SPECIES. 19 



ence of such individual differences is attested by the univer- 

 sal employment of the terms ' varieties ' and ' races.' Thus a 

 * variety ' comprises all those individuals which possess some 

 distinctive peculiarity in common, but do not differ 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 

 ' > n r ! etal,' 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 reproduc- 

 tive 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 



* 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. 



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