132-(102) GRADATION IN ANIMAL LIFE. 



rule, as to assume that we should be of a higher grade if 

 we had whalebone fringes in our mouths in place of 

 teeth. Again, the ascidians already mentioned have in 

 the embryo condition a dorsal cord, or notochord, as it 

 is called, which is a rudimentary back-bone, or verte- 

 bral column. The possession of such a column is bio- 

 logically the badge of admission to the highest division 

 of the animal kingdom. If an animal be ever so de- 

 graded in every other respect, yet can show even a 

 paltry rudiment of a backbone, he is at once received 

 with open arms into this legion of honor, the vertebrate 

 sub-kingdom. 



Burrowing in the mud of our bays, is a wretched 

 little worm. He does not look as resjoectable as one of 

 our honest earth-worms. Yet a scientist, a few years 

 ago prying into its inwards, discovered a long nervous 

 cord running fore and aft along his back. Forthwith 

 this eyeless, footless, idiotic worm was inaugurated 

 into the noble family of vertebrates, and from that time 

 has been privileged to look down upon his fellows. — 

 He is as great a nuisance in the family as a beggar would 

 be in a palace. We have to keep apologizing for his 

 presence there, for his meanness and emptiness ; but he 

 lias his patent for entrance, his little rudiment of a back- 

 bone, and we cannot turn him out. 



What shall we then think of an ascidian who posses- 

 ses the same birthright of a right of admittance to the 

 highest sub-kingdom and then abandons it when he is 

 on the high road to maturity \ Certainly we cannot so 

 closely follow our established law as to regard the pro- 

 cedures as any indication that the type of the dorsal 

 nerve-coid is really subordinate to the type lacking this 

 cord, which in this case is the following and maturer 

 form. 



It may be interesting here to call attention to the fact 

 that this little ascidian, who for a brief period of tender 

 youth temporarily fore-shadows the great vertebrate 

 sub-kingdom, has been considered by some eminent bio- 

 logists to be in the course of natural evolution, the con- 

 necting link with invertebrate organisms from which the 

 great and highest sub-kingdom of vertebrates has sprung. 



Again, the flounder, the halibut and other flat-fishes, 

 in their embryonic stages, have their two eyes placed on 



