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- j 

 toric times from the single primitive form, the wild or blue rock '\ 

 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 ! l 



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 1 

 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. 



