401 



2nd, polymorphism ; 3rd, local forms ; 4th, co-existing varieties ; 

 5th, races or sub-species ; and 6th, true species." The first is the 

 case of those insects " in which the specific form is to some extent 

 unstable," differences of form continually arising, between which 

 there are gradations that are not capable of being easily defined. 

 By polymorphism or dimorphism, he understands " the co-existence 

 in the same locality of two or more distinct forms, not connected 

 by intermediate gradations, and all of which are occasionally pro- 

 duced by common parents. These distinct forms generally occur 

 in the female sex only." Mr. Wallace believes " it will be found 

 that a considerable number of what have been classed as varieties 

 are really cases of polymorphism ;" and this may be suspected in 

 most instances " in which well-marked varieties occur in company 

 with the parent species, but without any intermediate forms." 

 Local forms " constitute the first step in the transition from variety 

 to species." They " occur in species of wide range, when groups of 

 individuals have become partially isolated in several points of its 

 area of distribution, in each of which a characteristic form has 

 become segregated more or less completely." On coexisting varieties, 

 or cases in which a slight variation of form " exists in company 

 with the parent or typical form, without intermediate gradations," 

 he does not lay much stress as of doubtful occurrence. Race or 

 sub-species is the name applied to " local forms completely fixed and 

 isolated," and it is a mere matter of opinion which are to be " con- 

 sidered as species and which varieties."* 



The above distinctions should be carefully attended to by the 

 local entomologist. For though Mr. Wallace's observations in the 

 paper refeired to below, relate to the Papilionidse alone, he has no 

 doubt that facts similar to those he has collected would " be found 

 to occur in other groups of insects, were local faunas carefully 

 studied in relation to those of the surrounding countries ; and they 

 seem to indicate that climate and other physical causes have, in 

 some cases, a very powerful effect in modifying specific form and 



* See Mr. Wallace's instructive paper, " On the Phenomena of Variation 

 and Geographical Distribution, as illustrated by the PapilionidsB of the 

 Malayan Kegion." — Linn. Trans., vol. xxv. p. 5. 



