72 STUDIES IN EVOLUTION 



for in plants, as in animals, spines may be developed by the 

 progressive differentiation of previous structures; as in the 

 angular edges of the leaf stems of many Palms becoming 

 spiniferous, or, as will be shown, suppressed structures may 

 arise from deficiency of growth force. In all cases spines 

 may or may not serve for protection. Thus, while they are 

 not always an indication of unfavorable environment, those 

 occurring on desert plants may generally be so considered, 

 for they are developed out of structures which are normally 

 of vital physiological importance. 



An animal or plant having spines and living in a favorable 

 environment, involving freedom of motion for animals and 

 abundance of nutrition without extremes of temperature or 

 dryness for both animals and plants, will, it is believed, 

 from the discussions and analyses of spine genesis in its 

 various phases, develop these features in most instances with- 

 out the sacrifice of organs and structures having important 

 physiological and motor functions. Thus, ordinarily, among 

 animals it is found that spines arise as excrescences or out- 

 growths of exoskeletal or epidermal tissues, without seriously 

 affecting the function of the organ or organs upon which 

 they are located. Such cases may clearly belong to the most 

 progressive series, and in fact usually occur there. 



On the other hand, if it is found that a leg, a wing, a 

 digit, or other organ is developed into a spine, this is always 

 accomplished by a process of retrogression, resulting in the 

 greater or lesser suppression of the part in question. It is 

 also seen that this kind of spine occurs most frequently in 

 retrogressive series or in others showing arrested develop- 

 ment, and the necessary interpretation seems to be either that 

 the environment is or has been unfavorable, at least so far as 

 the particular organ or set of organs is concerned, or that the 

 vital power has declined. Both influences are intimately 

 associated, and the latter is often the direct result of the 

 former. 



The stunting effects of aridity and barren soil on com- 

 mon plants is familiar to all. Among the plants of the 



