OUR SIX-FOOTED RIVALS. 355 



in their daily life, we can certainly see no grounds for supposing that 

 such processes may not have occurred often. In the case of larger 

 animals, where observation is easier, changes of habits, in accordance 

 with new facilities or new dangers, have been distinctly recognized. 

 There can be no necessity for us to quote the cases of alterations 

 in the nidification of birds given by Mr. Wallace. 1 Recent American 

 observations show that the habits of many birds, mammalia, and even 

 fishes, have undergone a very decided alteration in settled districts as 

 compared with less frequented regions. All species have become 

 more wary and circumspect in their movements, and are decidedly 

 more nocturnal. The birds build their nests on higher trees, or in the 

 densest thickets. Any unusual object placed in a river alarms the 

 fishes more than a similar object would have done some years ago, 

 and more than it does now in solitary parts of the country. A new 

 danger is recognized, and precautions are taken accordingly. 



On carefully examining the habits of ants, we find that there exist 

 among closely-allied species, and even in different colonies of one and 

 the same species, gradations which, to our mind, supply powerful evi- 

 dence that such habits cannot have been primordial. The slave- 

 making propensity, and the reliance placed upon slaves, occur in sev- 

 eral species, but not to the same degree. Poly erg us rufescens, for 

 instance, is absolutely dependent upon its slaves, and would, without 

 them, perish from sheer incompetence to manage its own affairs fur- 

 ther than by conducting slave-hunts. It is a military aristocracy, 

 which can fight, but will rather die than work. Formica sangicinea, 

 on the other hand, has much fewer slaves, and restricts them to a 

 much narrower sphere of duties, being itself capable of working as 

 well as of fighting. It is curious that the raids of slave-holding ants 

 are confined to worker-pupae of the species which they subjugate. No 

 instance has reached us of ants carrying off male and female pupae 

 with a view to raising a stock of slaves in their own city, without the 

 necessity of obtaining them by war. Surely, the most rational way 

 of accounting for this slave-making propensity is to suppose that, as 

 in the human race, it is a gradual outcome of war. Ants, in the wars 

 which they are known to wage against different species, as well as 

 against their own, would take prisoners an undeniable fact with 

 the original intention of killing and devouring them. Some few of 

 these victims, escaping immediate slaughter, might, if of a docile and 

 submissive disposition, be found useful, and might hence be allowed 

 to live in servitude. Prisoners of fiercer and more indomitable spe- 

 cies, if taken at all, are no doubt killed. The query here naturally 

 arises: "What happens in the not infrequent wars between two cities 

 of the same species? Are the prisoners slaughtered, or are they in- 

 corporated with the victorious nation ? " 



No less variation may be traced in the habits of the cattle-keeping 

 1 " Contributions to the Theory of Natural Selection," p. 227. 



