47 
possibilities in the evolution of new species are practically inhibited, and we 
get the same or very closely allied 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 arid conditions of this continent, 
of which the rainfall is less than 10” over 100,000,000 sq. miles! 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? 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 would Jimit 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 Koebré 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 cireum- 
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 hysiography of Arid Lands,” Report Brit. Assoc. 
Adv. Sci. for 1914, 303 (1915). Cae 
2 Domin, K., “Monographie der Gattung Didiseus (DC.).” Sitz. Kon, béhm, Ges, d, 
Wiss. ii. Cl. (1908) 13, 20-23, 
