60 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 otherwise 

 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- 



