This applies especially to true agriculturists (agriculturists of the higher stages) as opposed to hunting agriculturists. The latter bear much resemblance to true hunters, who hardly ever keep slaves, accordingly slavery is not so frequent among them as among true agriculturists.
Our general rule, however, requires an important qualification. Where all land has been appropriated, a class of free labourers commonly exists, and slaves are not wanted.
We have not entered into a thorough investigation of the influence of secondary or additional causes. A closer study of this matter will perhaps yield important results. What we have found with regard to these secondary causes is the following.
Slaves often perform female labour; on the other hand female labour sometimes serves as a substitute for slave labour.
Commercial agricultural tribes are far more likely to keep slaves than agricultural tribes among which commerce holds a very subordinate place.
Slaves are sometimes kept for military purposes, or as a mere luxury. These two circumstances, however, though they often lead to the keeping of a larger number of slaves than would otherwise be required, have not probably ever given rise to slavery.
Subjection of tribes as such, which among some pastoral tribes serves as a substitute for slavery, hardly ever occurs among agricultural tribes, except in Oceania, where slavery is already absent for the general cause mentioned above, the non-existence of free land.
Some external causes we had found before: fixed habitations, [[416]]living in large groups, preserving of food, and the neighbourhood of inferior races, call for no special notice here.
Intercourse with superior races, especially where these carry on the slave-trade, proves to be a factor greatly furthering the growth of slavery.
We shall not give here a list of the causes found up to the present. We think it more convenient to place such a list at the end of our general survey. [[417]]