2 OUTLINE OF THE ARACHNID THEORY. 



theory, therefore, in the form in which it is generally understood, could be in- 

 corporated into the arthropod theory, but it is evident that the conditions could 

 not be reversed, for no resemblance of annelids to vertebrates could either elimi- 

 nate, or account for, the resemblance of arthropods to vertebrates. 



The tunicate, echinoderm, balanoglossus, amphioxus, etc., theories have 

 similar inherent weaknesses, indicating that they must be subordinated to 

 some larger view. The bafHing resemblances between the embryonic stages of 

 these forms and vertebrates do not help us to explain vertebrate cephalogenesis, 

 or to account for the origin of the most characteristic vertebrate structures; and 

 so long as their own origin is unknown, and they have no fixed location in a 

 general system of classification, they can throw no light on the origin of verte- 

 brates, or on the still broader problems of the origin and inter-relations of the 

 other great subdivisions of the animal kingdom. 



All this is changed, however, as soon as we recognize that the echinoderms 

 tunicates, balanoglossus, and cephalodiscus are degenerate offshoots of a common 

 arthropod-vertebrate stock. In the light of this interpretation, the arachnid 

 theory not only recognizes and explains the resemblance of the echinoderms, 

 tunicates, and other acraniates to the vertebrates, but it fixes approximately 

 their position in the animal kingdom, and elucidates the salient features of their 

 morphology. It supplies, in the evolution of the arthropod cephalothorax, the 

 key to the analysis of the vertebrate head. It unites the apex of the arthropod 

 stock with the base of the vertebrate stock, and welds the entire series of seg- 

 mented animals into one homogeneous group. It shows that the great verte- 

 brate-ostracoderm-arthropod phylum forms the main trunk of the genealogical 

 tree of the animal kingdom; that, emerging from unsegmented, coelenterate-like 

 animals, as though driven by some mysterious internal power, moves with aston- 

 ishing precision, through broad, predetermined channels — from which neither 

 habit, nor environment, nor heredity, can cause it to diverge — toward its goal. 

 And finally it lays before us in their historic order the critical events of these age- 

 long periods, the succession of structural and functional changes that have fol- 

 lowed them, and that have in turn given rise to still other changes of form and 

 new conditions of growth. It thus reveals to us, as only the true science of mor- 

 phology can reveal, the important agents that have directed the course of evolution, 

 and that have determined the organic forms, or shapes, in which it is expressed. 



The arachnid theory thus not only unites and harmonizes these apparently 

 conflicting views as no other interpretation can, but it will, in my judgment, go 

 a long way toward restoring morphology to its former dominant position as the 

 expounder and prophet of the biological sciences. Morphology reduced to a 

 barnyard science, without its vast resources in comparative anatomy, its per- 

 spective in geological time, and its world-wide laboratory of Nature, is robbed of 

 its chief glory and power. 



One naturally looks on the arthropods as the probable ancestors of the 

 vertebrates, because they are the most highly organized of segmented invertebrates 



