164 THE ORIGIN OF VERTEBRATES. 



After this marvellous self-working, self-maintaining machinery 

 of life had worked successfully through the various geological 

 epochs, from the Cambrian to the Cretaceous Age, it was found 

 to be contrived on a faulty plan, and had to be amended by the 

 simple expedient of throwing overboard the digestive apparatus 

 which had hitherto grown and developed around the mouth of 

 ■every animal from the coral to the crab, while the Creator cor- 

 rected His creative work by constructing and substituting an 

 entirely new apparatus with all the latest improvements. 



This, by way of analogy. Dr. Gaskell does not, of course, 

 iDurden his thesis with any arguments either for or against the 

 creation, or any other origin of the universe. He is only con- 

 cerned with this planet, and only with life on this planet since 

 the palaeozoic era. The point is that, according to his view, 

 somewhere between the highest non-chordate and lowest 

 chordate there arose, in the steady progress of evolution, some 

 sort of crisis, or we might even say catastrophe. 



On the face of it one is naturally unwilling to accept any 

 theory involving a breacii in what we call the law of continuity 

 of design. It offends the innate instinct towards unity which 

 comprises a portion of our mental equipment. But, what is, 

 perhaps, a graver objection to Dr. ^Gaskell's theory is, that it 

 runs counter to what has been considered as a fundamental 

 axiom among geologists. 



The tissue of the original primitive cell consists of three 

 layers — the outer layer or epiblast, the inner layer or hypoblast, 

 and the intermediate tissue or mesoblast, though this is not re- 

 presented in the classes ranked below the annelids, i.e., sponges, 

 coelenterata, etc. Tn the higher animals the nervous system and 

 sense-organs are evolved from the epiblast or ectoderm ; the gut 

 and its outgrowths, lungs, liver, etc., from the endoderm or 

 hypoblast; the skeleton, muscles, etc.. from the mesoderm or 

 mesoblast. Now, Dr. Gaskell commits the heresy of evolving 

 the vertebrate gut from the arthropod nervous system ; that is, 

 he derives the gut from an epiblast, instead of from a hypo- 

 blast ; or, as he puts it, the epiblast of the arthropod may become 

 the hypoblast Of the vertebrate, and vice I'ersa. 



Needless to say, we shall not attempt to decide where 

 doctors disagree ; but we may, perhaps, be permitted to glance at 

 some of Dr. Gaskell's arguments. 



The general design in the construction of vertebrates is so 

 radically different from that of invertebrates that zoologists and 

 anatomists, although easily tracing the line from protozoa 

 through corals, zoophytes, etc., to worms, starfish, crustaceans 

 and insects, and while, with even greater ease, finding the an- 

 cestors of the mammals and birds in reptiles, amphibians and 

 fish-like animals, the remains of which have been discovered in 

 geological strata, have yet found the gulf between the highest 

 form of crayfish or scorpion, annelid or mollusc, and the lowest 

 vertebrate form well-nigh impassable. 



