ORIGIN AND SIGNIFICANCE OF SPINES 71 



growth force generally stands so directly as the result of an 

 unfavorable environment, that it is often difficult or impos- 

 sible to distinguish between their action. Furthermore, as 

 in the case of many parasites, it may be seen that the envi- 

 ronment may be quite favorable as regards temperature, 

 nutrition, etc. ; but unfavorable in respect to motion and use 

 of sensory and motive organs. From the almost universal 

 degradation and retrogression of parasitic forms, it is neces- 

 sary to consider these as intrinsically deficient, and therefore 

 lacking in the qualities of growth force which normally favor 

 a progressive evolution. Here, also, there are apparently 

 two intimately associated causes. In an attached animal 

 tlie absence of stimulus from disuse of an organ tends toward 

 atrophy, and the retrogressive development serves to affect 

 many organs in the same manner. The direct and indirect 

 results of the restraint of the environment may therefore be 

 expected to shade imperceptibly into each other, with only 

 the extremes sufficiently distinct for separation. 



The influence of an unfavorable environment as affecting 

 the character and growth of plants and animals is well shown 

 in desert or arid regions, and the flora has been made the 

 subject of especial study by Henslow.^ In such regions 

 the first thing to impress the observer is the small size of the 

 species. Next to diminutive size, the scantiness of life is a 

 striking feature, for large areas are common in which life is 

 almost wanting. An examination of these plants reveals a 

 series of characters not usually present elsewhere, among 

 which may be mentioned the development of a minimum 

 amount of surface, constituting what is known as consoli- 

 dated vegetation; next their uniform gray color, due either 

 to excessive hairiness or a coating of wax ; and lastly, their 

 frequent spinescent characters. 



The spines on desert plants are a feature of such general 

 occurrence that it has led to the notion that vegetable spines 

 are always associated with unfavorable conditions and are 

 therefore suppressed structures. This is probably incorrect, 



