CHAPTER III 



WHEN CHARACTERS 1 ARE COMBINED 



Between 1900 and 1905 about three thousand guinea pigs 

 and several hundred rabbits were reared and housed and 

 studied in the Zoological Laboratory of Harvard University. 

 Professor Castle kept them in this laboratory of living crea- 

 tures for the sake of seeing what he could do for descendants 

 when he himself chose ancestors for them. 



His main work was with guinea pigs, and he chose them 

 because they are small, because they do not eat much, do not 

 take up much room, are easily cared for, and multiply fast. 



Perhaps the last reason was really the most important of 

 all. The truth is, small animals have so short a time from one 

 generation to the next — that is, from parent to child — that 

 it is easy to trace resemblances between far-away ancestors 

 and present-day descendants. 2 



Rabbits have a new generation every eight months, and 

 guinea pigs multiply at the rate of four generations a year. 

 This means that guinea pigs can have children, grandchildren, 

 great-grandchildren, and great-great-grandchildren all within 

 the same year. Thus guinea pigs are particularly well fitted 

 to serve as helpers in answering questions about inheritance, 

 for within a very few years the descendants show just what 

 their long lines of ancestors have done for them. 



In size these animals are about as large as well-developed 

 rats. But instead of being all of one color, as are rats, the 



1 As used in books of this kind the word character means " characteristic." 



2 All children of the same parent belong to the same generation. 



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