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 succeeds in either defending itself or retaliating, 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, 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 consequence; certainly they not merely run the risk of but eagerly invite death.
The following day we descended the São Lourenço 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-
