and the horse;
but in some points of structure it may have differed considerably from
both, even perhaps more than they differ from each other. Hence in all
such cases we should be unable to recognize the parent form of any two
or more species, even if we closely compared the structure of the parent
with that of its modified descendants, unless at the same time we had a
nearly perfect chain of the intermediate links.

. . . . . .

"By the theory of natural selection [surely this is a slip for "by the
theory of descent with modification"] all living species have been
connected with the parent species of each genus, by differences not
greater than we see between the natural and domestic varieties of the
same species at the present day; and their parent species, now generally
extinct, have in their turn been similarly connected with more ancient
forms, and so on backwards, always converging to the common ancestor of
each great class; so that the number of intermediate and transitional
links between all living and extinct species must have been
inconceivably great. But assuredly if this theory [the theory of descent
with modification or that of "natural selection"?] be true, such have
lived upon the earth."[255]

To return, however, to Lamarck.

"Though Nature," he continues, "in the course of long time has evolved
all animals and plants in a true scale of progression, the steps of this
scale can be perceived only in the principal groups of living forms; it
cannot be perceived in species nor even in genera. The reason of this
lies in the extreme diversity of the surroundings in which each
different race of animals and plants has existed. These surroundings
have often been out of harmony with the growing organization of the
plants and animals themselves; this has led to anomalies, and, as it
were, digressions, which the mere development of organization by itself
could not have occasioned."[256] Or, in other words, to that divergency
of type which is so well insisted on by Mr. Charles Darwin.

"It is only therefore the principal groups of animal and vegetable life
which can be arranged in a vertical line of descent; species and even
genera cannot always be so--for these contain beings whose organization
has been dependent on the possession of such and such a special system
of essential organs.

"Each great and separate group has its own system of essential organs,
and it is these systems which can be seen to desc