SPECIES. 21 



termed "protean" or "polymorphic" genera that the deter- 

 mination of the different species and varieties becomes 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 dimorphic 

 plants a single species is composed of two distinct forms, 

 similar to one another in all respects except in their repro- 

 ductive organs, the one form having a long pistil and short 

 stamens, the other a short pistil with long stamens. In tri- 

 morphic plants, the species is composed of three such distinct 

 forms, which differ in like manner in the conformation of their 

 reproductive organs, though they are otherwise undistinguish- 

 able. (Darwin.) Similar cases are known in animals, but in 

 them the differences, though apparently connected with repro- 

 duction, are not confined to the reproductive organs. Thus 

 the females of certain butterflies normally appear under two 

 or three entirely different forms, not connected by any inter- 

 mediate 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 individuals 

 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, com- 

 munity of descent is hardly in a more satisfactory condition, 

 since the descent of any given series of individuals from a 

 single pair, or from pairs exactly similar to one another, is at 

 best but a probability, and is in no case capable of proof. In 

 the case of the higher animals it can doubtless be shown that 

 certain assemblages of individuals possess amongst themselves 

 the power of fecundation and of producing fertile progeny, 

 and that this power does not extend to the fecundation of in- 

 dividuals belonging to another different assemblage. Amongst 

 the higher animals, "crosses" or "hybrids" can only be pro- 

 duced between closely -allied species, and when produced 

 they are sterile, and are not capable of reproducing their like. 

 In these cases, therefore, we may take this as a most satis- 



