82 STUDIES IN EVOLUTION 



the necessary form, and the hard tissue the structure, so that 

 the whole will conform to the definition of a spine given early 

 in this paper; namely, a stiff, sharp-pointed process. 



The restraint of the environment was found to be one 

 * (cause for decrescence of organs. Another, which is properly 

 the subject matter of the present section, is disuse; and 

 lastly, it will be seen how the deficiency of growth force may 

 bring a similar suppression of structures. 



There is considerable difficulty in selecting particular 

 examples which will conform clearly to the strict require- 

 ments of these three categories. In a certain sense some of 

 the examples of spines produced by decrescence may belong 

 to more than one category. However, it does not prevent the 

 acceptance of any one of the three as primary causes. Thus 

 it may be urged that disuse has caused the atrophy of leaves 

 into spines among many desert plants, or produced a similar 

 reduction of the limbs in a Python. While this may be true 

 from one point of view, yet the manifest unfavorableness of 

 the environment in both seems to be a sufficient reason for 

 making it the primary factor. On the other hand many 

 parasites showing similar atrophies are not dependent upon a 

 large number of active organs for their food and maintenance. 

 After finding a host an abundance of food is at hand, and 

 the environment may be considered a favorable one. All the 

 organs, except those of nutrition and reproduction, then 

 become more or less useless and dwindle away, leaving 

 vestigial organs or disappearing altogether. Furthermore, 

 a change of habit, as from climbing to flying, will necessarily 

 cause the atrophy of some of the structures used for climbing 

 and the hypertrophy of others for flying. 



Most of the examples illustrating the production of a spine 

 through the atrophy of an organ by disuse are to be found in 

 the legs and digits of animals. The process bears consider- 

 able resemblance to the formation of spines on many plants 

 by the suppression of leaves, branches, etc. They will be 

 noticed here, although properly these vestigial structures 

 among animals are more strictly of the nature of claws, or, at 

 the most, spurs. 



