No. 375.] AGASSIZ’S WORK ON FOSSIL FISHES. 181 
embryology of the same; in fine, as he says, “The results of 
these two methods of inquiry complete and control each 
other.” 
In this memoir Agassiz also worked out the geological 
succession and distribution of the different groups of fishes, 
thereby greatly increasing the practical value of their remains 
as an aid in identifying strata. His observations upon the 
heterocercal tail, its duration in time, and, owing to accelerated 
development, its transitory appearance in the early stages of 
recent forms deserve notice in this connection. The principles 
of tachygenesis seem to have been fully grasped by him, 
although not distinctly formulated. To him properly belongs 
the credit, also, according to the testimony of one of his 
students at a later date,! of having first apprehended and 
expounded what is commonly called the biogenetic law of 
Haeckel. 
Yet another important feature of the Pozssons Fossiles was 
the proposal of an entirely new system of classification of fishes, 
fossil and living, based upon the different types of scales, which 
were found to coincide to a remarkable degree with certain 
skeletal differences, His system was the first to recognize the 
ganoids as an independent order, although it is true that the 
limits assigned it were much larger than we can at present 
allow. However, Agassiz did not himself overestimate the value 
of his classification, being fully aware of its empiric character; 
but he committed himself to it chiefly on account of its great 
practical convenience. His aim was quite as much to prove the 
succession of fossil fishes throughout the different geological 
horizons as to work out their anatomical structure; and for this 
purpose, as well as for enabling him to bring together in an 
intelligible order large quantities of fragmentary material, it 
succeeded admirably. It may not be amiss to cite in this con- — 
nection a letter of his to Humboldt, in which he disclaims 
attaching any special importance to his classification, and con- 
1 Cf. Alpheus Hyatt, On Cycle in the Life of the Individual (Ontogeny), and 
in the Evolution of its Own Group (Phylogeny). Science, N.S., vol. v (1897), pp- 
161-171. Hyatt considers that “ Agassiz’s introduction of the element of succes- 
sion in time laid the basis for all more recent [embryological] work ” (p. 163). 
