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, ‘he 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 meet 
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. 
