Double Adaptations 453 



adaptation, and on such a base a hypothetical 

 explanation could no doubt be rested. 



Instead of discussing this problem from the 

 theoretical side, I prefer to compare those spe- 

 cies which are capable of assuming a dwarf 

 stature under less uncommon conditions than 

 those of alpine and desert-plants. Many weeds 

 of our gardens and many wild species have this 

 capacity. They become very tall, with large 

 leaves, richly branched stems and numerous 

 flowers in moist and rich soil. On bad soil, or 

 if germinating too late, when the season is 

 drier, they remain very small, producing only 

 a few leaves and often limiting themselves to 

 one flower-head. This is often seen with thorn- 

 apples and amaranths, and even with oats and 

 rye, and is notoriously the case with buck- 

 wheat. G-auchery has observed that the ex- 

 tremes differ often as much from one another 

 as 1 :10. In the case of the Canadian horse- 

 weed or Erigeron canadensis, which is widely 

 naturalized in Europe, the tallest specimens 

 are often twenty-five times as tall as the small- 

 est, the difference increasing to greater ex- 

 tremes, if besides the main stem, the length of 

 the numerous branches of the tall plants are 

 taken into consideration. Other instances 

 studied by the French investigator are Ery- 

 thraea pulchella and Calamintha Acinos. 



