MR. BALL ON PLANTS COLLECTED IN MOROCCO. 269 
another; but there bigs a general agreement as to the criterion which 
should, if practicable, ts applied. All the forms that had sprung 
from the same common stock were to be ranked as varieties. Distinct 
species were races that had descended from an ancestor, or pair of © 
ancestors originally distinct. So long as this view prevailed there 
nothi wee 
vari 8 
regarded as the ‘ype, and the term subspecies had no resbgnised place 
in systematic w 
A vadtieally different conception of the relations of organised 
beings has now been accepted by the majority of naturalists. It is 
species, not to speak of the gos differences of structure that pei 
the groups which we call s, tribe, or natural order. For 
who have admitted this facchienanttel conception it is clear oar the 
absolute distinction hitherto supposed e betwe 1 
variety no longer exists. Allied species, as well as allied varieties, are 
linked together by th of geneti lation, and th 
differences may be far wider, it is impossible to assign a distinctive 
= nate or to draw a say ich the art ends 
es ppears certain that the union of _ s dis most of 
those ranked ate species some €s ertile 
tlh while there is reason to believe that undoubted varieties of 
the same stock occasionally produce none but infertile descendants. 
se cannot b ied that the prevalence of the theory of 
evolution—mainly due to the genius and industry of Mr in— 
wn sagaci 
sew exclusively of the vegetable world, we find that most 
widely diffused plants give rise to numerous varieties which reproduce 
themselves by hereditary descent, — sg are called races. In 
t 
