TEMPERATURE AND LIFE. 423 



Species of Crustacea, as the ajrus and hranckijms, which develop between 

 0° and -I-3UO, accomplishing their complete evolution in 24 hours at a 

 temperature of 30^, while between 10° and 20° it takes weeks to obtain 

 the same result. Tadpoles hatch in 10 days at a temperature of 15.5^; 

 at 10.5° it requires 15 days. Notice how various are the requirements 

 of different creatures in the matter of temperature. Thatof SGC', so 

 favorable to hrancMpus, is fatal to many, excepting the entire animal 

 life of Arctic seas, and also, as I have already shown, a number of spe- 

 cies of the Mediterranean, especially those which inhabit the seashore 

 and can not adapt themselves to temperatures in pools heated by the 

 summer sun. 



There is therefore for every species a certain temperat ure at which de- 

 velopment is most rapid and life most easy. The limits of this thermic 

 condition vary considerably according to the species and even the va- 

 riety. Subjected to the influence of a lower temperature than that 

 which is most favorable, each animal's development is retarded, in dif- 

 ferent degrees, and often fails to attain perfection. If exposed to a 

 higher temperature than that which is best adapted to them, disturb- 

 ances are produced, alimentation becomes impaired, and the animal — 

 or vegetable — begins to pine, as is also the case with man in excessively 

 hot climates. 



This influence of temperature on life is not only manifested in de- 

 gree and rapidity of development, it also appears in other phenom- 

 ena ; coloration, for instance. In this way Weissmann has shown that 

 two butterflies, Vanessa levana and Vanessa prorsolevana^ differing in 

 coloration npon certain points, have been looked upon as belonging to 

 two distinct species, whereas in reality they represent but one. The 

 difference is simply a question of temperature. One comes from an 

 egg laid during the winter, and one from one laid in the summer, but 

 it is easy to obtain at will either variety from the same egg by heating 

 or cooling artificially, according to the case. A more important ques- 

 tion is the influence which the temperature exerts upon sexual devel- 

 opment. Cold retards and sometimes arrests it ; a certain degree of 

 temperature favors and accelerates it; and it is well known that sexual 

 development in man himself is hastened by the influence of a hot 

 climate. In Cuba, and other warm climates, a girl attains maturity at 

 12 years. But the temperature must not be too high either. Crusta- 

 cea kept for several weeks at 19<^ do not acquire sexual activity, whereas 

 at 9° or 10° it is acquired in 2 days. 



Temperature thus exercises considerable influence upon all organ- 

 isms. An interesting proof of these effects on the intensity of life (if it 

 may thus be called) is furnished by a study of the influence exercised 

 by this factor on the action of poisons and medicines. Alexander von 

 Humboldt, and after him many investigators, have noted that this ac- 

 tion is more instantaneous and rapid in high temperatures (which are 

 neither fatal nor dangerous in themselves) than at a lower degree. 



