Louis Agassiz. 



ite theory ; and, lastly, that there is no evidence of a 

 direct descent of later from earlier species in the 

 geological succession of animals." 



In 1844 Agassiz completed a monograph on the 

 fishes of the Old Red Sandstone, which was begun 

 at the request of the British Association for the 

 Advancement of Science, and was a supplement to 

 the magnificent work in five volumes entitled 

 Recherches sur les Poissons Fossiles, At this time, he 

 made some observations that then and in later years 

 attracted no little attention. He knew that there 

 were certain living fishes, as the sturgeon, in which the 

 upper lobe of the tail was formed by the continuance 

 of the backbone, and he saw in these the descend-^ 

 ants of a large group that swarmed the ancient 

 Carboniferous and Devonian seas. Agassiz made 

 the remarkable discovery that certain other fishes, 

 not of the group of which the sturgeons may be 

 taken as a representative, had in the embryo state 

 tails that were heterocercal, which afterwards be- 

 came homocercal. From this discovery, which at 

 the time occasioned no little interest in the world of 

 science, Agassiz deduced the famous law that the 



embryo of the fish during its development, the 

 present class of fishes with its numerous families, 

 and the type of fish in its geological history, undergo 

 strictly analogous phases," and in his introduction to 

 a monograph of the fossil fishes of the Old Red 

 Sandstone he applies this in a general way to verte- 

 brates : The successive creations have undergone 

 phases of development analogous to those the em- 

 bryo passes through during its growth, and similar 



