Ants and Acacias 109 



full recognition. But the fact is also of great im- 

 portance theoretically, for the complex system of 

 inter-relations forms an external registration of 

 evolutionary gains, and a sieve by which new varia- 

 tions — sometimes subtle nuances, one might think — 

 are effectively sifted. 



ANTS AND ACACIAS 



In further illustration of the idea of the inter- 

 relations of organisms, we take the well-known link- 

 ages between ants and acacias — an old story on which 

 recent inquiry has thrown fresh light. 



When Dr. Hernandez was sent in 1570 by Philip 

 II of Spain to investigate the resources of Mexico, 

 he came across a "horn-bearing tree," and observed 

 that "within the horns there are generated certain 

 slender ants, tawny-colored and blackish, whose sting 

 is hurtful, causing pain which persists for a whole 

 day." The "horn-bearing tree" was an acacia, and 

 the paired horns are the stipules at the base of the 

 beautiful pinnate leaves. When the leaves fall off, the 

 thorns remain, and they are very formidable. 



Years and centuries passed, and, in 1872, Thomas 

 Belt, author of the famous book The Naturalist in 

 Nicaragua, was attracted to the "bull's horn thorn," 

 another acacia. He observed that certain ants bore 

 into a thorn and may even cut their way from one 

 thorn to the opposite member of the pair, that they 

 eat up the soft pulp and establish themselves in the 

 excavated interior, that they multiply exceedingly, 

 and that they form a standing army for the acacia, 



