li>i SUMMARY OF CURRENT RESEARCHES RELATING TO 



ally, persist, but have more or less lost their original function (78 

 cases !) ; (1) changes in which there has been change of function (G cases) ; 

 (5) changes associated with alterations in position (18 cases). 



The author then sums up the theoretical conclusions of his survey, 

 and occupying a position similar to that of his colleague Weismann, 

 emphasizes the importance of natural selection in maintaining structures 

 once established. As he says, natural selection has two sides — a positive 

 side establishing adaptations, a negative side allowing the latter to de- 

 generate when no longer essential. " As soon as changes in external 

 conditions exclude an organ from importance in the struggle, that organ 

 retrogrades. Panmixia, or general crossing, occurs between individuals, 

 some with the organ in question well developed, and others with it less 

 perfect ; the result is, a slow but constant degeneration of the organ." 

 The author's general conclusions as to the past and future of man are 

 very vivid, and backed as they arc by such an array of anatomical facts, 

 most valuable and suggestive to the general naturalist, as well as to the 

 anatomical expert. 



Degeneration.* — Prof. A. Weismann gives a vivid account of degene- 

 rative or retrogressive changes in animal organisms. He discusses the 

 wings of running birds, the blindness of cave animals, the rudimentary 

 olfactory organs of cetacea, the retrogression of parasites, the loss of 

 hair in some mammals, the sexual condition of worker ants, and many 

 other familiar illustrations. The point of the whole discussion is to 

 show that degenerations are not to be explained on Buffonian lines as 

 due to direct influence or absence of influence from environmental con- 

 ditions, nor on Lamarckian lines as due to the effect of disuse ; but on 

 Darwinian lines, by the action of natural selection, which is as necessary 

 to sustain as it is to establish adaptations. The process by which a 

 superfluous organ degenerates may be described as " panmixia," or 

 " general crossing," in which not those individuals alone reproduce 

 which possess the organ perfectly, but all, whether they have it developed 

 in greater or less perfection. " Nature endures no luxury, no impulse 

 or organ of the body has permanence, if it be not thoroughly necessary 

 for the preservation of the species. Panmixia, or, if you will, the (non-) 

 operation of natural selection, will secure that all the superfluous be 

 gradually reduced to the absolutely necessary." 



B. Histology.f 



Histological Elements of the Central Nervous System.} — Mr. F. 

 Nansen commences this important essay by a very full historical account 

 of the work of his predecessors. For the study of the structure of the 

 nerve-tubes in invertebrates he has made use of the lobster, Neplirops 

 norvegicus, various species of Nereis and other Polychaetes, Lumbricus 

 agricola, Patella vulgata, Phallusia venosa, and other Ascidians ; he 

 thinks that the general conclusions to which they have led him may be 

 applied to all bilaterally symmetrical invertebrates. In these the nerve- 

 tubes consist of an external consistent sheath with viscous contents ; the 

 sheaths are formed by, or belong to the connective substance extending 

 through the whole nervous system, which the author calls neuroglia, 



* Ber. Naturf. Gesell. Freiburg i. B., ii. (1886) p. 30. 



+ This section is limited to papers relating to Cells and Fibres. 



J Bergens Museum Aarsberetuing for 1886 (1887) pp. 29-215 (11 pis.)- 



