1894. HENSLOW AND NATURAL SELECTION. i8i 



circumstance that, with the exception of pahiis, few large trees are 

 spiny, and when they are so, as in the case of the Gleditschia, the 

 spines are most abundant on the trunk and on the younger branches. 

 In the same way, our holly, when it grows to a large size, usually has 

 the leaves towards the top spineless : the wild pear also is spiny 

 below but unarmed above. The climbing palms, on the other hand, 

 are armed to the very top, but in this case the spines assist climbing. 



The anomaly of the flora of the Brazilian Campos having most of 

 the true xerophilous characteristics, yet being almost wholly without 

 spiny forms, is quite in harmony with the fact of the great poverty of 

 this region in mammals destructive of woody vegetation. There are 

 really none but a few deer and cavies, which are mostly inhabitants 

 of the more wooded valleys, and which are kept from undue multipli- 

 cation by the considerable number of species of Felidae and Canidae 

 in the same area. 



We are, therefore, led to conclude that the apparent direct 

 dependence of an unusually spinescent vegetation on arid conditions 

 of soil or climate is to a great extent deceptive. Such conditions are 

 inimical to the growth of dense forest, and it is a well-known fact 

 that the larger mammalia abound most in partially wooded or open 

 country. Many of these animals are exceedingly destructive to 

 shrubby or aborescent vegetation, especially in districts which are 

 subject to occasional droughts ; and it is in such areas that so many 

 of these plants have acquired the protective armature of spines or 

 prickles, while others not so protected have sooner or later sucr 

 cumbed, thus leading to a preponderance of the former. But the 

 numerous instances in which considerable areas and extensive floras 

 are found to have hardly any spinous plants, as compared with other 

 areas in which the soil and climate are generally similar and where 

 such plants abound — the only important difference being the absence 

 or presence of destructive herbivorous or frugivorous mammals — 

 show us clearly that it is the latter rather than the former condition 

 which is the real starting point and efficient cause for the develop- 

 ment of spines, while the mode of their production has been through 

 spontaneous variation and Natural Selection.^ 



A few remarks may now be added on the general question of 

 adaptation in the vegetable kingdom. Reference has already been 

 made to the numerous cases in which the special adaptations of flowers 

 to insect-fertilisation can by no stretch of imagination be imputed to 

 the direct action of insects, and the same thing is equally clear in 

 many other directions. The whole group of insectivorous plants, for 

 instance, exhibit strange and complex adaptations which have no 



2 Professor A. Kerner gives an admirable account of the various forms of spiny 

 and pricl{ly plants, which are exceedingly numerous in the Mediterranean district, 

 and he adds : " In northern regions not exposed to summer drought, where grazing 

 animals find in summer enough green fodder, this form of plant is almost entirely 

 absent." ("The Natural History of Plants," English Translation, vol. i., p. 445.) 



