52 



THE STORY OF FISH LIFE. 



column, instead of being continued straight 

 backwards, turns sharply upwards, and the 

 single fin becomes differentiated into two lobes, 

 an upper and larger — the dorsal lobe ; and lower 



Fig. 6.— The Evolution of the Fish's Tail. A. B. C, Shows 

 how tail has passed from primitive elongated symmet- 

 rical form, through the unsymmetrical (5), to the modern 

 falsely symmetrical form (C). The false symmetry being 

 due to the excessive development of the lower lobe 

 marked 1. 1, in B. The upturned portitm of the tail in B 

 has gradually disappeared, and is represented only by the 

 black portion marked N, in G. D. E, F. Stages througli 

 which the tail of a modern fish successively passes dur- 

 ing its development, being practically a repetition of the 

 stages A. B. C. 



and much smaller — the ventral lobe. The ven- 

 tral lobe, it will be noticed, takes its origin, 

 entirely from the lower side of the upturned 

 vertebral column. 



The homocercal tail is found in the higher 



