BY R. J. TILLYARD. 31 



may occur; for instance, an ectogenic group may have spread 

 nearly all over a region, forming one or more secondary zoocentres 

 in it, and still exhibit connection with the parent group, entoyenic 

 in a neiglibouring region. As soon as that connection is definitely 

 broken, and the offshoot assumes its own distinguishing charac- 

 teristics, it becomes entogenic in the region of which it has taken 

 possession. Again, an entogenic group may gradually die out, 

 and so reach a stage at which it exhibits a contour intermediate 

 between an entogenic and a palseogenic one. Such a contour 

 w^ould not, perhaps, show any discontinuity, but the paucity of 

 contour-lines would indicate how very little more reduction was 

 needed to produce a typical palseogenic contour. 



It may be seen, also, how every group, in the course of time, 

 from its rise to its final extinction, may go through the three 

 stages of ectogenic, entogenic, and finally palseogenic contour in 

 any given region. 



Contours may exhibit flatness (in the case of groups with few 

 species) or steepness (in the case of groups with many species in 

 a small area). Several contour-lines may lie together in one 

 single line, as, for example, along the coast-line of a region, or, in 

 the case of several plant-feeding species which extend all together 

 to the utmost boundary of distribution of a single food-plant. 

 In such cases, it is probably best to exhibit the contours as a set 

 of close parallel curves arranged in the order in which they would 

 naturally come if the species did not end off quite coterminously. 

 In the case of a coast-line, these parallel lines may be drawn on 

 the map, actually over that part representing the sea, following 

 the coast-line in general direction, but not its irregularities. (See 

 the ectogenic contour in Transparency 1). Where the same 

 species occurs in a number of islands, a single contour-line may 

 be drawn round all the islands. 



When the contours of different groups have to be studied in 

 relation to the rainfall, temperature, or geological conditions of 

 the region, they should be drawn on transparent paper, so that 

 they can be placed over a map of the isohyets, isothermal s, or 

 geology of the region, as the case may be. This has been done 

 in the Plate given with this paper, the printed map showing 

 isohyets. 



