274 DOMESTICATED ANIMALS AND PLANTS 
any considerable length of time, and would soon become extinct 
were it not constantly renewed. Whether the apricot is a mu- 
tant from the plum, or the reverse, we could now only speculate, 
but from general reasoning we should regard the apricot as the 
original stock and the plum the mutant. 
The orange and the lemon. These two popular tropical fruits 
belong to a tangled group covered by the generic name C7trus, 
and including also the lime and the shaddock. While their 
original has not been identified in the wild, their origin is uni- 
versally credited to eastern Asia, probably China. The principal 
strains of this genus are as follows : 
The citron (Citrus medica proper), a large nonspherical fruit 
with an aromatic rind and a moderate amount of not very acid 
juice. 
The shaddock, or grapefruit (Cztrus decumana), large and 
round, juicy, slightly acid, extensively cultivated in southern 
Asia and in the tropics generally. 
The lemon (Citrus medica imonum), juice decidedly acid. 
The lime (Citrus medica acida), like the lemon, but much 
smaller ; juice very acid. 
The orange (Citrus aurantium), in two varieties, bitter and 
sweet, of which the latter is the cultivated, and of which the 
tangerine and the mandarin are minor strains distinguished by 
the easily separated rind, and for this reason often called “ kid- 
glove oranges.” 
The citrous fruits have a pronounced acid quality and a lurking 
tendency to be bitter, a tendency that crops out strongest in the 
bitter orange, which is wild, and in the shaddock, which is culti- 
vated.?, These fruits have been long in cultivation, as fruits go, 
1 This is because the mutant is more commonly destitute of some character 
that is present in the original. Nearly every pubescent species, too, has its 
smooth variety, which in some cases succeeds better than the original. The 
fact that we do not see the mutation is no argument that it has not taken 
place. Plain wheat, for example, has certainly arisen from the bearded, which 
is to be regarded as the original stock. See further in the chapter on Mutation. 
? It may be added that the common orange easily escapes from cultivation, 
in which case most of the trees bear insipid fruit, a few bitter, and occasionally 
