STRUCTURAL MODIFICATIONS 143 



displaced by the torsion of the body, regains its position on the 

 side abandoned. The first branchial slit, which remained in 

 its original place but enormously increased in size, is gradually 

 masked by a fold of the integument which comes down over 

 it like a shutter, and only leaves free a slit along the ventral 

 edge. The false lateral mouth, constituted by the first branchial 

 slit, becomes median, but is opposite to the nervous system 

 instead of being situated on the same side. In order to utilize 

 it, the organism is forced to turn itself towards the ground, 

 where it must, after all, find its food. It thus converts its old 

 dorsal surface into a ventral surface, and vice versa. The atti- 

 tudinal reversal pointed out by Geoffroy Saint-Hilaire is thus. 

 actually observed and explained in the most primitive of the 

 Vertebrates, and, as we have seen, it takes place twice over. 



The Vertebrates that are descended from the primitive types, 

 of which Amphioxus is the last remaining example, have 

 necessarily preserved their type, because their nervous system 

 has merely gone on increasing in size, and because it is the 

 volume of the nervous system which determined it as a natural 

 consequence of tachygenetic processes that have come in in its. 

 method of development. Dohrn has since demonstrated that 

 in the Lampreys and Sharks also the mouth was only a 

 modified branchial slit, and these animals have retained 

 numerous traces of dissymmetry. 



We must bear in mind from now on that the Vertebrates owe 

 their origin to the importance assumed by the nervous system. 

 It is by the subsequent development of this system that they 

 rise above the other animals, and arrive at last at Man, whose 

 evolution has thus been dominated by the rapid progress of 

 the organs in which intelligence resides. Here is a theme for 

 philosophers and above all for metaphysicians — a theme which 

 could provide a field for the adjustment of doctrines hitherto 

 regarded as irreconcilable. I have shown elsewhere how other 

 characteristics of Vertebrates, such as the formation at the 

 expense of the endoderm of a dorsal cord round which the 

 vertebral column develops, follow from these premisses. 1 



I will not here lay stress on the phenomena of degeneration 



which produced the type of the Tunicates, nor the interest their 



history presents, together with certain inferences to be drawn 



from it. I have treated this important subject in another work 2 



1 XXXVIII, 319. 2 XXVII, 358. 



