BY R. J. TILLYARD. 27 



A. 7%e general structure of a contour. 



It is evident that, in general, the lowest contour-lines will 

 enclose the largest areas, while, as the number-value of the 

 contour-line increases, the area it encloses will become smaller 

 and smaller. Finally, the nth, or highest, contour-line will 

 enclose a small area or series of areas surrounded by all the other 

 contour-lines. Such an area, representing a " summit " of the 

 contours, may be spoken of as a Zoocentre ; it being clearly 

 understood that in using this term no definite claim is put 

 forward that the area is also a centre of origin for the group. 

 The Zoocentre may be defined as the centre of present greatest 

 density for the group. It may be also a centre of origin, but in 

 most cases it is possible that such a claim cannot be maintained 

 for it. Sometimes the area of the zoocentre is elongated very 

 much in comparison with its breadth; it may then be termed the 

 Zoocentric Axis of the group. In the case where the contour 

 exhibits more than one separate zoocentre, that which contains 

 the highest number of species may be called the primary zoo- 

 centre, while those of lower value may be called secondary 

 zoocentres. 



It sometimes happens that the order of the contours is reversed, 

 so that the higher contours enclose the lower, until in the middle 

 may be found a small area in which perhaps only 2, 1, or even 

 no species occur. (Such a case, for example, is furnished by the 

 failure of a subtropical group to ascend a central mountain 

 range, though it may be spread abundantly all round it. As 

 one reaches a higher elevation, the number of species found will 

 diminish; until, perhaps, above a certain level no species of the 

 group will occur). In such a case, the area of lowest contour 

 may be called a Lacuna. The mapping of lacunse may be of the 

 very greatest importance in the study of a group. 



In constructing a contour, it is very important to leave out of 

 account purely local discoritinuity. To give an exaggerated 

 example : — Certain species of rush occur throughout Central Aus- 

 tralia, wherever there is a waterhole. The waterholes may be 

 fifty or a hundred miles apart. Nevertheless, the correct contour 



