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 evidence that such habits cannot have been primordial. The slave making propensity, and the reliance placed upon slaves, occur in several species, but not to the same degree. Polyergus rufescens, for instance, is absolutely dependent upon its slaves, and would, without them, perish from sheer incompetence to manage its own affairs further than by conducting slave-hunts. It is a military aristocracy, which can fight, but will rather die than work. Formica sanguinea, 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 species, 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 incorporated with the victorious nation?"
No less variation may be traced in the habits of the cattle-keeping
- ↑ "Contributions to the Theory of Natural Selection," p. 227.
