1917.] , 57 



ON THE RARITY AND RESTRICTED DISTRIBUTION OF 



ANIMAL— ES PECI ALLY INSECT— SPECIES. 



BY GEO. I!. WALSH, B.Sc. 



Most of US. whatever the designation we apply to ourselves — 

 biouoiuist, zoologist, entomologist, systeniatist, or, tmyissimvs tiir- 

 pis>tinioruiu, "mere collector" — take more pleasure and conceivably, 

 if such a fault can be laid to the charge of any of our fraternity, 

 more pride in the capture of a rare species such as, for exi mple, a 

 " Camberwell Beauty," than in that of its commoner relatives, such as 

 a " Small Tortoiseshell," although as cabinet specimens they fill up 

 equal blanks, and as living creatures the " commoner " may possibly be 

 more interesting than its " aristocratic " ally. In view of this very human 

 love of the uncommon, it may be worth while to consider for a short 

 time some of the intrinsic factors of which the rareness of a species 

 is the extrinsic manifestation. 



In the first place, the term "rai-eness" is rather loosely applied to 

 two somewhat different phenomena, the occurrence of a species in only 

 small numbers in any part of its range, and that of a species in more 

 or less restricted localities, where it may be common or rare in the first 

 meaning of the term, over a greater or smaller range. As examples of 

 the former we may quote, from the Order whose study I most affect — 

 the Coleoptera- — VeUeini' dilatatus Y., Bhizophagus aeneus Eicht. ( = 

 coeruleipennis Sahib.), and (in Britain) Lebia crux-minor L. ; and as 

 examples of the second, Pteros^fichus cristatus Dufts., and Acrulia 

 infinta Gyll. It is obvious, however, that these two divisions finally 

 merge into one another ; and as the two phenomena are governed by 

 the same laws, it will be convenient and proper to treat them together. 



It must be noted, however, that the evidence we use as the 

 criterion of the rarity of a species depends upon a variety of factors. 

 It is reasonably easy, for example, to gain rapidly a good idea as to 

 the frequency of occurrence of conspicuous organisms, especially if 

 they are very active, like birds or Initterflies, or fixed, like the fioweriug 

 plants ; even here, however, the problem may be rendered more difficult 

 by the habitat of the species being not entii'ely accessible, e.g., the 

 deep sea or mountain summits ; or by the shyness of an animal so 

 that it is rarely seen. For example, the otter is by no means un- 

 common in our northern rivers, and is said to occur within sound, and 

 certainly within sight, pf the great Elswick works, and yet compara- 

 tively few people have ever seen one. Then again a paucity of 



