524 discussion. 



ing to Dr. Kesteven, he said that the method was not intended to 

 show all the facts. It was intended to give a general 

 graphical view of the distribution of a group, and not to 

 try to map every tiny detail. With regard to the rushes, he 

 would draw his contour round the whole region in which they 

 occurred, not round each waterhole. He agreed that the term 

 "archipelagic" was unsuitable, and would substitute "palaeogenic." 

 He also claimed that Dr. Kesteven's two cutting ovals were as 

 inaccurate as his own representation, which did not claim to show 

 the details. The possibility of the 4, 3, 2, and 1 contours enclosing 

 ten species was only a theoretical possibility, and could not occur 

 in nature. He agreed with Mr. Baker's remarks. 



In conclusion, Mr. Tillyard emphasised the following points with 

 regard to the method: (a) It gives a density-contour, taking no 

 account of the separate species, as such, (b) It is not accurate in 

 small details, but is intended to show, at a glance, the general dis- 

 tribution of a group over a given region, exactly on the lines of a 

 rainfall- or barometric pressure-map. (c) The division into ecto- 

 genic, entogenic, and palaeogenic groups, represented the three 

 main stages in the evolution of any group, and was only a new 

 presentation of a well-known and accepted fact. (d)The question 

 of the further subdivision of each of these three types of contour 

 in any given region needed working out, but he felt sure that dif- 

 ferent subtypes could be shown to exist. (e)The zoocentres were 

 simply the areas of greatest density for each group, and it was 

 rmarkable, if Dr. Cleland's mutation-argument were true, that 

 practically all Australian entogenic groups should have similarly 

 located zoocentres ! This alone showed the fallacy of the mutation- 

 hypothesis. (/)The method is not restricted to regions, but cir- 

 cumtropic and circumpolar groups could be contoured over a map 

 of the whole world ; and even winter- and summer-distributions of 

 bird-groups could be exhibited in contrast, by means of it. 



