254 "^^^ DOCTRINE OF. DESCENT. 



primitive intestine becomes the branchial cavity. But 

 with reference to the vertebrate animals, the most im- 

 portant parts of the Ascidian larva are the following. 

 It possesses a true spinal cord with a vesicularly ex- 

 panded brain (ra). The distribution and position of 

 this organ agrees accurately with the corresponding 

 parts of the vertebrate animal, and Kupfer has even 

 discerned the rudiments of nerves (sss), which, if the 

 observation is confirmed, will still more incontroverti- 

 bly establish the homology of the organ in questidn with 

 the spinal cord of the Vertebrata and the nerves pro- 

 ceeding from it in pairs. But we know that it is not 

 the spinal cord alone, but its combination with the ver- 

 tebral column which constitutes the characteristic fea- 

 ture of the vertebrate animal. This vertebral column 

 the Ascidian larva likewise possesses (c) in the form 

 of the noto-chord, and, as in the vertebrate animal, this 

 embryonic vertebral column lies between the intestine and 

 the spinal cord. So far goes the accordance ; henceforth, 

 the development of this part, so important to the verte- 

 brate animal, becomes retrogressive in the Ascidian. The 

 rudder-like tail, with the spinal cord contained in it, 

 and the noto-chrord, are cast oflf when the animal be- 

 comes fixed; the larval brain which promised so well, 

 shrink into an insignificant nervous ganglion, and the 

 complete animal gives no cause for suspecting its analogy 

 with the Vertebrata. 



These laborious observations prove that the Verte- 

 brata are not the sole proprietors of the spinal cord and 

 vertebral column, but received these organs as a heritage 

 from lower grades of organization as their progenitors. 

 It does not occur to the Darwinists to regard man as the 



