212 PROCEEDINGS OF THE AMERICAN ACADEMY. 



erst spater iu die Reibe der Wirbelthiere, wenn gleich Spuren der 

 Wirbelsiiule schoa in den friihesten Perioden seinen Anspruch auf diese 

 Stelle in der Reilie der Tliiere beglaubigten." 



It is very obvious, from these statements of Meckel, that the correla- 

 tion of embryology and the epembryonic stages of the individual vpith 

 the permanent modifications of animals of simpler construction was under- 

 stood, as far as was possible with existing knowledge, from the time of 

 Aristotle, 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 probable (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 statement the 

 modern view of the affinities of the earliest stages of the embryo or its 

 repetitions 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 onto'^eny. Both Von Baer and Louis Agassiz were pupils of 

 Ignatius Bollinger, an embryologist who published nothing. Both of 

 these eminent men have recognized him as their master in embryology, 

 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 his work on fossil fishes, by the introduction 

 of the element of succession in time, and thus laid the basis for all 

 more recent investigations. 



Agassiz gave the fullest exoression of his views in " Twelve Lectures 

 on Comparative Embryology," Lowell Institute, Boston, 1848-49, subse- 

 quently published in pamphlet form. One wonders, as he reads, how any 

 man holdins 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 an hypothesis of evolution that 



* Entwickelung. d. Thiere, Scholion V. pp. 119, 120, etc. 



