Two years later, however, Darwin gave a new meaning and a new importance to Sclater’s work, by the new interpretation he caused to be placed on the words “centres of creation.”  Sclater’s facts and areas remained the same; Darwin rejected the idea of separate creations in the older sense of the words, and laid stress on the impossibility of accounting for the resemblances within a region and for the differences between regions by climatic differences and so forth.  He raised the questions of modes of dispersal and of barriers to dispersal, of similarities due to common descent, and of the modifying results produced by isolation.  He gave, in fact, a theory of the “creations” which Mr. Sclater had shewn to be a probable assumption.  It was in the nature of things that Huxley should make a contribution to a set of problems so novel and of so much importance to zooelogy.  In 1868, in the course of a memoir on the anatomy of the gallinaceous birds and their allies, he made a useful attempt, nearly the first of its kind, to correlate anatomical facts with geographical distribution.  Having shewn the diverging lines of anatomical structure that existed in the group of creatures he had been considering, he went on to shew that there was a definite relation between the varieties of structure and the different positions on the surface of the globe occupied at the present time by the creatures in question.  He made, in fact, the geographical position a necessary part of the whole idea of a species or of a group, and so introduced a conception which has become a permanent part of zooelogical science.

With regard to the number and limits of the zooelogical regions into which the world may be divided, Huxley raised a number of problems which have not yet reached a full solution.  Mr. Sclater had divided the world into six great regions:  the Nearctic, including the continent of North America, with an overlap into what is called South America by geographers; the Palaearctic, comprising Europe and the greater part of Asia; the Oriental, containing certain southern portions of Asia, such as India south of the Himalayas and many of the adjacent islands; the Ethiopian, including Africa, except north of the Sahara, and Madagascar; the Australian, containing Australia and New Zealand and some of the more southeastern of the islands of Malay; the Neotropical, including South America.  Huxley first called attention to certain noteworthy resemblances between the Neotropical and the Australian regions of Sclater, and held that a primary division of the world was into Arctogaea, comprising the great land masses of the Northern Hemisphere with a part of their extension across the equator, and Notogaea, which contained Australia but not New Zealand and South America.  Although this acute suggestion has not been generally accepted as a modification of Mr. Sclater’s scheme, it called attention in a striking fashion to some very remarkable features