6o ELEMENTS OF BIOLOGY. 



of varietal value ; and, in the nature of the case, it seems im- 

 possible to lay down any definite rules. To such an extent 

 do individual differences sometimes exist in particular 

 genera — termed " protean " or " polymorphic " genera — that 

 the determination of the different species and varieties be- 

 comes an almost hopeless task. 



Besides the individual differences which ordinarily occur 

 in all species, other cases occur in which a species consists 

 normally and regularly of two or even three distinct forms, 

 which cannot be said to be mere varieties, since no inter- 

 mediate forms can be discovered. When two such distinct 

 forms exist, the species is said to be " dimorphic," and when 

 three are present it is called " trimorphic." Thus in di- 

 morphic plants a single species is composed of two distinct 

 forms, similar to one another in all respects except in their 

 reproductive organs, the one form having a long pistil and 

 short stamens, the other a short pistil with long stamens. 

 In trimorphic plants the species is composed of three such 

 distinct forms, which differ in like manner in the conforma- 

 tion of their reproductive organs, though they are othenvise 

 undistinguishable. — (Darwin.) Similar cases are known in 

 animals, but in them the differences, though apparently con- 

 nected with reproduction, are not confined to the reproduc- 

 tive organs. Thus the females of certain butterflies normally 

 appear under two or three entirely different forms, not con- 

 nected by any intermediate links, and the same thing occurs 

 in some of the Crustacea. 



As regards, therefore, the first point in the definition of 

 species — namely, the external resemblance of assemblages 

 of individuals — we are forced to conclude that no two indi- 

 viduals are exactly alike ; and that the amount and kind of 

 external resemblance which constitutes a species is not a 

 precise and invariable quantity, but depends upon the value 

 attached to particular characters by any given observer. 



The second point in the definition of species — namely, 

 community of descent — is hardly in a more satisfactory con- 



