532 ASA A. SCHAEFFER 



is correct, then selection of food could be made in such cases upon 

 a chemical basis. Such (assumed) discrimination is, however, 

 much more restricted in its actual occurrence than might be sup- 

 posed; for many of the commonest proteins (the albumins) and 

 nearly all the common carbohydrates (the starches) are very 

 nearly or quite tasteless and odorless, whether in the soluble or 

 in the insoluble condition. If animals depended solely, there- 

 fore, upon the senses of taste and smell in each case where dis- 

 crimination occurs, these latter substances would never be eaten, 

 if they existed in the pure form. But since these substances 

 never exist in the pure form, since some tasting or smelling sub- 

 stance is always found associated with them, they find their way 

 into the stomachs of animals because they are supposed by the 

 animal to have tasting qualities which they do not actually 

 possess. 



The taste of a mass of food is thus almost invariably made to 

 extend, in the experience of the animal, to every particle of it; 

 while as a matter of fact, the tasting or smelling substance, such 

 as, for example, a meat extractive, or an essential oil, is usually 

 only a very small part of the food mass, and frequently its food 

 value is negligible. In by far the larger number of cases where 

 taste or smell function, an omnivorous animal reacts only to 

 these small quantities of substances which the animal has learned 

 are always associated with substances of real food value. 



In contrast to the discrimination in food which an animal as a 

 whole exercises, may be distinguished another form of selection 

 which obtains between the various tissues of an animal and the 

 food substances which come into contact with them. From the 

 same blood stream, as Verworn ('09, p. 175) has pointed out, the 

 muscle cells take certain substances, the cartilage cells others, 

 the ganglion cells still others, and so on. Even among the low- 

 est forms, the protozoa, there is a difference between the reac- 

 tions of the internal and the external protoplasm with regard to 

 solid objects. Metalnikow ('11) has shown that while a large 

 number of substances are readily eaten by Paramecium, some of 

 these are quickly excreted, while others remain for a consider- 

 able length of time in the body. Many similar observations are 



