Geology. 591 



sis * * * because we find no traces in the annelids of those illumi- 

 nating modifications of metamerism so characteristic of the arach- 

 nids, and that afford us the required data for filling in, and 

 explaining, the enormous gap between the unspecialized meta- 

 meres of an annelid and the groups of highly specialized meta- 

 meres in the head of a vertebrate." 



"The tunicate, echinoderm, balanoglossus, amphioxus, etc., 

 theories have similar inherent weaknesses, indicating that they 

 must be subordinated to some larger view." These groups are 

 therefore considered but degenerate offshoots of a common 

 arthropod-vertebrate stock. One naturally looks on the arthro- 

 pods as the probable ancestors of the vertebrates, because they 

 are the most highly organized of segmented invertebrates and 

 because the histological structure of their muscles, nerves, sense 

 organs, cartilages, etc., closely resembles that of the vertebrates. 



The problem Patten considers a perfectly simple one in princi- 

 ple although involving an enormous amount of detail in its appli- 

 cation. " We have merely to strip off the superficial disguise of 

 our hypothetical arachnid ancestors," he says, " 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 ances- 

 tors of the vertebrates. We venture to state at the outset, that 

 in our judgment they do harmonize with this assumption, and so 

 fully and in such detail as to leave no other conclusion open than 

 that the vertebrates arose from arachnid-like arthropods." 



The nature of the evidence presented includes : A, cephalogene- 

 sis in arthropods ; B, embryology ; C, arachnid cephalogenesis 

 prophetic of the vertebrate head ; D, paleontology ; and the 

 range in its complexity can readily be understood by the follow- 

 ing chapter headings : Outline of the arachnid theory ; Evolution 

 of the nervous system in segmented animals ; Subdivisions of the 

 brain ; Minute structure of brain and cord of arachnids ; Periphe- 

 ral nerves and ganglia ;«. General and special cutaneous sense 

 organs ; Larval ocelli and the parietal eye : The compound eyes 

 of arthropods and the lateral eyes of vertebrates ; The olfactory 

 organs of arthropods and vertebrates ; The functions of the 

 brain ; The heart ; Early stages of arthropod and vertebrate 

 embryos ; The old mouth and the new, locomotor and respira- 

 tory appendages ; Variation and monstrosities ; The dermal skele- 

 ton ; Endocranium, branchial and neural cartilages ; The middle 

 cord, the lemmatochord and the notochord ; Ostracoderms and 

 the marine arachnids ; The ostracoderms ; The vertebrates. 



Part II discusses the zoological position of the Acraniata, 

 including a contrast of craniates and acraniates ; the Cirripedes, 

 tunicates and echinoderms ; the Enteropneusta, Pterobranchia, 

 Polyzoa, Brachiopoda, Phoronida and Chaetognatha ; summary 

 and conclusion. 



The illustrations are beautifully done, being based upon mar- 

 vellously detailed preparations as well as upon numerous models 

 representing transitional stages and upon various specimens and 



