The Headwaters of the Paraguay 121 



which nature achieves the perpetuation of species. 

 Among the warrior and predaceous insects the prowess 

 is in some cases of such 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 practically never suc- 

 ceeds in either defending itself or retaliating, being cap- 

 tured and paralyzed with unerring efficiency and with 

 entire security to the wasp. The wasp's safety is abso- 

 lute. On the other hand, these fighting ants, including 

 the soldiers even among the termites, are frantically eager 

 for a success which generally means their annihilation; 

 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 conse- 

 quence; certainly they not merely run the risk of but 

 eagerly invite death. 



The following day we descended the Sao Lourengo 

 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 primitive 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, including a tiny woodpecker no bigger than 

 a ruby-crowned kinglet. They had collected two night 

 monkeys — nocturnal monkeys, not as agile as the ordi- 



