^'°'i923''"-] CA^TPBELL, A. G.. .Icanthhac or ThoruhUh. 195 



Only one sub-species oi a species can normally occur in any 

 one locality. 



Sub-specific characters are more variable than specific charac- 

 ters, owing to the fact that the inter- variations are now in pro- 

 gress between sub-species which have passed out of existence 

 between species. For the same reason, specific characters are 

 more variable than generic, intergradation of the latter being 

 lost in anti(]uity. 



A^VRIATION. 



It is remarked by even a casual observer that birds of one 

 species may vary in tone of colour according to their surround- 

 ings. There is the influence of heat and cold, light and shade, 

 humidity and dryness, the matter of food supply being co- 

 related. It is plain that under such external stimuli the living 

 bird reacts or adjusts itself to the requirements of the case. 

 This gives rise to variation, and under the influence of its en- 

 vironment a bird may become brighter or duller, lighter or 

 darker, larger or smaller than the type.* 



A. — Individual Variation. 



It is a fundamental fact that no two individuals of any one 

 species from one locality are identically alike. Little individual 

 variations occur within certain limits, however small they be, 

 that make it necessary for the observer to have several speci- 

 mens before him in order to obtain the average or typical 

 characteristics. 



This, the first step in variation, does not necessarily constitute 

 a specific difference because it may not be sufficiently fixed to 

 stand the experience of a movement in the reverse direction, 

 when it finds itself in alternating conditions of drought and plenty 

 — essentially phases of the Australian climate in the interior. 



B. — Regional Variation. 



This is a further stage, Avith an important bearing upon the 

 question of sub-species. Where variation has been in progress 

 for a long period of time in two localities that have some marked 

 difference in geographical environment, then the two branches 

 of original stock each take on some character or characters that 

 apparently apply, or belong to that particular region. And, if. 

 two such regions become separated from one another by some 

 natural feature, such as sea or desert, preventing an interchange 

 of stock, then regional variation may exhibit marked and per- 

 manent differences worthy of being classed as sub-specific. Many 



*Type is the first specimen of that species to be described and 

 named. A typical specimen is different, being one of average appear- 

 ance. 



