GIARD'S CRITICISMS 273 



gotten that homology does not necessarily mean an immediate 

 common origin or close relationship. There exist, doubtless, 

 homologies of great atavistic importance I consider as such, 

 for example, the formation of the cavity of Rusconi [the 

 archenteron] in Ascidians and lower Vertebrates. But there 

 are also adaptive and purely analogical homologies, such as 

 the interdigital palmation of aquatic birds, amphibians and 

 mammals. These are not purely analogous organs, for they 

 can be superposed one on another, which is not the case with 

 simply analogous structures (the bat's wing, for example, 

 cannot be superposed on the bird's wing) ; they are 

 homologous formations, resulting from the adaptation of the 

 same fundamental organs to identical functions. Such is, in 

 my opinion, the nature of the homology existing between 

 the tail of the ascidian tadpole and that of Amphioxus or of 

 young amphibians. The ascidian larva, having no cilia and 

 being necessarily motile, requires for the insertion of its 

 muscles or -contractile organs ... a central flexible axis, 

 a true chorda dorsalis analogous to that of Vertebrates " 

 (pp. 278-9). This point of view is strengthened by the fact 

 that in Molgula, studied by Lacaze-Duthiers, the embryo is 

 practically stationary, and forms no notochord, nor ever 

 develops sense-organs in the cerebral vesicle. 



Giard's general conclusion is that " the true homology 

 with Vertebrates ceases after the formation of the cavity of 

 Rusconi and the medullary groove : the homologies estab- 

 lished by Kowalevsky for the notochord and the relations 

 of the digestive tube and nervous systems are not 

 atavistic, but adaptive, homologies " (p. 282). There is 

 accordingly no close genetic relationship between Ascidians 

 and Vertebrates. 



Giard's criticisms did not avail to check the vogue of the 

 new theory, which soon became an accepted article of faith 

 in most morphological circles. 1 The fall of the Ascidians 

 from their larval high estate provided the text for many 

 a Darwinian sermon. 



1 For the later history of the Amphioxus-Ascidian theory the reader 

 may be referred to A. Willey's well-known work, Amphioxus and the 

 Ancestry of the Vertebrates, New York and London, 1894, and to Delage 

 et Herouard, Traite de Zoologie concrete. Tome viii., Paris, 1898. 



