26 



PROF. AGASSIZ'S 



(Plate XVII. fig. D.) that is, a crinoidal echinoderm 

 aping the sea-urchins by its spherical form and by 

 the regular arrangement of its plates and by the fact 

 that there are zones of holes,alternating with zones 

 of plates without holes. But that they are not 

 echini is shown by the fact,that they rest on a stem, 

 and that in each row of imperforated plates there 

 are four sets of plates instead of two, as in Echini. 

 Here crinoids are perfectly developed into the 

 form of higher types, but under the general char- 

 acter of the lowest group of these animals ; those 

 forms, however, become more and more individu- 

 alized in later periods. And here are other Cri- 

 noids, (Plate XV & VII) from which free star fishes 

 branch off during the subsequent geological times. 

 But what is most curious, is the fact, that among 

 the Echini, the oldest are the Cidaris{ Plate VI, D) 

 spherical bodies somewhat flattened, with large 

 plates, narrow rows of holes, and remarkably large 

 spines in proportion to their proper size, {Plate VII, 

 E) but precisely as we have them in the youngest 

 condition of the true Echinus. (Plate XI, G). 

 The Cidaris are numerous before any true Echinus 

 occurs. Next, those are developed and become 

 gradually more and more numerous, and they are 



soon succeeded by others of a more oblong form 

 and those greatly elongated Echinoderms which 

 we call Holothurise, occur only in the present pe- 

 riod (Plates XIV and V.) 



So that by all the facts to which I have briefly 

 alluded, I can come to the conclusion that the class 

 of Echinoderms presents, notwithstanding the im- 

 perfect condition of our information upon this 

 point, the most perfect agreement between the va- 

 rious embryonic forms observed and the different 

 permanent forms of the animals of that class in 

 their full grown condition ; that these embryonic 

 forms agree also with the different structures of 

 the fossil types through all the geological ages; and 

 that these again in their order of succession, agree 

 with the different appearances of the full ^rown 

 living animals, or more precisely with their grada- 

 tion as derived from a knowledge of their internal 

 structure. 



These various relations, so complicated, and nev- 

 ertheless so permanent in every respect, show the 

 same thought throughout the whole that struc- 

 ture, development and order of succession in time, 

 are regulated by one and the same unique princi- 

 ple. 



LECTURE IV. 



The result thus far obtained in the lectures 

 which I have delivered, can be expressed as fol- 

 lows : There is a gradation of types in the class 

 of Echinoderms, and indeed in every class of the 

 animal kingdom, which, in its general outlines, 

 can be satisfactorily ascertained by anatomical 

 investigation ; but it is possible to arrive at a 

 more precise illustration of this gradation by em- 

 bryological data. The gradation of structure in 

 the animal kingdom does not only agree with the 

 general outlines of the embryonic changes. The 

 most special comparison of these metamorphoses 

 with full grown animals of the same type, leads to 

 the fullest agreement between both, and hence 

 to the establishment of a more definite progressive 

 series than can be obtained by the investigation 

 of the internal structure. These phases of the in- 

 dividual development are the new foundations 

 upon which I intend to rebuild the system of zool- 

 ogy. These metamorphoses correspond, indeed, 

 in a double sense, to the natural series established 

 in the animal kingdom; first, by the correspond- 

 ence of the external forms, and secondly, by the 

 successive changes of structure ; so that we are 



here guided by the double evidence upon which 

 the progress in zoology has, up to this time, gen- 

 erally been based. 



Their natural series again correspond with the 

 order of succession of animals in former geologi- 

 cal ages ; so. that it is equally true to say thai the 

 oldest animals of any class correspond to their 

 lower types in the present day, as to institute a 

 comparison with the embryonic changes, and to 

 say that the most ancient animals correspond with 

 the earlier stages of growth of the types which live 

 in the present period. In whatever point of view 

 we consider the animal kingdom,we find its natural 

 series agree with each other : its embryonic phases 

 of growth correspond to its order of succession in 

 time; and its structural'gradation, both to the em- 

 bryonic development and the geological succes- 

 sion, corresponds to its structure ; and if the inves- 

 tigations had been sufficiently matured upon this 

 point, I might add that all these series agree also 

 in a general way with the geographical distribu- 

 tion of animals upon the surface of our globe ; but 

 this is a point upon which I am not yet prepared 

 to give full and satisfactory evidence, and which 



