The Making of Species 
The formule 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 /evana is the 
spring form and the pvorsa the summer and 
autumn form of the same species. The pupz of 
levana produce the prorsa form, but Weismann 
found that after being placed in a refrigerator 
they emerged, not as prorsa, but partly as devana 
and partly as another form intermediate in many 
respects between /evana and prorsa. Weismann 
also succeeded, by exposing the winter pupa to a 
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