The tracing of apparently widely divergent structures to a common origin has engaged the attention of many of our investigators. Not only has a large amount of evidence been offered to show a common origin of widely-separated structures, but memoirs of a speculative and theoretical character 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[1] 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[2] 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 larvæ, "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[3] has studied the law of nuclear displacement and its significance in embryology. In a discussion of this subject he says: "The mode of evolution of the yolk is of great interest, and doubtless occurred through the working of natural selection. It is evidently adaptive in character, and the necessity for its presence as an appendage of the egg grew out of the exigencies of the struggle for existence."
Mr. H. W. Conn,[4] in a paper entitled "Evolution of the Decapod Zoæ," gives a number of striking and suggestive facts explaining the reason of the multiform and diverse character of the larvæ of decapod crustaceans. He shows in what way 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,[5] on the significance of the "Larval Skin of Decapods," a very complete discussion of the views of authors is given. At the outset he shows that the crustaceans are a particularly favorable group for the study of phylogeny, and then suggests the character of the ancestral form of the Crustacea from the significance of the larval envelope. The author infers, from his studies, that "all decapods are to be referred back to a form similar to the Protozoæ (Zoæ), in which the segments of the thorax, and probably of the abdomen, were present, and whose antennae were locomotive organs."
Not the slightest justice can be done this admirable discussion in the
