January 6, 1909 ie 95 
DeVriese says species “have supplanted the genera which 
were the accepted units before Linnaeus. ‘They are now to be 
replaced, in their turn, by smaller types, for reasons which do 
not rest upon comparative studies but upon direct experimental 
evidence.” 
“Discussion” is Dr. Britton’s second heading, and under it 
he speaks of the difficulties attending the delimitation of species. 
“As long as species were generally understood to be rela- 
tively fixed in characters, their delimitation was relatively sim- 
ple, but the general understanding that all living organisms are 
descended from others which were different from them has 
greatly complicated the subject.” 
“Whether the evolution has been by imperceptible progres- 
sive modifications of structure or by mutation, or by both meth- 
ods, the result is essentially the same from the practical stand- 
points of taxonomy; from these standpoints, then, similarity of 
individuals must remain the consideration to which most weight 
will be given in taxonomic usage.” 
He says that species, “though necessarily difficult in delim- 
itation, will still remain the practical taxonomic groups, recog- 
nizing, nevertheless, that they are made up of either relatively 
constant or of widely fluctuating elementary components, which, 
in turn, will presumably yield the species of future geologic 
ages.” 
“The recognition of the existence of incipient or elemen- 
tary species or races within the composition of species explains, 
in large part, the multiplication of species and of groups of as- 
sumed lower rank, in many of the larger genera, nearly every 
taxonomist, except the most conservative, having taken more or 
less part in thns increasing the number of descriptions and of 
names.” 
Geographical distribution is also mentioned as an import- 
ant factor, the plants in certain areas differing from those of 
other areas; but in cases of contiguous land districts this method 
should be used with caution, as intermediate forms are almost 
