange- 

 ment of the nine atoms which compose the 

 molecule in each case. And to this different 

 arrangement the differing properties of the two 

 compounds are supposed to be due. A rough 

 illustration of the phenomenon of isomerism is 

 furnished by written language. Thus, three 

 different words can be made from the letters t, 

 a, and r, e.g. tar, art, and rat. They also form 

 tra, which does not happen to be an English 

 word, although it might have been one. 



Among organisms we sometimes observe a 

 phenomenon which looks very like isomerism. 

 The classical example of this is furnished by 

 the butterflies Vanessa prorsa and Vanessa 

 levana. 



At one time these were supposed to belong to 

 different species, since they differ so greatly in 

 appearance. Vanessa levana is red, with black 

 and blue spots. Vanessa prorsa is deep black, 

 with a broad yellowish-white band across both 

 wings. It is now known that the levana is the 

 spring form and the prorsa the summer and 

 autumn form of the same species. The pupae of 

 levana produce the prorsa form, but Weismann 

 found that after being placed in a refrigerator 

 they emerged, not as prorsa, but partly as levana 

 and partly as another form intermediate in many 

 respects between levana and prorsa. Weismann 

 also succeeded, by exposing the winter pupa to a 



