332 C. E. Beecher — Origin and Significance of Spines. 



through external and internal agencies. There still remain for 

 discussion the formation of spines by processes of decrescence 

 caused by extrinsic restraint (C), or intrinsic deficiency of 

 growth power (D). The lack of vitality or growth force gen- 

 erally stands so directly as the result of an unfavorable environ- 

 ment, that it is often difficult or impossible to distinguish 

 between their action. Furthermore, as in the case of many 

 parasites, it may be seen that the environment may be quite 

 favorable as regards temperature, nutrition, etc. ; but unfavor- 

 able in respect to motion and use of sensory and motive organs. 

 From the almost universal degradation and retrogression of 

 parasitic forms, it is necessary 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, the 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 effecting 

 the character and growth of plants and animals is well shown 

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

 ject of especial study by Henslow. 33 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 consolidated 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, for 

 in plants as in animals, spines may be developed by the progres- 

 sive 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 



