WORMS TO VERTEBRATES 77 



attempt to decipher his past history, though it may 

 lead to no sure conclusions, will yet be of use to us. 

 Practically all aquatic vertebrates lead a swimming 

 life, neither sessile nor creeping. The embryonic 

 development of our appendages leads to the same con- 

 clusion. We must never forget that the embryonic 

 development of the individual recapitulates briefly the 

 history of the development of the race. Now the legs 

 and arms, or fore- and hind-legs, of higher vertebrates 

 and the corresponding paired fins of fish develop in the 

 embryo as portions of a long ridge extending from 

 front to rear of the side of the body. 



This justifies the inference that the primitive verte- 

 brate ancestor had a pair of long fins running along 

 the sides of the body, but bending slightly downward 

 toward the rear so as to meet one another and continue 

 as a single caudal fin behind the anal opening. Such 

 fins, like the feathers of an arrow, could be useful only 

 to keep the animal " on an even keel " as it was forced 

 through the water by the lateral sweeps of the tail. 

 They would have been useless for creeping. 



But there is another piece of evidence that he was a 

 free swimming form. All vertebrates breathe by gills 

 or lungs, and these are modified portions of the diges- 

 tive system, of the walls of the oesophagus, from which 

 even the lung is an embryonic outgrowth. Now prac- 

 tically all invertebrates breathe through modified por- 

 tions of the integument or outer surface of the body, 

 and their gills are merely expansions of this. In the 

 annelid they are projections of the parapodia, in the 

 mollusk expansions of the skin, where the foot or creep- 

 ing sole joins the body. Why did the vertebrate take 

 a new and strange, and, at first sight, disadvantageous 



