47 



possibilities in the evolution of new species are practically inhibited, and we 

 get the same or very closely sillied representatives. 



But when cross-currents from the Antarctic (24, 222) cause precipitation 

 in Australia, the opportunity habitat is that of a large and open arid plateau 

 with little or no elevation, low rainfall, and a different and unlimited insect 

 fauna. Where constant conditions in illumination, temperature, and limited 

 rainfall prevail, there is practically nothing to limit the multiplication of 

 species able to survive the widely prevailing nrid conditions of this continent, 

 of which the rainfall is less than 10" over 100,000,000 sq. miles. 1 A 

 multitude of plants, so similar in appearance that it is difficult on casual 

 acquaintance to separate the different species, are evolved, especially as 

 annuals or ephemerals of the eremaea. To quote the genus Didiscus again, 

 which well exemplifies this theory : according to Domin 2 the Calycina 

 section represents the oldest form, limited to four species, of which two are 

 Papuan, one extending to N. Borneo and the Philippines ; the others occur 

 in Celebes and N.E. Australia respectively. Pseudo-calycina, considered 

 atavistic, is represented by one Papuan species and two in N.E. Australia. 

 The rest, or Eudidiscus, are all mostly annuals, younger types, represented 

 largely in W. Australia, with one in the Polynesian Islands, where restricted 

 conditions wouid .limit the evolution of the many closely allied species, so 

 marked a feature of Australian xerophytic types. 



Open plant associations at the Angi lakes belong to the " Opportunity " 

 category, as under normal conditions these areas would be in forest, being 

 only kept open by the means of artificial burning at regular intervals. It is 

 extremely interesting in this respect that it should be possible to compare 

 two distinct associations of this type, developed under absolutely opposed 

 conditions, viz. : 



1. The open marsh at 7000', sheltered in position, showing varying 

 conditions of soil and constant conditions of moisture. 



2. The Cladonia association of Koebre at 9000', where constant con- 

 ditions of exposure and drainage prevail with a hard surface soil, sterilised 

 by the constant burning. 



No more certain proof is afforded of the derived character of a flora 

 than that offered in a forest country by open spaces due to artificial circum- 

 stances, providing conditions, rainfall, and temperature are favourable. 

 This fact is well exemplified in New Zealand and the Pacific Islands, where 

 both rainfall and original plant covering once approached present Papuan 

 conditions, but now, under the stress of competition with ombrophobous 

 immigrant plants, not a single native species will be found in such areas. 



1 Griffith Taylor, " Discussion on the 1 hysiography of Arid Lands," Report Brit. Assoc. 

 Adv. Sci. for 1914, 303 (1916). 



2 Domin, K., " Monographic der Gattung Didiscus (DC.)." Sitz. Ktin. bo'hni. Ges. d. 

 Wiss. ii. Cl. (1908) 13, 20-23. 



