440 THE EVOLUTION OF MAN. 



of giviug us an approximate idea of onr oldest Silurian vertebrate ancestors. 

 But the latter are descended from Worms, the nearest blood-relatives of 

 •which are the Ascidians of the present day." — The Pedigree of the Human 

 Race (1868). 



The peculiarities in the structure of the body, Avhich dis- 

 tinguish Vertebrates from Invertebrates, are so striking, 

 that the relationship of these two main groups of the animal 

 kingdom formerly threw great difficulties in the way of 

 systematic classification. When, in accordance with the 

 Theory of Descent, the relationship of the various groups 

 of animals began to be regarded as not figurative, but as 

 really genealogical, this question came to the front, and 

 seemed to offer one of the greatest obstacles to the success 

 of the theory. Even at an earlier period, when without this 

 fundamental thought of the true g^ealogical connection 

 of the relationships between the great main groups of the 

 animal kingdom, the so-called " types " of Baer and Cuvier 

 were studied, investigators believed they had found, here 

 and there among Invertebrates, points connecting these 

 with Vertebrates ; some single species of Worms, in par- 

 ticular, appeared to approximate in the structure of their 

 bodies to the Vertebrates ; as, for examj^le, the oceanic Arrow- 

 worm (Sagitta). But the attempted analogy was shown, 

 by closer investigation, to be untenable. After Darwin 

 gave an impulse to a true tribal history of the animal 

 kingdom, by his reform of the Theory of Descent, this very 

 relation seemed to offer one of the greatest difficulties. 

 When, in 18GG, I attempted, in my Generelle Morpliologie, 

 to carry out the Theory of Descent in detail, and to apply 

 it to the natural system, no part of my task demanded i 

 more care than the connection of the Vertebrates with the 

 Invertebrates. 



