40 EVOLUTION AND DISEASE. 
The only interpretation which can be put on 
this fact is, that these organs or parts have been 
functional in the ancestors of such animals, but in 
consequence of the increased use of other parts and 
change in surrounding conditions, the organs in question 
are not so serviceable to the animal and have, in 
consequence of diminished use, slowly but gradually 
atrophied. This effect may be conveniently referred 
to assuppression. It is also a point of some importance 
to remember that in consequence of changed con- 
ditions in the surroundings and habits of an animal, 
organs originally used for one purpose may become 
so changed that they fulfil quite a different purpose: for 
example, the remora, or sucking-fish, is able to attach 
itself to the shark by means of a sucker-like disc on its 
head ; in the embryo this disc arises from the anterior 
part of the dorsal unpaired fin ; this is indicated through- 
out life by the arrangement of blood-vessels and nerves. 
Thus a locomotor-organ has become modified for 
attaching purposes. Change of function and atrophy is 
illustrated in a striking manner by the allantois. 
The Allantois—The embryos of reptiles, birds, and 
mammals, differ from those of fish and amphibians by 
the fact that at a very early date a vascular dilatation 
arises from the posterior end of the developing gut. 
This dilatation is known as the allantois, and in birds 
and reptiles spreads itself beneath the shell-membrane. 
The blood circulating in the membrane is in this way 
brought into favourable relations with the atmosphere ; 
the air diffusing through the shell, oxidises the blood in 
the allantoic capillaries. The allantois is the respiratory 
