124 
NATURAL SELECTION. 
Chap. IV , 
more parent-species are supposed to have descended from 
some one species of an earlier genus. In our diagram, 
this is indicated by the broken lines, beneath the capital 
letters, converging in sub-branches downwards towards 
a single point; this point representing a single species, 
the supposed single parent of our several new sub-genera 
and genera. 
It is worth while to reflect for a moment on the cha¬ 
racter of the new species f which is supposed not to 
have diverged much in character, but to have retained 
the form of (F), either unaltered or altered only in a 
slight degree. In this case, its affinities to the other 
fourteen new species will be of a curious and circuitous 
nature. Having descended from a form which stood 
between the two parent-species (A) and (I), now sup¬ 
posed to be extinct and unknown, it will be in some 
degree intermediate in character between the two groups 
descended from these species. But as these two groups 
have gone on diverging in character from the type of 
their parents, the new species (f will not be directly 
intermediate between them, but rather between types 
of the two groups; and every naturalist will be able to 
bring some such case before his mind. 
In the diagram, each horizontal line has hitherto been 
supposed to represent a thousand generations, but each 
may represent a million or hundred million generations, 
and likewise a section of the successive strata of the 
earth’s crust including extinct remains. We shall, when 
we come to our chapter on G-eology, have to refer again 
to this subject, and I think we shall then see that the 
diagram throws light on the affinities of extinct beings, 
which, though generally belonging to the same orders, 
or families, or genera, with those now living, yet are 
often, in some degree, intermediate in character between 
existing groups; and we can understand this fact, for 
