82 M. A. Giard on the Position o/'Sagitta, and on 



embryogeny of the Ascidia, Hackel has modified his opinions 

 upon this point : but we may say that the Sagittce were a 

 badly chosen group among the Nematoidea for the support of 

 this theory ; for we do not find in them the four muscular 

 bundles mentioned by Hackel, and their body is formed rather 

 of two antimera. 



On the other hand, the four primitive antimera occur with 

 wonderful distinctness in the tail of the larva? of certain Ascidia 

 {Perophora Listeri) ; and even in some adult Ascidia they are 

 clearly indicated by the quaternary symmetry of the buccal 

 aperture. 



Kowalevsky has himself expressed his opinion as to the 

 position of Sagitta in the animal tree : he does not hesitate to 

 place the Chaetognatha among the true Annelides*. 



Before examining these various opinions more closely, it 

 seems necessary to enter upon some general considerations 

 which will enable us the better to appreciate the causes of 

 their divergence. 



One of the most difficult problems of modern zoology, and 

 indeed that which must now-a-days preoccupy every thinking- 

 naturalist, is to determine in every peculiar arrangement of an 

 organism what belongs to heredity, and what must be attri- 

 buted to adaptation. Such inquiries present immense difficul- 

 ties, and can only be fruitfully attempted with groups of which 

 the embryogeny is sufficiently known. I speak, of course, of 

 stratological embryogeny, which only dates from ten years 

 back, and the general importance of which is unfortunately 

 not understood by all who are engaged in zoological investi- 

 gation. Every anatomical investigation that is not made with 

 the object of elucidating this new embryogeny, is a work which 

 may certainly possess some interest, but one which is no longer 

 of our epoch, and even loses an enormous portion of its value. 

 However, all naturalists of any merit have always been sus- 

 tained in their efforts by a philosophical idea ; and, although I 

 may thereby subject myself to bitter criticism, I regard the 

 memoirs of a Geoffroy Saint-Hilaire, a Wolf, or a Kowalevsky 

 as having contributed much more to the progress of science 

 than the anatomy of the cat by Straus-Durckheim, or that of 

 the tortoise by Bojanus. 



We shall endeavour to show what enormous influence the 

 external conditions of existence may have upon the form of 

 an animal, what astonishing resemblances may result from the 

 action of identical causes upon originally different organisms. 

 It will be the eternal glory of Lamarck that he was the first to 



* Nablioudenia nade rajvetierne Braclriopoda. Moscow, 1874, p. -'>!. 

 note, 



