( 24) 
employed “the word species as a designation for the 
totality of individuals differing from all others by marks 
or characters which experience showed to be reasonably 
constant and trustworthy, as is the practice of modern 
naturalists.” * 
This conception of a species is founded upon transition. 
Whenever a set of individuals can be arranged, according to 
the characters fixed upon by the systematist, in a series 
without marked breaks, that set is regarded as a species. 
The two ends of the series may differ immensely, may 
diverge far more widely than the series itself does from other 
series; but the gradual transition proclaims it a single 
species. If transitions were all equally perfect of course there 
would be no difficulty. But transitions are infinite in their 
variety ; while the subjective element is obviously dominant 
in the selection of gaps just wide enough to constitute 
interspecific breaks, just narrow enough to fuse the species 
separated by some other writer,—dominant also in the choice 
of the specific characters themselves.t Looking back upon the 
interval between Linnzeus and Darwin, it seems remarkable 
that the mutability of species was not forced upon systematists 
as the result of their own labours. It is astonishing that 
many a naturalist was not driven by his descriptive work to 
the conclusion which Darwin stated to Asa Gray on July 20, 
1856: ‘“‘— as an honest man, I must tell you that I have 
come to the heterodox conclusion, that there are no such 
things as independently created species—that species are only 
strongly defined varieties.” { 
For, as I have said above, every describer of species made 
continuity and transition in characters the test of a variety, 
discontinuity the test of a separate species. And in diflicult 
cases no two of them agreed in their conclusions. Many 
passages in Darwin’s correspondence convincingly prove how 
essential an element is this continuity, and how inevitable 
* Le. p. 870. 
+ How important this choice may be is well shown by Karl Jordan in 
‘*Novitates Zoologice,” vol. iii, Dec. 1896, pp. 428-480. Characters are 
subject to independent variation as well as correlated variation. Hence 
there may be the widest discrepancy between the transitions constructed 
by naturalists making use of different characters. 
+ ‘‘Life and Letters,” vol. ii, p. 79. 
