Vo2 ^ VARIATION 



Plants, and animals too for that matter, growing in cold 

 climates or under hard conditions suffer profound changes, to 

 which they become accustomed (acclimated) and which are ever 

 afterward constitutional. We become accustomed to cold or to heat 

 and are thereafter less affected by extremes. Recent calorimeter 

 tests show that the temperature of the human body is lowest from 

 three to five o'clock in the morning and highest from one to three 

 in the afternoon, thus following fairly close the minimum and 

 maximum of outside temperatures. These conditions continue 

 even if the subject works at night and sleeps in the daytime. 



Two conditions tend to produce hard and spiny growth in 

 vegetation. These are intense light and extreme dryness. Both 

 are found in tropical regions, and when they occur together their 

 maximum results follow as to harshness and spines. These condi- 

 tions can be verified in the laboratory, showing conclusively 

 that the character of growth is, in a measure at least, dependent 

 upon surroundings. 



Speaking generally, plant lice reproduce parthenogenetically 

 during the growing season of the summer, and during this time 

 only wingless females are produced. With the approach of cold 

 weather, however, a winged bisexual brood is produced that lives 

 over winter. 



These conditions can be produced artificially in the greenhouse 

 at any time by lowering the temperature and allowing the plants 

 on which the lice feed to dry up. Thus we may say that wings 

 and sex may be developed at will by the manipulation of the condi- 

 tions of life. 



The so-called conversion of one species into another by influ- 

 encing its environment has been largely overstated, and yet the 

 facts are that when Schmankewitsch l grew Artemia satina in 

 water whose saline content was gradually increased, the caudal 

 fins and their bristles " progressively degenerated " until, in many 

 cases, these appendages had disappeared, the animal thus assum- 

 ing the character of A. milhausenii, which normally lives in waters 

 of extreme density. These experiments were undertaken because 

 he seemed to have observed this transformation taking place 

 naturally in a lake crossed by a dam, and which was inhabited 



1 Bateson, Materials, etc., p. 96. 



