THE GEOGRAPHICAL DISTRIBUTION OF LIFE 445 



and in this none of our plants are found. As we go- influence of 

 from one environment to another, we observe that our 



plants differ in their ability to exist in them, though the develop- 

 there is no doubt that their seeds have reached these adaptations 

 places. The dandelion is able to endure surroundings 

 quite impossible for the sunflower. It also spreads 

 more easily on account of its parachutelike fruits, and 

 when once established lasts a long while, being a 

 perennial. Thus we find ourselves discussing all the 

 characters of the plants in their relation to the sur- 

 roundings ; the study of distribution becomes a broad 

 study of dynamic botany, of forces rather than of mere 

 structures. As in the other case, we find that precise 

 answers to our questions are often impossible. They 

 could be reached only through the knowledge of facts 

 which we perhaps have neither time nor ability to 

 ascertain. This must not prevent us from doing our 

 best; the human mind must always face the unknown 

 in the process of education. 



From time to time we shall be rewarded by discoveries 

 which will reveal the wonderful machinery of Nature. 

 Thus it was found out that the spread of the Spanish 

 bayonet or Yucca was strictly limited by the range of a 

 little white moth which carries the pollen and brings 

 about fertilization. Conversely, of course, the range 

 of the moth is limited by that of the Yucca. These two 

 partners have to go together; they cannot spread inde- 

 pendently of each other. The Eastern United States, 

 and particularly the Mississippi Valley, are remarkable 

 for the great abundance and variety of large fresh-water 

 clams. We know from fossils that similar shells were 

 once more widely distributed, as they occur abundantly 

 in certain deposits of the Rocky Mountain region. It 

 might appear that they could live wherever there was 



