34 REPORT—1874, 
remoteness of the common parent, and more or less marked according to the 
extinction or preservation of unaltered primary or less altered intermediate 
forms. ' 
As there is thus no difference but in degree between a variety and a 
species, between a species and a genus, between a genus and order, all disputes 
as to the precise grade to which a group really belongs are vain. It is left 
in a great measure to the judgment of the systematist, with reference as 
much to the use to be made of his method as to the actual state of things, 
how far he should go in dividing and subdividing, and to which of the grades 
of division and subdivision he shall give the names of Orders, Suborders, 
Tribes, Genera, Subgenera, Sections, Species, Subspecies, Varieties, &c., with 
the consequent nomenclature. In the limitation of his orders, genera, spe- 
cies, &c. he must carefully observe those cases where the extinction of races 
has definitely isolated groups having a common parentage; and in other 
cases where the preservation of intermediate forms has left no such gaps, he 
is compelled to 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 exist- 
ence, and that genera, orders, &c. were more arbitrary, established for prac- 
tical use, and founded on the combination of such characters as appeared the 
most constant in the greater number of species, and therefore the most im- 
portant ; 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. 
In valuing these evidences, in estimating the comparative value of cha- 
racters, a new difficulty has arisen, that of distinguishing the two classes of 
characters to which Professor Flower has appropriately given the names of 
essential and adaptive, the former the result of remote hereditary descent, 
the latter the more recent effect of external influences. This distinction is 
often the more difficult, as the essential ones are often only to be found in 
embryos, in the early stages of organs, or are merely indicated by slight 
rudiments requiring close observation to: detect them; whilst the adaptive 
ones, of comparatively small systematic importance, are often developed in 
external form, in ramification, spinescence, foliage, &c., and are the most 
striking to the eye. Oue consequence is, that the systematist of the present 
day sees more and more the necessity of preparing a double arrangement of 
his genera, species, and other groups—a natural one according to the best 
evidences of affinity for the purpose of scientific study, and an artificial clavis 
by which the student can be led to identify genera or species by the more 
readily observed characters, which may only form part, or be but chance 
accompaniments, of the essential ones. The greatest change, however, which 
the adoption of the doctrine has effected in the methodical study of plants 
is the having 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 absolute as to obviate the need of con- 
sidering 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 his judgment, it gives equal increase to the value of the results obtained. 
The principal works through which the systematic botanist contributes to 
the scientific study of the vegetable kingdom are:—1. General treatises or 
descriptive reviews of the natural orders (Ordines Plantarum); 2. Methodical 
enumeration and descriptions of genera (Genera Plantarum); 3. Methodical 
enumeration and descriptions of species (Species Plantarum); 4. Monographs 
of separate orders or genera, subgenera or species; 5. Floras of separate 
