560 William Re iff 



tropics are the original home of its ancestors, atavism could be 

 produced only through the influence of heat. I could obtain no 

 reversional forms by this means, but, on the contrary, all the but- 

 terflies departed still further from the series of forms which we 

 have been considering; hence, the supposition lies near at hand 

 that in the north temperate zone coenia has become so far detached 

 from its original home that the species has already lost the ability, 

 or has already become too old to produce atavistic forms. This 

 would agree with the view which I have previously stated, that 

 coenia had already become a stable species in the nearctic fauna 

 before the glacial period, and that it continued to exist further 

 south as coenia during this period, and then again took possession 

 of the north after the expiration of the ice age without change in 

 its coloration or markings. Another fact that lends support to 

 this view is that the nearly related Pyrameis atalanta L., and P. 

 cardui L., react much more decidedly to the influence of tempera- 

 ture, so that the. phylogenetic age of these species' must be much 

 less than that of J. coenia. Nor does the distribution of these 

 species contradict this view, for atalanta, and especially cardui, 

 belong to the best flyers among the butterflies. 



If, therefore, the forms which one obtains through heat, and 

 which are more or less decidedly modified in one direction, show 

 no reversions, they must necessarilv be regarded as progressive 

 forms. It could be objected, perhaps, that heretofore in the experi- 

 ments of European investigators, progressive forms, so far as heat- 

 ing the pupae is concerned, have been produced by constant and 

 only moderately high temperatures, but we must bear in mind 

 that in the middle zone of the United States, where J. coenia is 

 most abundant, the temperature that prevails during by far the 

 greater portion of the year is the one which was artificially produced 

 in the European experiments. Hence I was, of course, obliged 

 to expose the pupae to a relatively higher temperature. The 

 experiments of European investigators have shown, however, that 

 in one and the same species, pupae exposed for a certain time to 

 moderate abnormal temperatures, may produce in a correspond- 

 ingly shorter time the same varieties as are produced by employ- 

 ing high temperatures. If the heat forms that have been obtained 



