THE ORIGIN OF VERTERRATES. 165 



Tliis may, perhaps, be owing to the imperfection of the 

 geological record ; but scientists have for some time endeavoured 

 to trace some line of descent between some annelid, like the 

 earthworm or lobworm, on the one hand, and the unique balano- 

 glossus or lancelet on the other; the latter being regarded as a 

 primitive ancestor or degenerate descendant of a fish, the former 

 rather doubtfully ranked as an archaic vertebrate on account of 

 its being the happy possessor of a rather questionable notochord. 

 Another attempt ^through a different channel, connects the mol- 

 luscoid lampshell with the tunicate or sea-squirt, whose larval 

 form also boasts a notochord " in the tail ! " ^ 



These theories between them may be said to have held the 

 field until Dr. Gaskell's " earthquaking hypothesis " effected a 

 linking uj) of the broken chain in the persons of two solitary 

 survivors of their respective families, the king-crab and the 

 lamprey, by the simple process, as already explained, of con- 

 verting the gastric system of the arthropod into the brain ven- 

 tricles of the fish, which obligingly constructs for itself, proprio 

 iiiotu, a fresh digestive system out of a portion of its respiratory 

 apparatus. 



In spite of its revolutionary character, Dr. Gaskell's conten- 

 tion is supported by two considerations : First, the extreme pro 

 bability, on quite other grounds, that some such metamorphosis 

 actually took place at a t'me when the arthropods were at their 

 greatest period of develo})ment, since it coincides with the first 

 recorded appearance of representatives of the class Pisces in 

 form greatly resembling in some respects the king-crab, in 

 others, the hagfish or lamprey. Secondly, that such a funda- 

 mental alteration can be regarded as a parallel, if not a prece- 

 dent, to the change which undoubtedly must have taken place 

 when marine vertebrate organisms first developed into am- 

 phibious and land-frequenting animals ; a change noj. perhaps, so 

 .drastic, nor involving the heretical conversion of hypoblast into 

 epiblast, but still entailing nothing less than the transformation 

 of the air-bladder of the fish into the lung of the amphibian, and 

 the atrophy of the gills, or their employment in other than 

 respiratory functions. 



Says Dr. Gaskell : — 



" This transition from the gill-bearing to the lung-bearing vertebrates 

 is most interesting, for it has been proved that the hmgs are formed 

 by the modification of the swim-bladder of fishes ; and in a group of 

 fishes, the Dipnoi, or lung-fishes, of which three representatives still exist 

 on earth, tlie mode of transition from the fish to the amphibian is plainly 

 visible; for they possess both lungs and gills, and yet are not amphibians 

 but true fishes. But for the fortimate existence of Ceratodus in Australia, 

 Lepidosiren in South America, and Protopteris in Africa, it would have 

 been impossible from the fossil remains to have asserted that any fish 

 existed which possessed at the same moment of time the two kinds of 

 respiratory organs, although from our knowledge of the amphibian we 

 niiglit have felt sure that such a transitional state must have existed." 



The foregoing has been brought forward by Dr. Gaskell, 

 :not, of course, as his own discovery, as it is a commonplace of 



