194 CAMPBELL, A. G., Acanthhae or ThornbiUs. ["^^f Jan." 



Geological change has been more violently retrograde in this 

 portion of y\ustralia than in any other part. This is the cause, 

 which, in efifect, produced the pallid and desert forms of the 

 genus. Possibly the region became diy gradually, and birds 

 steadily adapted themselves to the harder conditions. 



The other Tertiary area, embracing, as it does, the whole of 

 the Murray basin, consisting of more than two-thirds of New 

 South Wales and about half of \^ictoria, though extremely flat, 

 is a land of i)lenty, well covered with light to medium forests, 

 well watered, and in the southern half, with a good rainfall — 

 real "Australia Felix" harbouring a great variety of birds of 

 all classes. The hypothesis is that on the higher lands in East- 

 ern, South, and Western Australia the direct descendants of the 

 original stock of Acanthica will be found, while on the flat inter- 

 vening country the sub-species, or later varieties, occur. 



WHAT LS A SPECIES ? 



This is the next cjuestion.* If intergradations can be found 

 between west and east forms, these intergradations occurring in 

 intervening localities, then it is obvious that the species is one 

 and the same throughout. If intergradations are found in one 

 and the same locality, this fact should be still more obvious. But 

 if complete intergradations cannot i)e found (or have not yet 

 been found), or, if two species, even though grading toward one 

 another, are yet separated by a reasonable gap, we must conclude 

 they are worthy of separate specific rank. 



A species in scientific classification is a group of individuals 

 of similar nature and constitution. A species may vary within 

 reasonable limits. Reasonable limits being — 



(a) One species must not intergrade with another species, 

 connecting links may have existed in the past, but have 

 all died out. 

 (h) The gap between two closely allied species must be 

 consistent and wide enough to be easily recognisable. 



Two or more species of a genus may occur in any one locality. 



A species may collectively be formed by two or more sub- 

 species. 



A sub-species is a local race which, owing to change in en- 

 vironment or geographical location, differs consistently from the 

 race first discovered. 



Sub-s])ecies may grade into one another. 



Sub-species should exhibit some recognisable and consistent 

 characters in the normal average, though a much narrower 

 margin of demarcation be allowed than with species. 



*Sct' Tho .^iik, vol. xxxix., p. 147. 



