CHAPTER XII. 



THE RELATION OF THE INDIVIDUAL TO ITS 

 SURROUNDINGS. 



136. The Individual and the Environment. For several 

 chapters we have been considering the single animal as 

 an individual, in relation to its own parts, and these 

 parts in relation to one another. It is now necessary 

 that we return to the consideration of the individual in 

 relation to its total environment. The student will 

 find a use here for all his former observations in the field. 



The forces within the animal, which come from its 

 parents through the ovum and sperm and tend to guide 

 and determine its development, we call hereditary. But 

 this takes account of only one side of the question. The 

 conditions of the outside the environment must equally 

 be taken into account. Heredity determines that a 

 frog's egg, if it develops at all, shall produce a frog; but 

 the conditions outside largely determine whether the 

 egg will develop at all, whether it will be well nourished 

 or dwarfed, at what rate the development will proceed, 

 and many other similar things. The conditions may be 

 such as to prohibit entirely the development of the 

 organism or they may merely modify it in various ways 

 from what would normally be expected. 



Some of the external conditions are so fundamental 

 and powerful in their influence that all living things 

 must become adjusted or adapted to them in some way 

 in order to preserve their existence. 

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