The present paper was read in the first general meeting of the International Congress of Zooelogists at Leyden on September 16, 1895. Several points, which for reasons of brevity were omitted when the paper was read, have been re-embodied in the text, and an Appendix has been added where a number of topics receive fuller treatment than could well be accorded to them in a lecture. The address was first printed in _The Monist_ for January, 1896, and afterwards in a German pamphlet.

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