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1859
1860
1861
1866
1869
1872

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1860
1861
1866
1869
1872

on this earth have to be classed together, 1859 1860 1861
on this earth, have to be classed together, 1866
can be arranged within a few great classes; 1869 1872

have 1859 1860 1861 1866
within each class have, according to our theory, formerly 1869
within each class have, according to our theory, 1872

by the finest 1859 1860
by fine 1861 1866
together by fine 1869 1872

on my view 1859 1860 1861
on this view 1866
OMIT 1869 1872

groups 1859 1860 1861
or more groups 1866 1869 1872

if 1859 1860 1861 1866 1869
in their adult condition, if 1872

the same or 1859 1860 1861
closely 1866 1869 1872

the same or nearly similar parents, 1859 1860 1861
the same parent-form, 1866
one parent-form, 1869 1872

in that degree 1859 1860 1861
OMIT 1866 1869 1872

descent. 1859 1860 1861
descent; but dissimilarity in embryonic development does not prove discommunity of descent, for in one of two groups all the developmental stages may have been suppressed, or may have been so greatly modified as no longer to be recognised, through adaptations, during the earlier periods of growth, to new habits of life. 1866
descent; but dissimilarity in embryonic development does not prove discommunity of descent, for in one of two groups the developmental stages may have been suppressed, or may have been so greatly modified through adaptation to new habits of life, as to be no longer recognisable. 1869 1872

1 blocks not present in 1859 1860 1861 1866 1869; present in 1872
Even in groups, in which the adults have been modified to an extreme degree, community of origin is often revealed by the structure of the larvæ; we have seen, for instance, that cirripedes, though externally so like shell-fish, are at once known by their larvæ to belong to the great class of crustaceans.

It will reveal this community of descent, however much the structure of 1859 1860 1861
Community of descent will, however, often be revealed, although the structure of 1866
Even in groups, in which 1869

modified and obscured; 1859 1860 1861
greatly modified and thus obscured; 1866
modified to an extreme degree, community of origin is often revealed by the structure of the larvæ; 1869

can 1859 1860 1861
though externally so like shellfish, can 1866
though externally so like shell-fish, are 1869

embryonic state of each species and group of species partially 1859 1860 1861
embryonic state of each species and group of species 1866
structure of the embryo generally 1869
embryo often 1872

the 1859 1860 1861
more or less completely the 1866
more or less plainly the 1869 1872

ancient progenitors, 1859 1860 1861 1866
and ancient progenitor, 1869
and ancient progenitor of the group, 1872

of life should resemble 1859 1860 1861 1866
so often resemble 1869
so often resemble in their adult state 1872

their descendants,— our 1859 1860 1861
our 1866
OMIT 1869 1872

species. 1859 1860 1861
species, their descendants. 1866
species in the same class. 1869
species of the same class. 1872

but I am bound to confess that I only hope 1859 1860 1861 1866
and I hope 1869
and we may hope hereafter 1872

the law hereafter proved 1859 1860 1861 1866
it hereafter shown in most cases 1869
the law proved 1872

now supposed to be represented in many embryos, 1859
now supposed to be represented in existing embryos, 1860 1861 1866
of the progenitor of the group 1872
OMIT 1869

in a long course of modification 1859 1860 1861
OMIT 1866 1869 1872

have ever
lived,
lived
on this earth have to be classed together, and as all have been connected by the finest gradations, the best,
or,
and,
or indeed,
if our collections were nearly perfect, the only possible arrangement, would be
genealogical;
genealogical.
descent
Descent
being on my view the hidden bond of connexion which naturalists have been seeking under the term of the
Natural
natural
System.
system.
On this view we can understand how it is that, in the eyes of most
natu- ralists,
naturalists,
the structure of the embryo is even more important for classification than that of the adult. For the embryo is the animal in its less modified state; and in so far it reveals the structure of its progenitor. In two groups of
animals,
animal,
however much they may
at present
at present
differ from each other in structure and
habits
habits,
if they pass through the same or similar embryonic stages, we may feel
almost assured
assured
that they
have
all are
have both
descended from the same or nearly similar parents, and are therefore in that degree closely related. Thus, community in embryonic structure reveals community of descent. It will reveal this community of descent, however much the structure of the
adults
adult
may
may
have been modified and obscured; we have seen, for instance, that
cirripedes,
cirripedes
can at once
known
be recognised
by their larvæ
to belong
as belonging
to the great class of crustaceans. As the embryonic state of each species and group of species partially shows us the structure of
its
the
their
less modified ancient progenitors, we can
clearly
clearly
see why ancient and extinct forms of life should resemble the embryos of their descendants,— our existing species. Agassiz believes this to be a
universal law
law
of nature; but I am bound to confess that I only hope to see the law hereafter proved true. It
can,
can
however, be
be
proved true
only in
in
those cases
alone
alone
in which the ancient
state
state,
now supposed to be represented in many embryos, has
not
not
been
obliterated
wholly obliterated
wholly obliterated,
obliterated,
neither
either
by
the
the
successive variations in a long course of modification having
supervened
super- vened