chap, xvn.] HABITS AND INSTINCTS. 419 



destruction by the large lizards, snakes, or other animals 

 which abound in the district ; because such large birds 

 must roam about a good deal in search of food. Here then 

 we seem to have a case, in which the habits of a bird may 

 be directly traced to its exceptional organization ; for it 

 will hardly be maintained that this abnormal structure 

 and peculiar food were given to the Megapodidse, in order 

 that they might not exhibit that parental affection, or 

 possess those domestic instincts so general in the Class 

 of birds, and which so much excite our admiration. 



It has generally been the custom of writers on Natural 

 History, to take the habits and instincts of animals as fixed 

 points, and to consider their structure and organization as 

 specially adapted to be in accordance with these. This 

 assumption is however an arbitrary one, and has the bad 

 effect of stifling inquiry into the nature and causes of 

 " instincts and habits," treating them as directly due to a 

 "first cause," and therefore incomprehensible to us. I 

 believe that a careful consideration of the structure of a 

 species, and of the peculiar physical and organic conditions 

 by which it is surrounded, or has been surrounded in past 

 ages, will often, as in this case, throw much light on the 

 origin of its habits and instincts. These again, combined 

 with changes in external conditions, react upon structure, 

 and by means of "variation" and " natural selection" both 

 are kept in harmony. 



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