58 THE ORIGIN OF VERTEBRATES 



by means of which blood should be brought to the surface, so as to 

 interchange its gases with those of the external medium ; and it is 

 significant to find that of all vertebrates the Amphibia alone are 

 capable of an effective respiration by means of the skin. 



As to the circulatory system, it is exceedingly easily modified. 

 An animal such as Amphioxus has no heart ; in some the heart is 

 systemic, in others branchial ; in some there are more than one heart ; 

 in others there are contractile veins in addition to a heart. There 

 is no difficulty here in altering and modifying the system according 

 to the needs of the individual. 



For a digestive system all that is required is an arrangement for 

 the digestion and absorption of food, a mechanism which can arise 

 easily if some of the cells of the skin possess digestive power. Now 

 Miss Alcock has shown that some of the surface-cells of crustaceans 

 secrete a fluid which possesses digestive powers, and she has also 

 shown that certain of the cells in the skin of Ammocoetes possess 

 digestive power. 



The difficulty, then, of forming a new digestive system in the 

 passage from the arthropod to the vertebrate is very much the same 

 as the difficulty in forming a new respiratory system in the passage 

 from the water-breathing fish to the air-breathing amphibian — a 

 change which does not strike us as inconceivable, because we know it 

 has taken place. 



The whole argument so far leads to the conclusion that vertebrates 

 arose from ancient forms of arthropods by the formation of a new 

 alimentary canal, and the enclosure of the old canal by the growing 

 central nervous system. If this conclusion is true, then it follows 

 that we possess a well-defined starting-point from which to compare 

 the separate organs of the arthropod with those of the vertebrate, 

 and if, in consequence of such working hypothesis, each organ of the 

 arthropod is found in the vertebrate in a corresponding position and 

 of similar structure, then the truth of the starting-point is proved as 

 fully as can possibly be expected by deductive methods. It is, in 

 fact, this method of comparative anatomy which has proved the 

 descent of man from the ape, the frog from the fish, etc. 



Let us, then, compare all the organs of such a low vertebrate as 

 Ammocoetes with those of an arthropod of the ancient type. 



