Chap. X. 
AFFINITIES OF EXTINCT SPECIES. 
333 
other by a dozen characters, in this case the genera, at 
the early period marked VI., would differ by a lesser 
number of characters; for at this early stage of descent 
they have not diverged in character from the common 
progenitor of the order, nearly so much as they sub¬ 
sequently diverged. Thus it comes that ancient and 
extinct genera are often in some slight degree inter¬ 
mediate in character between their modified descendants, 
or between their collateral relations. 
In nature the case will be far more complicated than 
is represented in the diagram; for the groups will have 
been more numerous, they will have endured for ex¬ 
tremely unequal lengths of time, and will have been 
modified in various degrees. As we possess only the last 
volume of the geological record, and that in a very broken 
condition, we have no right to expect, except in very 
rare cases, to fill up wide intervals in the natural system, 
and thus unite distinct families or orders. All that 
we have a right to expect, is that those groups, which 
have within known geological periods undergone much 
modification, should in the older formations make some 
slight approach to each other ; so that the older mem¬ 
bers should differ less from each other in some of their 
characters than do the existing members of the same 
groups; and this by the concurrent evidence of our best 
palaeontologists seems frequently to be the case. 
Thus, on the theory of descent with modification, the 
main facts with respect to the mutual affinities of the 
extinct forms of life to each other and to living forms, 
seem to me explained in a satisfactory manner. And 
they are wholly inexplicable on any other view. 
On this same theory, it is evident that the fauna of 
any great period in the earth’s history will be inter¬ 
mediate in general character between that which pre¬ 
ceded and that which succeeded it. Thus, the species 
which lived at the sixth great stage of descent in the 
