158 Changing Plant and Animal Nature 



of maintaining the unity of the organism in the presence of 

 constantly changing conditions. It is these minor actions 

 and reactions that in their totahty constitute the essential 

 and continuous struggle for existence. 



It is well to keep in mind that in so far as a plant or ani- 

 mal continues to remain alive this struggle is successful. On 

 the other hand, the adjustments in detail can never be alto- 

 gether perfect. Certain of the mechanisms may be relied 

 upon to operate automatically and effectively, but in so far 

 as new situations arise, there must be in the life of every 

 individual a relative failure in one detail or another. This 

 is illustrated by the reactions of higher animals to a change 

 in diet. While it is true, for example, that the animal's 

 choice of food is adequately guided by its " instincts," this 

 guide is never perfect. In the case of many birds and mam- 

 mals the individual learns by trial and error what food is 

 acceptable to the taste and it is by trial and error that many 

 of them discover that some food is acceptable to the stomach 

 and the blood. If the adaptation at this point were quite 

 perfect it would never be possible to poison an animal. The 

 individual human being learns to accept new kinds of food 

 not only as a matter of taste, but also as a matter of adequate 

 management by the stomach and intestines. Acute indiges- 

 tion would be impossible in an organism perfectly adapted 

 to its environment. 



Kinds of Y aviation 



We are told that with some attention one can learn to 

 know individual pigs or geese or chickens apart just as we 

 know many people *' by name." Most of us are unable to 

 analyze the make-up even of human beings to say just what 

 is distinctive about each individual. We commonly recog- 

 nize differences in size or stature, in proportion, in the shape 

 of the features, differences in color of hair and skin and eyes. 

 Most of these differences are measurable, as are many others 

 that we observe in common plants and animals. We can 



