NATURE OF THE EVIDENCE. 3 



and because the histological structure of their muscles, nerves, sense organs, 

 cartilages, etc., closely resembles that of the vertebrates. This view was, there- 

 fore, the first to be entertained by the older anatomists (Leydig and Dohrn); 

 but in more recent years it has not been regarded with favor. 



So far as I have been able to determine, most zoologists of to-day, who make 

 any attempt to justify their deep rooted prejudice against the arthropod theory, 

 base their objections on the a priori ground that the arthropods, being highly 

 specialized animals, cannot have given rise to the vertebrates, because the verte- 

 brates must have come from some generalized type. This objection clearly has 

 but little weight, for the general application of such a law would exclude the 

 possibility of any evolution. Every animal is a specialized one when compared 

 with its ancestors, and at the same time a generalized one when compared with 

 its descendants. Even the most primitive vertebrate is a highly specialized 

 animal, and its immediate ancestors were also highly specialized. It is clear, 

 therefore, that in order to solve our problem we must discover not some gener- 

 alized ancestor but a specialized one, and the only evidence of value in deter- 

 mining whether we have found the right one or not, is the degree to which its 

 particular kind of specialization agrees with that of a vertebrate. 



II. NATURE OF THE EVIDENCE TO BE PRESENTED. 



Our problem then is a perfectly simple one in principle, although it is one 

 that involves an enormous amount of detail in its application. We have merely 

 to strip off the superficial disguise of our hypothetical arachnid ancestors and see 

 whether either their underlying structure, their mode of growth, the general 

 direction and historic sequence of their evolution, does or does not harmonize 

 with the assumption that they are the ancestors of the vertebrates. We venture 

 to state at the outset, that in our judgment they do harmonize with this assump- 

 tion, and so fully and in such detail as to leave no other conclusion open than that 

 the vertebrates arose from arachnid-like arthropods. 



A. Cephalogenesis in Arthropods. We shall show, with the aid of com- 

 parative anatomy, that the process of cephalizing the anterior part of the body, 

 that is, the transformation of a large number of independent metameres into a 

 compact, organized group of unlike structures that may be called a "head," is 

 the dominant process in the evolution of arthropods, and that this process has 

 already definitely established in the higher forms the more characteristic features 

 of the vertebrate head. The process is initiated in primitive arthropods either 

 by the division of the anterior part of the body into regions, or by the addition 

 from time to time of distinct groups of like metameres, or tagmata. The succes- 

 sive appearance of new groups of metameres at the tail end of the body marks 

 .distinct epochs in the evolution of the arthropods, and they constitute the under- 

 lying basis for the characteristic subdivision of the body into pre-oral, oral, tho- 

 racic, vagus, abdominal, and caudal regions. We shall call them the procephalon, 



