HEADWATERS OF THE PARAGUAY 117 



type as to render the possessor practically immune from 

 danger. In other cases the condition of its exercise may 

 normally be the sacrifice of the life of the possessor. There 

 are wasps that prey on formidable fighting spiders, which 

 yet instinctively so handle themselves that the prey prac- 

 tically never succeeds in either defending itself or retaliat- 

 ing, being captured and paralyzed with unerring efficiency 

 and with entire security to the wasp. The wasp's safety 

 is absolute. On the other hand, these fighting ants, in- 

 cluding the soldiers even among the termites, are fran- 

 tically eager for a success which generally means their an- 

 nihilation; the condition of their efficiency is absolute 

 indifference to their own security. Probably the majority 

 of the ants that actually lay hold on a foe suffer death in 

 consequence; certainly they not merely run the risk of 

 but eagerly invite death. 



The following day we descended the Sao Louren^o to 

 its junction with the Paraguay, and once more began the 

 ascent of the latter. At one cattle-ranch where we stopped, 

 the troupials, or big black and yellow orioles, had built 

 a large colony of their nests on a dead tree near the primi- 

 tive little ranch-house. The birds were breeding; the old 

 ones were feeding the young. In this neighborhood the 

 naturalists found many birds that were new to them, in- 

 cluding a tiny woodpecker no bigger than a ruby-crowned 

 kinglet. They had collected two night monkeys — noctur- 

 nal monkeys, not as agile as the ordinary monkey; these 

 two were found at dawn, having stayed out too late. 



The early morning was always lovely on these rivers, and 

 at that hour many birds and beasts were to be seen. One 

 morning we saw a fine marsh buck, holding his head aloft as 



