HORN KXPKDITION — SUMMARY. 141 



In cf-rtain gi-oups, osppcially in tlio Colcoptcra, niimeious spocios lia\p 

 bof'ii alicady (lescrilj(><l, l»ut excluding the latter tiie list given indieatcw within 

 naii'ow limits our piesent knowledge of the numeiical proportions of the Central 

 Australian fauna. 



That the nuniber.s will l)e .steadily increased in time is of course cei-tain, 

 as many of the rarer forms can only be secured at intervals and undei' \eiy 

 favourable circumstances such as the successive occurrence of two (ir more good 

 seasons. 



It is highly proliable, in fact certain, (hat the fauna varies to a great extent 

 with the climate. Central Australia may be described as possessing a permanent 

 and a fluctuating fauna ; the former, which may b(\ regaidcnl as the nucleus of its 

 fauna, consists of species winch ha^e lieconie especially adapted to life in an arid 

 region ; the latter consists of immigrant .species not so hardy and f)nly to be met 

 with when more favourable seasons have rendered their innnigration fr^m f)ut^ 

 lying regions possible. 



Probably the permanent fauna is fairly well represented in the collection 

 made and amongst certain groups, such especially as the Land ]\b)llusca, there is no 

 fluctuating fauna, l)ut in the case of others, such as the Insecta especially and to a 

 lesser extent the Mannnalia, tlie fluctuating fauna, dependent as it is primarily 

 upnn tiie vegetation, is an important factor. With a succession of had seasons 

 tlie vegetation dwindles and tiie animals, except the most hardy species, disappear, 

 and even tlie latter become very much thinned out. With a recui'rence of good 

 seasons first of all the sur\ iving inhabitants iiicrea.se in numtiers, and then if the 

 good seasons last long enough a gradual immigration takes place. 



The permanent fauna again may be divided into two groups, the first contain- 

 ing those animals which can always be found during the diy season, the second 

 containing those which only appear during the short wet season. 



The collector who sets to work as we did in a dry season, especially if he has 

 been accustomed to the iiioister coastal district, is first of all struck with the fact 

 that there is a wonderful poverty of animal life except so far as regards ants, flies, 

 grasslioppers and certain beetles, birds and lizards. He naturally misses almost 

 all forms of life associated, as the Plialangerid;e for example, with well wooded 

 districts, or the Platypus with the sheltered pools of permanent rivers, and he 

 rapidly appreci;ites the influence of a climatic Ijarrier. 



After turning over every availalile stick and hundreds of stones and finding 

 no trace of moisture he realizes how impossible it is for creatures such as land 



