The causes which lead to the keeping of slaves, and those which prevent it, have been divided by us into internal and external causes. These terms do not perhaps quite answer the purpose, but we cannot find other terms which would express our meaning better, without requiring a prolix circumlocution. We think, however, that it is clear what we mean by these terms. Slavery cannot exist, where there are no internal causes requiring it, i. e. where there is no use, economic or non-eco-mic, for slave labour. A tribe will not keep slaves, even though its coercive power would enable it to do so, if there is no employment for them; in such case positive internal causes fail. The same obtains, where negative internal causes are found, i. e. where there are definite circumstances which make slave labour useless. The positive internal causes may also be called motive-forces. Slaves will not be kept, even where the best opportunities of procuring them exist, if there is no motive-force which requires the keeping of slaves, i. e. if they are not wanted.