272 



NA TURE 



[July 



1892 



ing types of life, since the most critical steps in evolution 

 have been taken on the land, and terrestrial fossils are of 

 the rarest occurrence. 



In illustration we may select the author's treatment of 

 the Arthropoda, which have their origin in some ancient 

 Annelid, probably a marine Polychaete, and not an 

 Oligochaete, since no Arthropod possesses the red blood 

 which the Oligochceta have acquired as an adaptation to 

 land life. 



The absence of cilia and a thoroughgoing chitinization, 

 which are the most striking peculiarity of the Arthropoda, 

 are a direct adaptation to land life ; the chitinous envelope 

 furnishing on the one hand protection against desiccation, 

 and on the other organs of support, whilst its extensive 

 development necessarily involves the disappearance of 

 cilia, and the development of fresh contrivances for 

 respiration. 



Another important character common to the Arthropoda 

 is the transverse striation of the muscle fibre ; but trans- 

 verse striation is generally admitted to be correlated with 

 excessive functional activity, from which, according to the 

 author, it results. Encase an animal in chitin, and its 

 movements will, from the mechanical conditions of the 

 case, be " acrobatic," — to move at all it must move 

 strenuously, by this excessive exercise transverse striation 

 will develope in all the voluntary muscles, and "by cor- 

 relation " in those of the alimentary canal as well. So 

 much is the author impressed by the cogency of this 

 reasoning that he regards the striation of the musculature 

 as a direct indication of the terrestrial origin of the 

 animal possessing it, and ventures to apply this formula 

 to Sagitta, the direct development of which he gives as an 

 additional argument for its descent from some terrestrial 

 species. 



The parapodia of the Annelida naturally gave rise to 

 the appendages of the Arthropods, and it was while these 

 were still short, scarcely-jointed stumps that the 

 Trilobites branched off in one direction, converting all 

 their parapodia into legs, and the Scorpions and 

 Merostomes, which discarded their abdominal append- 

 ages, in another. The Crustacea, retaining like the 

 Trilobites all their appendages, branched off at about the 

 same level, and their connection with the Arachnida is 

 confirmed by Jaworowski's recent observation of the 

 exopodital and endopodital splitting of the appendages 

 in Tarentula. A confirmation of the terrestrial habitat 



-Pedipalp of an smhvyo oi Trochosa singoriensis ; 

 ex, exopodite ; h, hairs (after Jaworowski). 



7, endopodite; 



of the primitive Crustacea is suggested by the fact that 

 the most archaic existing forms are the Branchiopoda, 

 which still live in fresh-water and salt marshes, can sur- 

 vive drying up, and indeed seem to require it for the pro- 

 duction of sexual eggs. The remarkable diversity of the 

 respiratory organs in the Crustacea is another important 

 piece of evidence, since it points to their having been 

 acquired as secondary adaptations. 



Of Arachnoid forms, some entered the sea, probably the 

 majority of the Merostomata and the Xiphosura, but 

 Limulus still gives evidence of its original home, since it 



NO. 1186, VOL. 46] 



comes to the shore for begetting, and lays its eggs at 

 the highest tide-mark. 



No doubt the notion that the immediate ancestors of 

 Limulus were land animals will excite scorn in prejudiced 

 minds ; but it is one that Balfour long ago suggested (the 

 author does not seem to be aware of this), led to it prob- 

 ably by his recognition of the close relationship between 

 Limulus and the Arachnoids on the one hand, and the 

 Arachnoids and Insects on the other — the latter connec- 

 tion lately so much strengthened by Jaworowski's remark- 

 able discovery of rudimentary antennas in Tarentula. In 



this direction may be looked for a reconciliation of the 

 views of Lankester and Lang. 



The mild surprise with which we learn that Trilobites 

 and Crustacea were originally denizens of the land has 

 scarcely given place to conviction before we encounter the 

 chapter on fishes. We shall be prepared to find that 

 these can claim terrestrial ancestry too. The earliest 

 fossil vertebrates of which we know anything are the 

 Placoderms ; these were dwellers in the Old Red Sand- 

 stone lakes, and, as our author remarks, " from fresh water 

 to the land is only a step." That the Placoderms were 



