Recent Progress and present State of Systematic Botany. 298 
draw arbitrary lines of distinction wherever it appears to be 
most convenient for use. In the pre-Darwinian state of the 
science we were taught, and I had myself strongly urged, that 
species alone had a definite existence, and that genera, orders, 
c., were more arbitrary, established for practical use, and 
founded on the combination of such characters as appeared the 
most constant in the greater number of species, and therefore 
the most important. We must now test our species, as well as 
genera or other groups, by such evidences as we can collect of 
affinity derived from consanguinity. 
“Tn valuing these evidences, in estimating the comparative 
value of characters, a new difficulty has arisen, that of distin- 
guishing the two classes of characters to which Professor Flower 
has appropriately given the names of essential and adaptive,— 
) 
e 
form, in ramification, spinescence, foliage, &c., and are the most 
striking to the eye. One consequence is, that the systematist 
the — day sees more and more the necessity of 6 a ol 
the more readily observed characters, which may only form 
i or be but chance accompaniments, of the essential ones. 
he greatest change, however, which the adoption of the doc- 
trine has effected in the methodical study of plants is the hav- 
ing rendered it necessary, in the case of every genus or other 
group, to take into account and specially to estimate the value 
of all the characters observed—no one can be taken as so abso- 
lute as to obviate the need of considering others, no one can be 
passed over as theoretically worthless; and whilst this adds 
immensely to the labour of the systematist and to the calls on 
is judgment, it gives equal increase to the value of the 
results obtained.” 
