468 Dr. C. Semper on the Identity in Type 



But even then, of course, only the identity in type of the 

 three segmented classes would be proved, and not, as was for- 

 merly supposed, the near relationship of the Arthropods and 

 Vertebrates, which would stand rather in the position of cousin - 

 ship to one another, while the Annelids would have to be con- 

 sidered as their common ancestors. For only in these last 

 does one find all the relations in the structure of the blasto- 

 dermic layers as well as in their gradual segmentation, by the 

 more or less partial transformation of which the typical single 

 segments of the Vertebrates and of the Arthropods are to be 

 explained. 



If, therefore, only the relative positions of the organs are 

 taken into consideration, the correspondence in type between 

 the three segmented classes is to be regarded as proved. The 

 result is otherwise, hov\^ever, if one employs the purely phy- 

 siological consideration of the position with regard to the 

 earth's surface in order, as Baer has again recently done, to 

 demonstrate the identity of ventrum or dorsum in all bilaterally 

 symmetrical animals. Then, of course, there appears an abso- 

 lute distinction between Articulates and Vertebrates ; what in 

 the latter is turned upwards, lies in the former on the ventrum ; 

 and a similar direct inversion appears in all the organs, 

 although " evolutio bigemina " is typical in both cases. 



But how is the identity of the ventrum* in the Articulates 

 and the Vertebrates demonstrated ? I have sought in vain to 

 find a proof of it in Baer's latest work. It could only be 

 established in one of two ways — either by proving that the 

 same organs lie on the ventral side in both gTOups of animals, 

 which is in this case impossible, or by showing that (perhaps 

 in consequence of the influence of gravity upon the developing 



* I should like in this place to be allowed to mate a second small cor- 

 rection in Baer's reproduction of my remarks. Baer says that I had com- 

 menced my reasoning with the proposition that " dorsum and ventrum are 

 not morphological ideas " in order to smooth my way. This is not quite 

 accurate ; for in the complete work, which appeared in October 1874, I 

 introduced this in the course of the discussion of the other arguments 

 against my views, and I did it purposely in order to avoid the appearance 

 of wishing to smooth my way by a dogma : and, further, I did not put the 

 proposition forward as a dogma, but attempted to prove it by the use of 

 various arguments. It may be doubted whether this attempt has suc- 

 ceeded ; but no one is justified in ascribing to me an intention of establish- 

 ing a foundation for discussion which cannot be found in the wording of 

 my paper. 



I must confess that this misinterpretation of my words (which, I repeat, 

 is in no way justified) has pained me.; or has Baer possibly not read my 

 ' Stammverwandtschaft ' ? Besides, Baer has completely ioiisunderstood 

 me when he supposes that I wished to deny the existence of a marked 

 morphological distinction between dorsum and ventrum in the same 

 animal or in the same group. 



