PRACTICAL LAWS OF LIFE 333 



everyone cooperated, beginning with present numbers, or with ten pairs,, 

 to increase them to limits of insect and weed-seed food supply? 



Figuring the number of buds produced by a grape, peach, apple, 

 i-trawberry, or other fruit, the number of eyes by a potato, the number 

 ( >f seeds by a grain or vegetable plant, how long would it take to supply 

 t very farm or garden with a favorable variation ? This introduces us to 

 the second practical law of life. 



Law of variation. No two liviny things are exactly alike. 

 Oan we find two forest leaves, blades of grass, or human 

 'aces exactly alike ? Living organisms are too complicated 

 for it to happen, even by chance, that any two should be 

 ilike. So this universal law of living nature has given us all 

 our different kinds of plants and animals. 



Domesticated plants and animals early attracted Darwin's 

 attention as showing variations most clearly. 1 Horses, cattle, 

 sheep, dogs, pigeons, and all manner of cultivated plants have 

 varied in the brief centuries of human control, and are still 

 varying, in most wonderful fashion. We have horses, from 

 Clydesdales and Norman Percherons to Shetland ponies, all 

 produced by human breeding and selection. Ages before 

 man appeared on the earth little Eohippus, not much larger 

 than a fox, with five toes, four of them hoofed, trotted over 

 the bogs of the times ; and we can now trace in successive 

 strata of rocks how the modern horse developed from this 

 earliest form. The story of other animals and even of man 

 himself we have not as yet been able to trace so clearly. 



The great practical values attaching to variations in relation 

 to agricultural productions are touched upon in Chapter IX. 

 Since these depend so largely upon the possibilities of increas- 

 ing and propagating favorable variations, we must consider 

 this subject further in connection with the greatest of all 

 biological laws. 



i Darwin, Variations of Animals and Plants under Domestication, 



