sterile and its propagation effected almost wholly by vegetative means, the 
unknown factor of hybridism is thereby practically eliminated. While it would 
be useless to deny the possibility of still lower and still higher forms appear- 
ing, yet, so far as this series goes, it is without a break or a missing link in 
the evolutionary chain. Parts of series, broken series, the so-called groups, 
are always in evidence, but a series in which the connection of intergrading 
forms is so well kept up as in the one before us, is not so common. This 
difficulty however is probably to be attributed more to a scarcity of material, 
at least in many cases, than to an absence of such connecting forms in 
nature. 
It is generally assumed that the lowest forms of a series are the oldest, 
and therefore properly represent the specific type, and that the highest forms 
are the latest and mark the evolution of this type. While this proposition 
is probably well founded, it scarcely admits of a demonstration: and while 
it is reasonably certain that plants appear in series, it is an open question 
whether these series are always ascending: possibly some are ascending, 
some comparatively stationary and others descending, inline for final extinc- 
tion. 
It is however more for its practical application in systematic work than 
for any theoretical consideration that this series is presented at this time, as 
I am fully convinced that if we are to make any considerable progress in 
systematic bryology, it must be along these lines. The younger bryologists 
among us know quite well what difficulties they have to contend with in 
determining the mosses they collect. If they succeed in tracing, with a 
reasonable degree of certainty, a given specimen to its genus, they are not 
infrequently baffled, notwithstanding the most persistent efforts, in making 
a satisfactory reference as to the species, two or three descriptions agreeing 
equally as well as any one. By not a little hard work and many bitter fail- 
ures, the older ones among us have learned to discount these descriptions to 
their actual value, and are thereby enabled to approximate a determination 
with a reasonable degree of confidence. 
In every system of classification, dealing with plants, the species is 
the unit of aggregation. It is therefore of the utmost importance that 
we should have a definite conception of what a species really is. In 
botany there is probably nothing so unscientific as the looseness with 
which species are usually made. Huxley defines a species as “the 
smallest group to which distinctive and invariable characters can be 
assigned.” This is the traditional species, originating in a special act of 
creation, (whatever that may mean,) and continuing through all time with 
but limited variation, sexual trespass among the individuals of which being 
punished with annihilation. Huxley’s species may be both logical and theol- 
ogical, but it is objectionable for the reason that it is too restrictive. Spe- 
cies with invariable characters are rare and probably do not exist outside of 
a single individual. Alphonse de Candolle says: “They are mistaken 
who repeat that the greater part of our species are clearly limited and 
that the doubtful species are in a feeble minority. This seemed to be 
