78 
Sudden Appearance and Constancy. 
once in a million) can not be excluded. The object of 
experimentation cannot therefore be to demonstrate ab- 
solute constancy. The best plan usually is to be content 
with a few hundred individuals; it is even often impos- 
sible to get sufficient seed for more. Experiments on a 
smaller scale should only serve to confirm the results ob- 
tained in other cases; but even if they only do this they 
are, in my opinion, by no means without value. 
Fig. /. Bidcns tripartita. Type without ray florets. 
The nearest that we can get to demonstration of ab- 
solute constancy is to make observations on races which 
grow in vast quantities in certain districts and are never- 
theless true to their type. In these cases the constancy 
is so striking as to induce many systematists to regard 
the form as a species. Amongst the better known ex- 
