INTRODUCTORY LETTER TO SENATOR MO550xix 



than that of a larger animal, an important considera- 

 tion, as upwards of one hundred were kept at a time 

 for several years. 1 Another important advantage de- 

 pends on the fact that nearly every individual is 

 marked with spots and blotches of brown and black 

 differently from all others, so that they all can be 

 readily told apart without any artificial marks, and 

 hence it is easier to follow the growth of individuals. 

 Occasionally there is one all white, but such white 

 ones can be marked with spots of nitrate of silver on 

 the hair. Guinea-pigs are so unintelligent that I 

 have been unable to feel any interest except scientific 

 in them, which perhaps also has been advantageous. 



Later, as recorded in Chapter III., a limited 

 number of determinations of the weight of growing 

 rabbits and chickens was also made. 



All these animals were kept in summer in suitable 

 spacious pens in the country ; in winter, in large 

 boxes in well lighted and ventilated rooms, warmed 

 by artificial heat. They were carefully tended most 

 of the time by myself ; the endeavour was to secure 

 continuously the best hygienic conditions by unremit- 

 ting attention ; it was my habit to make two visits 

 daily. They were fed with the best food obtainable. 



To measure the growth the weights were taken of 

 the growing and adult individuals, the weight being 

 the only available measure for the whole animal, 

 and the only one permitting comparisons between 



1 During one winter upwards of eighteen barrels of carrots, three tons of hay, 

 twenty-six bushels of oats, and some other food were eaten by my guinea-pigs. 



