20 MANUAL OF ZOOLOGY. 



they employ the term species ; one of these being a certain 

 amount of resemblance between individuals, and the other 

 being the proof that the individuals so resembling each other 

 have descended from a single pair, or from pairs exactly simi- 

 lar to one another. The characters in which individuals must 

 resemble one another in order to entitle them to be grouped 

 in a separate species, according to Agassiz, "are only those 

 determining size, proportion, colour, habits, and relations to 

 surrounding circumstances and external objects." 



On a closer examination, however, it will be found that 

 these two leading ideas in the definition of species external 

 resemblance and community of descent are both defective, 

 and liable to break down if rigidly applied. Thus, there are 

 in nature no assemblages of plants or animals, usually grouped 

 together into a single species, the individuals of which exactly 

 resemble one another in every point. Every naturalist is 

 compelled to admit that the individuals which compose any 

 so-called species, whether of plants or animals, differ from 

 one another to a greater or less extent, and in respects which 

 may be regarded as more or less important. The existence 

 of such individual differences is attested by the universal 

 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 

 " 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 



* 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. 



