20 GENETICS 



The kea parrot, after the introduction of sheep into 

 New Zealand, changed from herbivorous to carnivorous 

 habits and consequently became a pest. Many animals 

 in captivity are less fertile than when free, while differ- 

 ent individuals are well known to vary widely with re- 

 spect to their susceptibility to disease. Nageli, for 

 example, reports the presence of tubercles in 97 per 

 cent of the cases in five hundred autopsies, although a 

 majority of the deaths in question was not due to 

 tuberculosis at all, a fact which indicates a great 

 diversity in the resistance of different individuals to the 

 tubercle bacillus. 



Psychological variations in man, such as those which 

 determine the disposition or mental traits of individuals, 

 are apparent to every one. 



b. With respect to their duplication variations may 

 be single or multiple. A legless lamb 1 is an example of 

 a single variation or "sport." Four-leaved clovers, on 

 the contrary, are multiple for the reason that this 

 variation, although not common, nevertheless occurs 

 frequently. 



c. With respect to their utility variations may be 

 useful, indifferent, or harmful to the organism possess- 

 ing them. Useful variations are of the kind emphasized 

 by Darwin as being effectively made use of in natural 

 selection. Indifferent variations, on the other hand, 

 are those which apparently do not play an important 

 part in the welfare of their possessor, such, for ex- 

 ample, as the color of the eyes or of the hair. Finally, 

 the degree of degeneration in certain organs may be 



1 "A Peculiar Legless Lamb." Stockard. Biol. Bull, xiii, p. 288. 



