316 HEREDITARY ADAPTATION TO HIGHER TEMPERATURE 



therefore, is to lower the upper temperature limit for the development 

 of the succeeding generation and not to elevate it as would be expected 

 if the adaptation to temperature was hereditary. 



It might be objected that the results shown in Table I are not due 

 to any difference either in the eggs or imago but merely to the fact 

 that, in the case of the imagos raised at 20°, the eggs which are to 

 give rise to the succeeding generation pass through the early stages 

 of development within the female while at the lower temperature and 

 so escape injury; while in the case of the cultures kept continuously 

 at 32° the early stages of the eggs must necessarily be passed at this 

 temperature and the eggs are thereby injured. If this was the case, 

 only those eggs produced by the 20° culture immediately after being 

 transferred to 32° should develop, and the ones produced later 

 should fail to develop. This, however, is not so. Imagos raised at 

 20° and transferred to 32° can produce eggs, capable of developing 

 at this temperature, for a week or 10 days after having been trans- 

 ferred from the lower temperature. 



It was found that imagos, raised and kept permanently at a tem- 

 perature of 30°, are unable to produce eggs capable of development at 

 this temperature. If, however, they are removed from the 30° incu- 

 bator within a week after emerging from the pupee and placed 

 at a temperature of about 20°C. for 24 hours or longer, they be- 

 come able to produce eggs capable of development at 30°C. when 

 replaced at this temperature. Table II is a summary of an 

 experiment illustrating this point. It is necessary to remove the 

 imagos from the higher temperature within a week or 10 days after 

 they have emerged from the pupae. If they are left longer at 

 the higher temperature, the injury becomes permanent and they are 

 no longer able to produce eggs capable of development at any 

 temperature. 



It is therefore not possible to raise more than one generation of 

 Drosophila at a temperature of 29° or over unless the culture is removed 

 to a lower temperature for at least 24 hours every generation. ^ A 

 culture has been continued at 30° by this method of intermittent 



^ This fact may seem surprising since the organism is a tropical form. The 

 explanation is probably that the temperature even in the tropics does not stay 

 continuously higher than 28 or 30° for more than a week or 10 days. 



