THE 



EVOLUTION OF THE VERTEBRATES 



AND THEIR KIN. 



CHAPTER I. 



OUTLINE OF THE ARACHNID THEORY. 



In the two following chapters we shall present a brief outline of the arachnid 

 theory, showing the broad foundations upon which it rests and the relation of 

 the principal organs in the arachnids to those in the vertebrates. 



I. Its Scope and Relation to Other Theories. 



The arachnid theory, like every other large problem in descent, should be 

 based on comparative physiology, anatomy, embryology, and paleontology, 

 and should be constructed in accordance with the established principles of these 

 sciences. This particular theory has the additional task of reconciling, eliminating, 

 or absorbing the claims of strongly entrenched rival theories, some of which con- 

 tain certain elements of truth. It is important, therefore, to at once determine 

 which supplies the greatest volume of evidence; which draws its evidence from 

 the widest fields; which can eliminate the others, or include the others within 

 itself. 



We shall show that in these respects the arachnid theory stands in a class 

 by itself, for it is the only one that is securely built on the natural science trinity 

 of structure, function, and historic sequence. It not only has its own distinctive 

 merits upon which it claims recognition, but it is the only theory that can either 

 ■ eliminate the others, or incorporate them within itself, where they become rein- 

 forced and revitalized. 



The essential features of the annelid theory, for example, are included in 

 the arachnid theory, because both arachnids and annelids agree in the funda- 

 mental nature of their metameric structure. But when standing alone, the anne- 

 lid theory ceases to be of value as a working hypothesis, or as a touchstone to 

 solve the problems of vertebrate morphology, because we find no traces in the 

 annelids of those illuminating modifications of metamerism so characteristic of 

 the arachnids, and that afford us the required data for filling in, and explaining, 

 the enormous gap between the unspecialized metameres of an annelid and the 

 groups of highly specialized metameres in the head of a vertebrate. The annelid 



