EXPEKIMENTS IN HYBRIDIZATION AND TEMPERATURE. 163 



L., Polyommatus phloeas, L,, Pararge egeria, L. — all species 

 which exhibit seasonal dimorphism — led to analogous results. 



As fa.r as Dorfmeister, Weismann, and Edwards are concerned 

 up to the year 1875, all three of them dealt with a species hy and 

 for itself, as an isolated type relative to the action of various 

 degrees of temperature, during the pupal stage on the resulting 

 changes in character of the species, within its own limits, with- 

 out considering their relationship to those of other species. As 

 a matter of fact, the species experimented with up till that 

 time were not especially suited to open up further phylogenetic 

 research. 



Exactly ten years later I commenced analogous experiments, 

 although not to any great extent until 1893, as my hybridization 

 experiments, which were commenced in 1873, and carried on 

 later in conjunction with the temperature experiments, took up a 

 good deal of my time until then. 



Ever since the year 1888 the English entomologist Merrifield 

 has been making experiments in the same direction, and since 

 that time has published a number of papers on this question in 

 the ' Transactions ' of the Entomological Society of London. 

 Weismann has also continued his experiments lately, and 

 numerous other younger entomologists are now making similar 

 experiments. 



The temperature experiments made by me can be naturally 

 divided into two groups. On the one hand, by using constant 

 high temperatures of +37° to +39° C, in which the pupae were 

 placed three or more days ; or by constant low ones of +4° to 

 + 6° C, which lasted four to eight weeks. We will call these, 

 shortly, warmth and cold experiments. 



The first were made in the developing apparatus of the 

 Polytechnic Seed Culture Station, Zurich, whose Director, Dr. Gr. 

 Stebler, has assisted me in the kindest and most willing manner. 

 For the second, I have used an ordinary ice safe, as is used for 

 keeping food in many households. 



After the treatment with cold the pupae remained for some 

 time in an ordinary temperature till the imagines emerged. 

 This was also usually the case with the warmth experiments, 

 very few pupae remaining in the forcing apparatus until they 

 were fully developed. 



On the other hand, I made experiments which could be 

 termed heat and frost experiments, the temperature being only 

 intermittently applied — two to seven hours at a time — because 

 only capable of being endured for short intervals. 



The heat experiments were carried out with the help of the 

 forcing apparatus at from +40° to +45° C. 



The degrees of cold, 0° to -18°, exceptionally to -20° C, 

 were procured at the freezing apparatus of the Institute for 



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