A PROBLEM IN EVOLUTION 419 



The first vertebrates to make their appearance on the face of the 

 earth were fishes. They are still wonderfully well preserved as fossils 

 in the rocks of the Devonian period ; and it is perfectly clear that, when 

 alive, they were practically identical in structure with certain fishes 

 now living. But we have no records of true fishes from an earlier 

 period ; from this point downwards into the abyss of time, without 

 warning or apparent reason, the vertebrates drop from the records, 

 although the records themselves remain, and they contain, both after 

 that period and for an immeasurably long time previous to it, a full, 

 even a detailed, account of nearly every known group of invertebrates. 

 Why do the vertebrates disappear at this point? Where did they come 

 from? What kind of invertebrates were their ancestors? How did 

 the anatomical structures peculiar to all vertebrates originate? Here- 

 tofore no one has been able to give even an approximately satisfactory 

 answer to these questions. Here indeed is a great gap in the evolution 

 of the animal kingdom. It is not merely one link that is missing; 

 the whole middle section, perhaps two thirds of the entire animal 

 kingdom, is either absent, or, if present, it has not been recognized and 

 properly located. As there is no apparent resemblance between the 

 structural plan of any known invertebrate and that of a vertebrate, 

 there is no way of uniting the higher animals with the lower; no way 

 of deciding what was the great trunk line of evolution. 



This is a serious defect in the very foundations of the biological 

 sciences. While it remains we are compelled to admit that, with all 

 our boasted schemes of classification according to genetic relationships, 

 the whole class of vertebrates hangs in mid-air over an unknown and 

 apparently inaccessible abyss ; that we are totally ignorant of the great 

 creative period in the evolution of the highest type of animals; that 

 we know nothing of the way in which the fundamental structural 

 features of man arose; that we have no basis for the interpretation of 

 the early stages of his embryonic development; and no clue to the 

 initial significance of a single one of his characteristic organs, such as 

 the mouth, notochord, skeleton, lungs, jaws, appendages, heart, thymus, 

 thyroids, pituitary body, pineal gland, sense organs and brain ! 



III. The Origin of Vertebrates Abandoned as a Hopeless 



Problem 



During the generation following the appearance of Darwin's 

 '•' Origin of Species," many attempts were made to bridge this great gap 

 in our knowledge of evolution. The best known theories were defended 

 by the most distinguished zoologists of their time ; but they were, after 

 all, mere suggestions, and their authors were compelled to unite the 

 nearest probable extremes by long arrays of purely imaginary animals. 



