120 
EVOLUTION 
little competition, a very peculiar mode of life, or some other 
advantage. It therefore appears in the top line, represent- 
ing the later period. D is supposed to be a species with 
many individuals, and therefore one in which favourable 
variations are more likely to arise. It gives rise to four 
varieties, but ultimately only to two species, X and Z. E is 
supposed to die out like B. 
The four species T, U, V, W are nearly allied. After a 
long period they will again give rise to new species, repre- 
sented by the letters in the top row. The species K, L, M 
derived from W will be nearly allied in character, but will 
differ much from U and still more from C, from X, and from 
the two species H and I (derived from Z). The divergence 
between these various groups will now be so great that they 
will be regarded as genera , one with three species, one with 
two, and the rest each with one species. The common an- 
cestor of the species of a genus is far back, but the common 
ancestor of two allied genera farther still. Genera agree in 
fewer characters than do species. The characters that mark 
genera should be studied in a flora (see also Coniferae in 
Pt. II.); it will be noticed that floral characters enter more 
largely into the diagnoses than in the case of species. The 
genus represented by the one species C will be what is 
called an archaic genus, a type belonging to a former age. 
Several examples of this kind may be met with, eg. Casua- 
rina, Isoetes, Phylloglossum, &c. 
Just as varieties gradually diverge into species and these 
into genera, so these again form groups of genera, or natural 
orders. The characters of all the natural orders are given in 
full in Part II. and should be studied there, especially by the 
method of comparison of allied orders. The orders may be 
grouped into cohorts , these into series , and so on. 
It is necessary to keep clearly in mind the fact that the 
ancestral forms die out as species diverge, and can only be 
found, if at all, as fossils. The archaic forms of vegetation 
now existing are of special interest as types of a preexisting 
flora, but they must not be regarded as representing the 
ancestral forms from which our existing vegetation of higher 
types is derived, though very possibly often nearly related to 
them. The study of fossil botany has brought to light 
