FISHES. 119 



acter. This theory was based essentially upon the fact that 

 (1) the existing fishes have mostly " homocercal " tails when 

 adult, but have " heterocercal " tails when in the embryonic 

 condition ; while (2) the Palaeozoic and older Secondary fishes 

 have almost invariably " heterocercal " tails when fully 

 grown : and we do not meet with homocercal fishes till we 

 reach the Jurassic period. Many arguments have been 

 adduced against this theory, the most important being the 

 fact, as shown by Huxley, that the so-called " homocercal " 

 tail of the majority of existing fishes is only symmetrical by 

 deceptive external appearance, and that it is in reality 

 " heterocercal." On the other hand, the adult Flasmohranchii 

 of the present day have heterocercal tails, whereas in their 

 embryos the caudal fin is homocercal. The new observa- 

 tions of A. Agassiz would, however, go to show that the 

 theory of the elder Agassiz is essentially correct. Thus it 

 now appears that the really earliest stage of the tail in the 

 Bony fishes and Elasmobranchs is the " leptocardial " stage, 

 in which the tail is symmetrical and the notochord is straight, 

 and that the " heterocercal " condition constitutes a second 

 stage, superseded in the former by the permanent " homo- 

 cercal " or " diphycercal " condition, but remaining through- 

 out life in the latter group. Moreover, some of the ancient 

 Devonian fishes have, as adults, a " leptocardial " tail ; while 

 others have a more or less marked " heterocercal " tail ; but 

 none of them reach the staoe of a " homocercal " tail, such 

 as that of the majority of living Bony fishes. 



As regards their general distribution in time, the geologi- 

 cal history of fishes presents some points of peculiar interest. 

 Of all the classes of the great sub-kingdom Vertebrata, the 

 fishes are the lowest in point of organisation. It might 

 therefore have been reasonably expected that they would 

 present us with the first indications of vertebrate life upon 

 the globe ; and such is indeed the case. After passing 

 through the enormous group of deposits known as the 

 Laurentian, Huronian, Cambrian, and Lower Silurian forma- 

 tions — representing an immense lapse of time during which, 

 so far as we yet know for certain, no vertebrate animal had 

 been created — we find in the Upper Silurian rocks the first 



