BY R. J. TILLYARD. 29 



the parent group in Y, and the dilFerences may be accentuated 

 by the evolution of the two groups along divergent lines. Thus 

 tlie group in Z gradually takes on a distinct or regional form, 

 and becomes part of the native or autochthonous fauna of tlie 

 region. We thus obtain a group whose (jroup-charactei's, as nofv 

 recognised, ivere actually evolved within the recjion Z. Such 

 (jroujjs form the characteristic fauna or Jiora of a. given region, 

 and it is on the evidence of such groups that regional distinc- 

 tions are based. Their contours are recognisable by the fact 

 that their zoocentres he ivithin the region, while the lower con- 

 tour-lines spread out farther and farther around, and may even 

 overlap into surrounding regions (Transparency 2). 



For a contour of this type, the name Entugenic Contour is 

 proposed. 



We can now go one stage further, and assume that a particular 

 entogenic group in Z is faced with newer and stronger invasions 

 of ectogenic groups from other regions, due, perhaps, to the 

 removal of old barriers. In the struggle for existence, the older 

 group will go under, and, if it is preserved at all, will appear as 

 a remnant in one or more areas of the region Z. These areas may 

 be the original zoocentres of its former entogenic contour; for it 

 is reasonable to suppose that the group would be able to hold 

 out longest in those areas where its density is greatest. They 

 may, however, be simply "refuge" areas into which the remnants 

 have been driven, and, in such cases, will not afford any eAidence 

 of the position of the original entogenic zoocenti-es. The contour 

 of the group will now appear as a f-eries of discontinuous ovals 

 with no contour-lines of high value. 



Such a contour may be termed a Palceogenic CoiUour. These 

 are the contours of archaic groups. They may be sufficiently 

 numerous to furnish part of the distinctive character of the fauna 

 or flora of the region, but are usually of less importance, though 

 not necessarily of less interest, than the entogenic and ectogenic 

 groups of the region. Owing to the great changes in land dis. 

 tribution throughout long geological epochs, true paheogenic con- 

 tours may very often be, and indeed usually are, discontinuous 

 over more than one region. Hence their contours should be 



