K.E. VON BAER. — PHILOSOPHICAL FRAGMENTS. 189 



its adversaries were partly directed only against the exaggerations, 

 partly were weak in not being supported by careful pergonal 

 observation of the development of the same animal form. 



A few exceptional cases can have but little weight, unless a 

 completely new doctrine can be substituted. The gradual de- 

 velopment of the embryo out of a delicate homogeneous mass, 

 called to mind so strongly the construction of the body of the 

 lowest animals, that all objections appeared wasted against un- 

 important minutiae, so long as the attempt was not made, re- 

 cognizing this agreement, to demonstrate another and a differ- 

 ent relation. 



Quite recently, in fine, the doctrine of the agreement of in- 

 dividual metamorphosis with the ideal metamorphosis of the 

 whole animal kingdom obtained an additional importance, by 

 Rathke's brilliant discovery of gill-clefts in the embryos of Mam- 

 mals and Birds, in which very soon afterwards the appropriate 

 vessels were found. 



always more cautious and indefinite, wliile those who followed them were much 

 more decided. The whole doctrine appears to me to be more a phase of the 

 development of natural science than the property of any single man. Different 

 degrees of development were observed in the different forms of animals. It 

 was recognized that these animal forms are to be considered as modifications 

 of one another. It was natural, in fact necessary, that the attempt should be 

 made to carry out the simplest mode of conceiving these modifications, that a\\ 

 forms are immediately developed out of one. The assumption of this develop- 

 ment as an historical fact, can be considered but a small step further, and is a 

 logical deduction ; a comparison with individual development came necessarily 

 into the same circle of ideas ; and in any case it is a service to have made the 

 experiment how far our knowledge of development can be introduced therein. 

 With the full conviction that the view in its whole extent is a necessary 

 transitional phase of our knowledge of nature, it seemed unnecessary to the 

 short exposition given above, to follow an exact chronological order. Many 

 of the attempts indicated, to show the development of the classes of ani- 

 mals one out of the other, are older than the more important endeavours to 

 refer the stages of development of the embryo to the class-differences. All 

 this I know very well, and I expect no objections on this head. I chose only 

 the shortest mode of exposition. The whole circle of ideas, which I here hope 

 to define more exactly, by drawing attention to the distinction between the 

 higher and lower stages of development of the animal body and the type of 

 organization, has so large a share in all investigations, that we meet with it in 

 a very gi'eat number of works, and it is therefore quite unimportant to make 

 a selection here. 



