228 OUTLINES OF EVOLUTIONARY BIOLOGY 



The aim which the zoologist or botanist sets before himself in 

 classification is the expression of the " natural affinities " of the 

 plants or animals investigated. The existence of such natural 

 affinities was clearly recognized long before the explanation of 

 them was known, and even by upholders of the doctrine of special 



creation. Thus it was 

 Individual 

 vn Bears 



Individuals 



Species 



Genera 



Families 



Orders 



Classes 



Species A re cos 



t 



Genus Ursus 



Family Ursidae: 



Order Carnivora 



Class Mammalia 



Phylum Vertebrata 



well known to Linnaeus 

 that individuals fall 

 naturally into species, 

 that species may be 

 grouped in genera, 

 genera in families, and 

 so on in ever widening 

 circles. It also soon 

 came to be recognized 

 that natural affinities 

 could be best determined 

 by taking into account 

 as many characters as 

 possible instead of rely- 

 ing merely on one or 

 two. A system of classi- 

 fication based on a small 

 number of characters 

 only is always more 

 or less artificial, and 

 though it may serve its 

 purpose for a time it 

 will have to be amended 

 by future workers who 

 are able to bring a more 

 complete knowledge to 

 bear upon the problem. 

 Thus it comes about 

 that our views on the 

 classification of the animal and vegetable kingdoms are always 

 undergoing change, old systems are constantly being discarded 

 as too artificial and more natural ones proposed in their stead. 



As our knowledge progresses it becomes more and more evident 

 that a natural classification assumes a tree-like form. The 

 division of the whole organic world (with the exception of the 



Phyla 



Kingdoms 



Kingdom Animalia 



The Organic World 



FlG. 86. Diagram, to show the Tree-like Form 

 assumed by a natural Classification. 



