INBREEDING EXPERIMENTS 101 



decreased size and in the appearance of definite abnormal 

 or pathological conditions. 



Obviously it is not possible to formulate any definite 

 rule by which to judge under what condition the good 

 effects of inbreeding may be expected to outweigh the 

 evil, by generalizing from a series of isolated facts. What 

 is needed is controlled experimentation to determine just 

 what inbreeding involves, and interpretation of the re- 

 sults in keeping with general biological knowledge. Dar- 

 win 38> 39 was the first to appreciate this. After 

 endeavoring rather unsuccessfully to generalize on the 

 subject with a collection of published records as a basis, 

 he himself carried on a series of inbreeding experiments 

 on plants extending over a period of eleven years. Plants 

 were probably selected as the material with which to work 

 primarily because the experiments could be carried out 

 easily and with little expense. But there is another rea- 

 son why plants serve the purpose better than animals in 

 such an investigation; the animals commonly available 

 are bisexual ; hence, they cannot be inbred as intensively 

 as hermaphroditic plants. Nevertheless, Darwin's ex- 

 periments served to stimulate more interest in the subject 

 among zoologists than among botanists, and the quanti- 

 tative experiments carried on during the next thirty 

 years were nearly all upon animals. 



The rat has been a favorite species, it having served 

 as material for the extended researches of Crampe, 34 

 Ritzema-Bos 184 and Kin. 119 > 12 - 121 The first two investi- 

 gations, together with that of Weismann and von 

 Guaita, 86 ' 8T on mice have been the classic examples of the 

 adverse effects of inbreeding. 



Crampe 's experiments started with a litter of five 



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