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 as suppression. 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 

 iand 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 



