JanuAEY 29, 1897.] 



SCIENCE. 



163 



aus dieser einen Form sich alle, die niedrig- 

 sten wie clie hochsten so entwickelu, dass 

 diese die perraanenten Formen der ersten 

 nur als voriibergehende Perioden durch- 

 laufen. Aristoteles, Haller, Harvey, Kiel- 

 meyer, Autenrieth und mehrere andere 

 haben diese Bemerkung entweder im Vor- 

 iibergehen gemacht oder, besonders die 

 letzen, hervorgehoben und fiir die Phj^sio- 

 logie ewig denkwiirdige Eesultate daraus 

 abgeleitet. 



" Von diesen niedrigsten Wirbelthieren 

 an bis zu den hochsten Geschlechtern lasst 

 sich die Vergleichung zwischen dem Embryo 

 der hohern Thiere und den niedern im 

 vollkommenen Zustande voUstandiger und 

 treffender durchfiihren. 



" In der That giebt es ja eine Periode wo 

 der Embryo des hochsten Thieres, wie 

 schon Aristoteles sagt, nur die Gestalt einer 

 Made hat, wo er ohne ausere und innere 

 Organisation, bloss ein kaum geformtes 

 Kliimpschen von Polypensubstanz ist. Un- 

 geachtet des Hervortretens von Organen 

 bleibt es doch noch wegen des ganzlichen 

 Mangels eines innern Knochengerilstes 

 eine Zeitlang Wurm und MoUusk und tritt 

 erst spater in die Eeihe der Wirbelthiere, 

 wenngleich Spuren der Wirbelsaule schon 

 in den friihesten Perioden seinen Anspruch 

 auf diese Stelle in der Eeihe der Thiere 

 beglaubigten." 



It is very obvious, from these statements 

 of Meckel's, that the correlations of embry- 

 ology and the epembryonic stages of the 

 individual with the permanent modifica- 

 tions of animals of simpler construction 

 was understood, as far as was possible with 

 existing knowledge, from the time of Aris- 

 totle and that it was, to a greater or less 

 extent, a working hypothesis at that time 

 and, as declared by him, had been helpful 

 in giving a clearer understanding of the 

 development of the individual and of the 

 relations of the individual to the whole 

 animal kingdom. 



The next step was taken by von Baer, in 

 dividing the animal kingdom into four 

 types and in limiting this general statement 

 to animals occurring within each of these 

 types. He also considered it highly prob- 

 able (not barely possible, as it is quoted by 

 some writers) that the earliest stages of the 

 embryo resemble in aspect the adult stages 

 of the lowest grade of forms in the animal 

 kingdom. He had in mind in this state- 

 ment the modern view of the affinities of the 

 earliest stages of the embryo or its repeti- 

 tions of the characteristics of Protozoa,* so 

 far as the knowledge of his time permitted. 



Von Baer endeavored to prove that each 

 of the four types had similar embryos and 

 that the type characters were determinable 

 at early stages in the ontogeny. Both von 

 Baer and Louis Agassiz were pupils of 

 Ignatius Dollinger, an embryologist who 

 published nothing. Both of these eminent 

 men have recognized him as their master in 

 embryologjr, but have given no definite 

 statement of what they were taught by him. 

 Louis Agassiz accepted von Baer's opinions 

 and subsequently enlarged them, when he 

 published on his fossil fishes by the intro- 

 duction of the element of succession in time 

 and thus laid the basis for all more recent 

 work. 



Agassiz gave the fullest expression of his 

 views in ' Twelve Lectures on Comparative 

 Embryology,' Lowell Institute, Boston, 1848 

 -49, subsequently published in pamphlet 

 form. One wonders as he reads how any 

 man holding such views could have held 

 his mind closed to the conclusion that 

 animals were evolved from simpler or more 

 primitive forms. The effect of theoretical 

 preconceptions in closing the mind to the 

 reception of new 'ideas never had a stronger 

 illustration. Louis Agassiz, in 1849, had 

 all the facts essential for building up a 

 hypothesis of evolution that would have 



* Entwickelungsgescli. d. Thiere, Scholion V., p. 

 199, p. 120, etc. 



