82 DOMESTICATED ANIMALS AND PLANTS 



Summary. Infinitely more individuals are born into the world than can 

 possibly find room and food. This sets up a struggle for food and room 

 and the right to live, under which the fittest alone survive to reproduce 

 their kind. 



In this way the race is modified or improved, because each succeeding 

 generation is born, not from average individuals, but from those that are 

 best able to meet the demands. If conditions remain constant, in a few gen- 

 erations the " fit " becomes close ; but if the conditions change, the standard 

 of fitness and selection changes also, which necessarily results in a modi- 

 fication of the race in a new direction, the principle being that whatever 

 happens to individuals^ the race as a whole will respond to selection from 

 whatever standard administered. This is the principle on which the 

 breeder operates, though his standards of selection are the ones that 7neet 

 his needs, and may not be the same as those of nature. 



Exercises. 1. Estimate the number of seeds in a robust plant of purslane, 

 pigweed, or plantain. 



2. Ascertain the number of kernels on a single ear of corn, and calcu- 

 late how long it would take one ear to produce seed enough to plant the 

 entire state. 



3. Outline the causes that prevent the unlimited increase of various 

 species, especially man and the animals and plants most closely related to 

 his affairs. 



4. Make original studies into the different methods by which the most 

 troublesome weeds persist in spite of our most persistent efforts to eradicate 

 them ; for example, Canada thistle, morning-glory, ragweed, purslane. 



5. How is it that weeds " come up " in new lands never before culti- 

 vated, and what are the various ways by which birds and other animals 

 carry weed seeds ? 



6. Go to the fields and observe the various ways by which seeds trans- 

 port themselves, especially by wind and water. Make studies of definite 

 species and describe carefully their habits of seed distribution ; for example, 

 wild cherry, thistle, cocklebur. 



References. 1. " Origin of Species " (especially chaps, iii and iv). Darwin. 



2. " Darwiniana." Asa Gray. 



3. " Darwinism." Wallace. 



4. " Color of Animals." Beddard. 



