50 ADAM SEDGWICK. 
must not be forgotton that some of them are functional and 
that these resemble organs of larve in retaining ancestral 
features, e. g. the ductus arteriosus, &c. | 
To put the matter in another and more general way, the only 
functionless ancestral structures which are preserved in develop- 
ment are those which at some time or another have been of 
use to the organism during its development after they have 
ceased to be so in the adult. In this way I should be inclined 
to explain the hair of the human fcetus and the teeth of the 
foetal whale—that is to say, I should be inclined to suppose that 
the possession of the lanugo is due to the fact that there was a 
time in the evolution of man when the babe required this 
protection against the cold after the necessity for it had dis- 
appeared in the adult, and that the young whale in the days 
when whalebone was first acquired still retained the ancestral 
habits which required teeth. It is, however, possible that 
these and other similar cases of the retention of rudimentary 
organs in late embryonic life have another explanation, and it 
becomes necessary to collect and examine as many cases as 
possible of the undoubted retention, as embryonic rudiments, of 
organs which we have reason to know have recently disappeared 
from the adult stage. 
The retention of such organs in the embryo may, as I have 
hinted, be due to the fact that they have been retained func- 
tionally by the young animal after they have been lost by the 
adult; but another explanation is possible, which is that 
organs which are becoming functionless, and disappearing at 
all stages, may in some cases disappear unevenly; that is to 
say, they may remain at one stage after they have totally dis- 
appeared at another. In this manner we might get an organ 
which had become quite functionless and had quite disap- 
peared in the free stage, still persisting, though with a much 
reduced development, in the embryo. It is possible that the 
lanugo and the teeth of foetal whales may be explicable in this 
manner. But that such a retention of organs in the embryo 
is not an important or permanent one is shown by the fact of 
their comparative scarcity in embryonic histories. This is a 
