45 ^ 
Our Six-footed Rivals. 
[October, 
changes of habits in accordance with new facilities or new 
dangers have been distinctly recognised. There can be no 
necessity for us to quote the cases of alterations in the 
nidification of birds given by Mr. Wallace.* 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 fre- 
quented 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 recognised, 
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-pupse of the species which it 
subjugates. 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 fac5t — 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 
* Contributions to the Theory of Natural Selection, p. 227. 
