96 DOMESTICATED ANIMALS AND PLANTS 
balance of natural condition of climate, room, and food supply. 
If these fundamentals are provided for, selection is able to 
modify type in many directions at the same time, so that from 
a single original stock a multitude of diverse forms may be 
built up. 
There are no better instances of this than the pigeon, the 
many and diverse varieties of which have been bred within his- 
toric times from the single primitive form, the wild or passenger 
pigeon (see Fig. 13). Hardly second to this is the wonderful 
variety in the different breeds of the dog, well known to all 
observers. 
If this can be done with these species, what a future of possi- 
bilities is opened up for still further developing and improving 
our animals and plants of field, orchard, and garden! 
Summary. The marvelous effects of natural selection and its power to 
modify type to fit the surroundings simply through the extermination of the 
inferior individuals, suggests to man a means of still further adapting these 
species to his own needs. 
In nature the basis of selection is simply the power to live and repro- 
duce fast enough to keep up with the death rate. Man, on the other hand, 
is interested in something besides mere life and reproduction. 
For example, he keeps the cow for her milk, and he is interested in the 
amount she can give. In nature she needed only to give enough for the 
calf, and that only until he could wholly or partly shift for himself. In do- 
mestication, on the other hand, man considers the cow as a machine that 
should give all the milk possible and give it continuously. Manifestly, 
therefore, man must set up some additional standards of selection, and all 
the evidence is that he does this; the domestic cow reacts, and increases her 
output. This does not mean that a poor cow can be made into a good one 
by any process known to man, but does mean that if the dairyman breeds 
only from his best cows, the calves will develop into a better lot, on the 
average, than they would have been if he had bred from good, bad, and 
indifferent. 
This is artificial selection, copied after nature’s plan. It has been prac- 
ticed from the earliest times, and is the process that has produced about all 
the improvement that has been made up to near the present day. 
This plan of improvement by selection will be considered later in detail 
under the head of systematic improvement of animals and of plants. 
