190 THE EVOLUTION OF CONTINUITY 



could not supply the Fish with oxygen, it may well be 

 that the accumulation of impurities within the closed vessels 

 brought on recurring contractions of part of the vessels 

 themselves. The primitive heart, and the arterial pulse, 

 would on this supposition originate as wave-like " convul- 

 sive " spasms of the walls of part of the vessel system, the 

 spasms being the result of waste-product accumulation, 

 and recurring regularly owing to the regularly recurring 

 accumulation of waste-products. 



In a similar way, it may be remarked, we might account 

 for the first beats of the human fcetal heart ; for, as develop- 

 ment proceeds, the maternal circulation alone is insufficient 

 for the removal of the impurities and poisons of fcetal 

 metabolism, and these substances may well directly force the 

 foetal circulation to evolve its own driving powers. It is to 

 be noted that when the mother's excretory functions fail to 

 act properly spasmodic contractions of her muscular system, 

 in the form of eclampsic convulsions, can occur. 



With respect to the systemic circulation of the Fish we 

 would detect in the main branches arising from the main 

 venous and arterial trunks ancestral megazooidal circular 

 canals. 



So far we have tried to show how the primitive Fish's 

 respiratory and circulatory systems would evolve, an 

 anterior mouth being formed at the same time ; and if our 

 theory is reasonable it should help to explain the evolution 

 of the Fish's nervous system. 



In Invertebrate segmental organisms, such as the earth- 

 worm, it has been shown (page 120) that terminal compression 

 provided a primitive form of brain by bringing together 

 the ganglia of successive anterior segments ; while pos- 

 teriorly the ganglia of successive uncompressed segments 

 formed for the animal a ganglionic chain or primitive cord. 

 The anterior ganglionic mass is situated above the mouth, 

 or dorsally, but the chain runs ventrally or below the 

 alimentary canal, a connecting nerve-collar encircling the 

 oesophagus. 



On the other hand, in Vertebrates the brain and spinal 

 cord occupy a dorsal position, and when it is presumed that 

 the vertebrates have evolved from some primitive inverte- 



