44, ADAM SEDGWICK. 
the embryonic stage! is the same as it is in the adult. It may 
be altered relatively more or it may be altered relatively less ; 
the point is that it is altered in the same direction as the 
adult organ. And this is surely what we should expect when 
we remember that embryonic development is the preparation 
of the free form in the most perfect state and at the least 
expense. If this view is correct that variations are present in 
the embryo—that an organ which is enlarged, diminished, or 
suppressed in the adult is correspondingly, or nearly so, en- 
larged, diminished, or suppressed in the embryo,—then I ask, 
how are we to account for those cases which most undoubtedly 
occur in which records of previous states of structure are 
present in the embryonic history, e.g. the pharyngeal slits of 
Sauropsids, the tubular heart, the vascular arches, the em- 
bryonic kidney of the same group, and manysuch. The point 
is this: organs which have been recently altered show a 
similar alteration in the embryo, whereas some organs, like the 
gill-slits, which must have been altered very far back, do not 
show a corresponding embryonic alteration, but persist more 
or less in their old form without discharging the original 
functions or being of any use to the embryo. In other words, 
some ancestral organs persist in the embryo in a functionless 
rudimentary (vestigial) condition and at the same time without 
any reference to adult structures, while other ancestral organs 
have disappeared without leaving a trace. The latter arrange- 
1 It appears that in some cases, at least, it is less in the embryo. LE. g. 
sternal ribs of ostrich are generally five in adult, rarely six; in embryo, 
they appear always to be six. In birds the fibula reaches the tarsus in 
embryos, but very rarely does so in adults. 
A case of this kind which might be investigated is tlis:—In the golden 
plover the hallux is entirely absent, whereas in other plovers it is present. 
Has the golden plover any trace of it in the embryo? 
I am aware that it is often held—Darwin held it—that rudimentary organs 
are, relatively to the adjoining parts, larger in the embryo than in the adult. 
But unless this fact can be shown to be universal, it has but little value 
because it applies to many other organs in the embryo which are not rudimen- 
tary, e. g. brain, eye, heart, and kidney. This difference in relative size is 
probably simply owing to the fact that the bulk of the skeletal, muscular, 
and connective tissues of the embryo is relatively less than in the adult. 
