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THE EVOLUTION THEORY 



dwelling-place in its hollow, chambered stem (Fig. 37, A), and feeds 

 on a brown sap which oozes from the inside. On the stem there are 

 even little pits regularly arranged in definite places (E), through 

 which the female of Azteca can easily bore her way into the interior. 

 There she lays her eggs, and soon the whole interior of the trunk 

 teems with ants, which come trooping out whenever the tree is 

 shaken. 



This alone would not suffice to protect the tree against the leaf- 

 cutting ants, for how should the Aztec ants living inside notice the pre- 

 sence of the lightly climbing leaf-cutters 1 But that is provided for, for 



the Aztecs also frequent the outside 

 of the trunk, and just where attack 

 would be most disastrous, namely, 

 at the stalks of the young leaves. 

 At these places there is a peculiar 

 velvet-like cushion of hair (P), from 

 which grow little stalked white 

 papillae (Fig. 37, B), which are rich 

 in nourishment, and are not only 

 eaten by the ants, but are harvested 

 by them, being carried off into 

 the ants' dwellings, presumably to 

 feed their larvae. In this case, 

 then, a particular organ, offering 

 special attraction to ants, has been 

 developed by the plant at the 

 places more especially threatened; 

 while, as regards the ants, it is pro- 

 bable that only the instincts of 

 feeding and habitat require to be 

 modified, since courage and thirst 

 for battle are present in all ants, 

 almost any species being ready at any time to throw itself oh any 

 other which intrudes into its domain. 



It should be noted that not all the candelabra-trees live in 

 symbiosis with ants, and so secure a means of defence against the 

 leaf-cutters. Schimper found in the primitive forests of South 

 America several species of Cecropia which never had ants in the 

 chambers of their hollow stem. But these species did not exhibit the 

 nutritive cushions at the base of the leaf-stalk; these contrivances for 

 attracting and retaining the presence of partner ants were altogether 

 absent. Indeed, only one species, Gecrojna 23eltata, has produced these 



Eia. 37. A, a piece of a twig of an 

 Imbauba-tree {Cecropia adenopus), with 

 the leaves out off. At the leaf-bases are 

 the hair-cushions (P). E, the opening 

 for the associated ant (Azteca instabilis). 

 B, a piece of the hair-cushion with the 

 egg-shaped nutritive corpuscles (nk). 

 After Schimper. 



