254 COOK 



range of development. Some of the coffee mutants have ex- 

 tremely short internodes. None of these complications need 

 obscure the fact that the phenomena of artism can be viewed as 

 entirely distinct from those of heterism, though neither phe- 

 nomenon excludes the other. 



DIFFERENCES OF USE AND DISUSE. 



One of the reasons for the persistence of the belief that 

 adjustments to external conditions represent direct effects of 

 environment, lies in the fact that several other kinds of intra- 

 specific differences have been confused with environmental 

 adjustments. Most of these additional types of diversity are 

 rather uncommon, but they are well calculated to confuse 

 thought and even to vitiate experiments, especially when these 

 are undertaken without fully considering all the sources of 

 possible error. 



If an animal or a plant be kept in captivity or placed other- 

 wise under conditions where its normal activities are not called 

 into use, muscles or other organs may fail to reach their normal 

 development, or they may actually decline in size and deteriorate 

 in structure under continued disuse. There are certain senses, 

 of course, in which it may be said that the environment, by 

 determining the use of parts, causes them to prosper or decline, 

 but closer attention will show that these are phenomena of 

 growth and nutrition rather than of environmental adjustment. 

 The use of a muscle is as truly a condition of its development 

 as the food from which the tissue is nourished, and the decline 

 of such a part may be reckoned as a starvation phenomenon, or 

 interference with the normal processes of growth. 



The fact that so much has to be learned through precept and 

 practice by the young of the human species has led some to 

 overlook the existence of definite instincts and muscles which 

 develop without use, just as the internal organs and functions 

 develop in the embryo before birth. 



The idea that there is a natural and general tendency to 

 evolutionary motion, to change of organic form and structure, 

 need not be confused with the predication of a principle of evo- 

 lutionary perfection by which some writers have thought that 



