218 EVOLUTION IN BIOLOGY VI 
3. In a remarkable essay ' Meckel remarks— 
“‘There is no good physiologist who has not been struck by 
the observation that the original form of all organisms is one and 
the same, and that out of this one form, all, the lowest as well as 
the highest, are developed in such a manner that the latter pass 
through the permanent forms of the former as transitory stages. 
Aristotle, Haller, Harvey, Kielmeyer, Autenrieth, and many 
others, have either made this observation incidentally, or, 
especially the latter, have drawn particular attention to it, 
and deduced therefrom results of permanent importance for 
physiology.” 
Meckel proceeds to exemplify the thesis, that 
the lower forms of animals represent stages in 
the course of the development of the higher, with — 
a large series of illustrations. 
After comparing the Salamanders and the 
perennibranchiate Urodela with the Tadpoles and 
the Frogs, and enunciating the law that the more 
highly any animal is organised the more quickly 
does it pass through the lower stages, Meckel goes — 
on to say— 
‘From these lowest Vertebrata to the highest, and to the — 
highest forms among these, the comparison between the embry- — 
onic conditions of the higher animals and the adult states of the — 
lower can be more completely and thoroughly instituted than if 
the survey is extended to the Invertebrata, inasmuch as the latter © 
are in many respects constructed upon an altogether too dissimilar 
type ; indeed they often differ from one another far more than 
the lowest vertebrate does from the highest mammal; yet the 
1 « Entwurf ciner Darstellung der zwischen dem Embryozus- 
tiinde der hoheren Thiere und dem permanenten der niederen 
stattfindenden Parallele,” Beytrdge zur Vergleichenden Anatomic, 
Bd. ii, 1811. 
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