AMERICAN ZOOLOGISTS AND EVOLUTION. 495 



The tracing of apparently widely divergent structures to a com- 

 mon origin has engaged the attention of many of our investigators. 

 Not only has a large amount of evidence been offered to show a com- 

 mon origin of widely-separated structures, but memoirs of a specula- 

 tive and theoretical chai-acter have given us a possible clew to the 

 avenues we may follow in further establishing a proof of the unity of 

 origin of forms and parts. 



Dr. Francis Dercum* gives an interesting review of the structure 

 of the sensory organs, and urges that the evidence goes to prove the 

 common genesis of these organs. 



Professor A. Hyatt f has presented an interesting study of the 

 larval history of the origin of tissue. He attempts to show a phyletic 

 connection between the Protozoa and Metazoa, and also to show that 

 the tissue-cells of the latter are similar to a sexual larvse, " and are 

 related by their modes of development to the Protozoa, just as larval 

 forms among the Metazoa themselves are related to the ancestral 

 adults of the different groups to which they belong." Dr. John A. 

 Ryder J has studied the law of nuclear displacement and its signifi- 

 cance in embryology. In a discussion of this subject he says : "The 

 mode of evolution of the yolk is of great interest, and doubtless oc- 

 curred through the working of natural selection. It is evidently 

 adaptive in character, and the necessity for its presence as an append- 

 age of the egg grew out of the exigencies of the struggle for exist- 

 ence." 



Mr. H. "NY. Conn,* in a paper entitled " Evolution of the Decapod 

 Zoae," gives a number of striking and suggestive facts explaining the 

 reason of the multiform and diverse character of the larvae of decapod 

 crustaceans. He shows in what Avay natural selection has affected the 

 young. What has seemed an almost insoluble mystery, as to why the 

 early stages of closely allied crustaceans should be so often diverse in 

 their varied armature of long spines, their powers of rapid flight, etc., 

 are explained on the ground of natural selection. In another memoir 

 by the same author, || on the significance of the " Larval Skin of Deca- 

 pods," a very complete discussion of the views of authors is given. 

 At the outset he shows that the crustaceans are a particularly favor- 

 able group for the study of phylogeny, and then suggests the charac- 

 ter of the ancestral form of the Crustacea from the significance of the 

 larval envelope. The author infers, from his studies, that " all deca- 

 pods are to be referred back to a form similar to the Proiozore (Zow), 

 in which the segments of the thorax, and probably of the abdomen, 

 were present, and whose antennje were locomotive organs." 



Not the slightest justice can be done this admirable discussion in the 



* "American Naturalist," vol. z'u, p. STO. 



\ " Proceedings of the British Society of Natural History," vol. xxiii, p. 45. 



X " Science," vol. i, p. 273. * Ibid., vol. iii, p. 513. 



|] " Studies from Biological Laboratory," Johns Hopkins University, vol. iii, No. 1. 



