Vol. X, No. 6.] Ind(h Australian Passalid Goleoptera. 207 



[N.S.] 



lies in the nature of the conditions postulated towards the 

 centre of distribution in each cas3. Clark says, speaking of a 

 potential genus as yet scarcely distinguishable from a species 

 (Amer. Nat. xlv, p. 373): "There is somewhere within the 

 range of this young genus, normally at or near the centre, an 

 area of optimum conditions, where life is easy and there is no 

 severe struggle for existence." And again (Indian Ocean 

 Crinoids, p. 18): " The crinoids of Australia came from the 

 northward, from the great East Indian Archipelago ; but here 

 continual changes in the distribution of land and sea have con- 

 stantly rejuvenated the fauna so that none of its component 

 species has been permitted to drift into the peaceful old age so 

 obvious in almost all of the species along the Australian 

 shores." 



In the case of Indo Australian Passalidae, on the other 

 hand, I have been forced to regard the centre of distribution 

 as the site of keen competition among forms well adapted 

 for survival; resulting in a radial pressure of more, on les< 

 highly specialized species — the latter surviving chiefly when 

 cut off from the former by some barrier which gives a check, 

 probably oi a more or less temporary nature, to the distribu- 

 tion of the newer forms. 



Clark does not find it necessary to point out the radial 

 character of the relation between the structure and distribu- 

 tion of Crinoids, although it exists, as deduced above from his 

 data ; but that he recognizes its frequent occurrence, and also 

 the occurrence of a certain amount of radial pressure, is shown 

 by the following passages from " The Ontogeny of a Genus " 

 (loc. cit., p. 373): — " Here [in the central ''area of optimum 

 conditions "] various more or 1 k ss aberrant types arise and are 

 able to perpetuate themselves, spreading out in every direction 



as did the orginal stock, but never so far, as they are not so 

 well prepared to encounter adverse conditions/' And fl The 

 forms occupying the limits of the range of a genus as a whole 

 (geographical or bathymetrical) are continually trying to colo- 

 nize new territory, both from their own initiative and as the 

 result of pressure from behind." 



Clearly the two hypotheses are not mutually exclusive ; for 

 the initiative of the relatively primitive forms living on the 

 limits of the range of the " genus" as a whole may in some 

 groups be as nothing compared to the pressure from behind, 

 while in others the reverse may be the case [ . Each may be true 

 of certain groups ; or possibly, when a £i mature ' ? group reaciies 

 a certain maximum of vigour, a temporary increase in competi- 



1 The exceptional richness of the fauna of the East Indian Archipelago 

 indicates the presence there, of conditions which may well be responsibl 

 for a very great increase, in the Tndo- Australian area, of this pressure 



from behind. 



