12 MANUAL OF ZOOLOGY. 



distinctive peculiarity in common, but do not diflfer in other 

 respects from another set of individuals sufficiently to entitle 

 them to take rank as a separate species. A " race," again, is 

 simply a permanent or "perpetuated" variety. The question, 

 however, is this — How far may these differences amongst in- 

 dividuals obtain without necessitating their being placed in a 

 separate species ? In other words : How great is the amount 

 of individual difference which is to be considered as' merely 

 " varietal^' and at what exact point do these differences become 

 of '^specific" value? To this question no answer can be given, 

 since it depends entirely upon the weight which different 

 naturalists would attach to any given individual difference.* 

 Distinctions which appear to one observer as sufficiently great 

 to entitle the individuals possessing them to be grouped as a 

 distinct species, by another are looked upon as simply of 

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

 sible 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 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 intermediate 

 links ; and the same thing occurs in some of the Crustacea. 



* As an example of this, it is sufficient to allude to the fact that hardly 

 any two botanists agree as to the number of species of Willows and Bram- 

 bles in the British Isles. What one observer classes as mere varieties, 

 another regards as good and distinct species. 



