SPECIES AXD VARIETY. 141 



on closer investigation, suffice to distinguish the individual forms. 

 Combinations of modified characters are often present in the same 

 species, and occasion important variations (varieties) which can be 

 inherited by the descendants. The more important of such variations 

 which are maintained by reproduction are called constant varieties or 

 subspecies, or races, and are divided into natural races and artificial 

 or domesticated races. 



The former are found in free natural life, and are usually confined 

 to definite localities. They have arisen in course of time in conse- 

 quence of conditions of climate, and under the influence of variations 

 in manner of life and nourishment. The domesticated races, on the 

 other hand, owe their origin to the care and cultivation of man. 

 They comprise only domestic animals whose origin is still unknown. 



Varieties, however, which have arisen from one species may differ 

 very surprisingly from one another ; in fact, they may present 

 more important features of difference than do distinct natural 

 species. An example of this is found in the domesticated race of 

 pigeons, whose common descent from Columba livia (the rock 

 pigeon) was shown by Darwin to be very probable. They are 

 capable of such striking alterations, that their varieties, known as 

 tumbler pigeons, fan tail pigeons, etc., were held by ornithologists, 

 who were without knowledge of their origin, to be real species, and 

 were even divided into different genera. 



In the natural state, too, it often happens that varieties cannot 

 be distinguished from species by the quality of their characteristics. 

 It is customary to consider that the essential of a character is to be 

 found in the constancy of its occurrence, and to recognise varieties 

 by the fact that their characteristics are more variable than those 

 of species. If forms which are widely different can be connected 

 by a continuous series of intermediate forms, they are held to be 

 varieties of the same species. But if such intermediate forms are 

 absent, they are held to be distinct species, even when the differences 

 between them (so long as they are constant) are less. 



Under such circumstances we can understand that in the absence 

 of a positive test, the individual judgment and the natural tact 

 of the observer decides between species and variety ; * and how it 

 is that the opinions of different observers have differed so widely in 



* The establishment of the conception of sub-species is completely at variance 

 with the common conception of species, and is the most striking proof that 

 systematists themselves recognize that the distinction between Bpecies and 

 sub-species is a relative one. 



