( ^41 ) 



course of time, which need not be long, the species A and B develop into A^ and B', 

 with the characters a.' and b^ respectively ; this change in characters may perhaps be 

 due to the respective areas occupied by the species being suddenly extended or 

 restricted. The characters a and b are, therefore, not inherited by A^ and B' ; hut 

 A and B (and A^ and B') are nevertheless specifically distinct. 



III. Wallace's definition of the term " species " * is a combination of definitions 

 I. and II., with the addition that the specific characters are of an adaptive kind. As 

 the objections raised under I. and II. apply also to Wallace's definition, we can restrict 

 our remarks to a short discussion of that latter point. As a species is not only 

 opposed to every other species, but is also to be distinguished from variety, the 

 definition of the term " species " must be a guide for the distinction of species from 

 variety. The kind of characters, therefore, to be mentioned in the definition must 

 exclusively be distinctive of species ; a certain quality alleged to be required to make 

 a group of individuals specifically distinct must not be a quality that distinguishes 

 variety from variety. The question is, therefore, are there varieties (as opposed to 

 species) the distinguishing characters of which are adaptive ? The theory of Natural 

 Selection, so much supported by Wallace, gives an aflirmative answer. All the 

 varieties which are selected as the fittest are varieties with special adaptive char- 

 acters. If we accept Natural Selection as a factor in evolution, we have consequently 

 to concede that both sjjecies and varieties are " adapted to slightly different con- 

 ditions of life." Hence it is evident that Wallace's definition of the term " species " 

 includes a quality which is not exclusively specific, but applies also to the term 

 " variety," and that we have to cancel altogether the restriction that specific characters 

 are of an adaptive kind. 



The principal objection here raised against those three definitions, which may 

 be taken as fairly representing the various views of modern authors, is that the 

 definitions, even when accepted as giving the general distinction between any two 

 species, do not furnish us with a general criterion between species and varieties. 

 This sounds baroque, but is a fact. There are varieties (as opposed to " species ") 

 which do no longer have sexual intercourse with the other individuals of the same 

 species, and we must also assume that sometimes such an intercourse is not jiossible ; 

 there are varieties which exhibit hereditary distinguishing characters; and there are 

 also varieties with adaptive distinguishing characters. The consequence of accepting 

 a definition of the term " species " which does not exclude every kind of variety (yarietas 

 as opposed to species) leads naturally to the conclusion that there is no real dis- 

 tinction between " species" and " variety," and that it is purely conventional whether 

 we call a form species or variety, an opinion by no means rarely met with even 

 amongst us species-makers. For example, Butler says : f " For some years past I 

 have held the view that what is generally understood by the term " species " (that is to 

 say, a well-defined, distinct, and constant type, having no near allies) is non-existent 

 in Lepidopter a, a,m\ that the nearest approach to it in this order is a constant, though 

 but slightly differing, race or local form — that genera, in fact, consist wholly of a 

 gradational series of such forms." 



According to our definition there is, however, a real distinction between the 



• Dariolnhm 1S8U. p. 1(!7. 



t Ann. Mat/. A'. JI. (5). XIX. p. Iu:i. It is easily pcTceivable fioiii Dr. Hutler's work on Li-pidoptcm, 

 for instance from liis revision of a group of butterflies callctl Euphkinai', that what Butler regards above 

 as '* species " is subspecies or geographiavl variety, the gradational series of which constitute the *' species," 

 and that his " genua " is the species. 



ao 



