1 46 THE NA TURE-STUD V RE VIE W [3 : s-may, 1907 



ones are removed by death or other causes. ' This is accompHshed 

 by the habit of plundering neighboring colonies. Recalling the 

 simple conditions of the Sanguinaries, the reader will remember 

 the important part this habit played. For considerable periods 

 the colonies seem to be on a continuous slave-hunt, the captured 

 young forming the chief food of the robbers and those that are 

 not devoured being reared into slavery. Many other species 

 not slave-makers at all, like Eciton for example, are constantly 

 guilty of this same habit. So pronounced is this that Darwin 

 in his attempt to explain the slave-making instinct among ants 

 was tempted to look upon this as in itself sufficient to explain 

 the conditions then known. Thus in the "Origin of Species" 

 he wrote : 



'*By what steps the instinct of Formica sanguinea originated I will not 

 pretend to conjecture. But as ants, which are not slave-makers, will, as I 

 have seen, carry off pupae of other species, if scattered near their nests^ it is 

 possible that such pupae originally stored as food might become developed;, 

 and the foreign ants thus unintentionally reared would then follow their 

 proper instincts, and do what work they could. If their presence proved 

 useful to the species which had seized them — if it were more advantageous to 

 this species to capture workers than to procreate them — the habit of collecting 

 pupae, originally for food, might by natural selection be strengthened and 

 rendered permanent for the very different purpose of raising slaves. When 

 the instinct was once acquired, if carried out to much less extent even than in 

 our British F. Sanguinea which, as we have seen, is less aided by its slaves 

 than the same species in Switzerland, natural selection might increase and 

 modify the instinct — always supposing each modification to be of use to the 

 species — until an ant was formed as objectly dependent on its slaves as is the 

 Formica (Polygergus) rufescens." 



These two habits, then, originally distinct and each widely 

 distributed throughout the ant-world, once combined or rather 

 superimposed in the life cycle of any one species, would give at 

 one bound conditions such as are at present observable in San- 

 guinea. In the hands of the forces of Natural Selection, coupled 

 with the effects of use and disuse, the later and more complex 

 conditions would in the course of time readily enough arise. It 

 must not, of course, be assumed that the processes of Natural 

 Selection have given origin to this institution. In the field of 

 Instincts quite as in that of Structures, these processes alone 

 can do nothing. There must first be some material basis for- 



