618 ON THE DISTINCTION OF SPECIES. 
nature appears to have made permanently distinct, and those 
which are described in books under a supposition that they 
are so. The former I shall beg here to designate natural 
species ; applying to the latter the epithet of book species. A 
book species and a natural species may be strictly iden- 
tical, or one natural species may be improperly divided into 
two or more book species. Fraxinus excelsior, as now un- 
derstood in this country, is an example of identity; but when 
Fraxinus heterophylla was absurdly made into a second book 
species, the identity was destroyed. Probably, it does still 
sometimes occur, that two or more natural species are incor- 
rectly described as one in our books; the Myosotis scor- 
pioides of Linnæus was formerly an example of this. It is 
my present object to enumerate some additional tests, less 
precise than the foregoing, but which may assist a decision 
upon the validity of book species; that is to say, how far 
they are truly identical with natural species. 
Thirdly, then, in continuation of the former and better 
tests, it may be said that characters which are mutually inter- 
changed between two book species, must be insufficient to 
prove them really distinct species; while the fact of the 
interchange goes far to establish their identity as a single na- 
‘tural species. When a complete series of intermediate forms 
is produced by this interchange of characters, which gradu- - 
ally become more and more unlike one of the book species, 
by assuming more and more the characters of the other book 
species, until varieties of the one coalesce with varieties of 
the other; the series may then be taken as equivalent to a 
demonstration, that the two book species constitute together 
only a single natural species. Thus, Viola arvensis (Murr.) 
is so completely united with Viola tricolor, by a continued 
series of intermediate forms, that no doubt can remain re- 
specting their identity as one species in nature, although they 
have been described as two species in books. Again, among 
the few specimens of Erica Mackaii (Hook.) which I have 
had the opportunity of inspecting, a gradual transition to- 
wards Erica Tetralix is so decidedly shown, that I greatly 
