Definition of "incentively"

Should she kill this magnetic spider, and so end its baleful influence? Incentively the trees hissed 'Yes, yes.' Motioning for them to be silent, she steadily watched the insidious spinner, now as if divining her purpose, merely an indrawn inert black speck but acutely watching her.

1907, Barbara Baynton, edited by Sally Krimmer and Alan Lawson, Human Toll (Portable Australian Authors: Barbara Baynton), St Lucia: University of Queensland Press, published 1980, page 289