■74 STUDIES IN EVOLUTION 



This secondary influence by grazing animals may have had 

 some effect in determining the particular abundance of spiny 

 plants in certain desert regions, and their comparative infre- 

 quency in other similar regions. In either case the unfavor- 

 able environment brings about a suppression of structures, 

 and one type of this action results in the production of 

 spines. These represent the limits of retrogression before 

 the part becomes entirely obsolete. 



Wallace has criticised Henslow's views on the origin of 

 xerophilous plants and their distribution. It is believed 

 that the views here offered remove some of the objections, 

 and bring the opinions of these authors into greater accord. 



Under arid conditions bracts, stipules, leaves, and even 

 branches may become spinescent. Some forms in which the 

 spinose character has not as yet become permanently fixed by 

 heredity, when transported or found living in moister and 

 richer soils, develop normal leaves or branches, and lose their 

 spinescence ; others, like the Cactus, retain their spines under 

 similar changes; while still others, as Acayitliosicyo^ hor- 

 rida,^^ cannot be artificially cultivated, and have become 

 truly xerophilous types. 



As examples of plants which lose their spines by cultiva- 

 tion, the Pear, species of Rose, Plum, etc. (Henslow), may 

 be cited. According to Henslow, ^^ others, as Onomis sphiosa, 

 have an especially spiny variety (horrida) living on sandy 

 sea-shores, while in more favorable natural situations the 

 same plant becomes much less spiny, and under cultivation 

 loses its spines. M. Lothelier *2 ^Iso found that by growing 

 the Barberry (^Berheris vulgaris) in moist air, the spines dis- 

 appeared, the parenchyma of the leaves being well formed 

 between the ribs and veins. Dry atmosphere and intense 

 light both favored the production of spines. 



Henslow ^^ cites the genus Zilla as a desert plant in which 

 the branches are transformed into spines, JEcJiinops for a 

 similar modification of the foliage, Fagonia for spiniform 

 stipules, and Centaurea for spinescent bracts. As further 

 illustrations taken not only from desert plants but also from 



