430 SUMMARY OF CURRENT RESEARCHES RELATING TO 



the central processes of the fibre-cells and their nuclei. At the level 

 of the deeper ends of the cylinder-cells the axis-cylinders divide. The 

 medullated fibres lose their medullary sheath and assume a horizontal 

 direction, soon dividing aud anastomosing with the neighbouring 

 bundles of fibres. There is thus formed, beneath the cylinder-cells, 

 a plexus of pale nerve-fibres. From this plexus arise fine bundles of 

 fibres, which trend towards the deejicr ends of the cylinder-cells, and 

 then course along their surfaces to the cuticular border, but do not 

 perforate the latter. 



Thus the fine fibres investing the cylinder-cells are to be inter- 

 preted as a terminal nervous apparatus, while the cells are mere sup- 

 ports for the nerves [?]. 



Fauna of the Austral Regions.* — M. Alph. Milne-Edwards 

 gives in this paper an instalment of a large work in the hands of the 

 French Academy of Sciences, on the present geographical distribution 

 of animals. He refers chiefly to the superior animals of the land and 

 sea, but takes account also of the lower forms, and while comparing 

 them with forms found elsewhere, seeks for physical or other circum- 

 stances capable of explaining their presence at, or absence from, this 

 or that part of the globe. 



He considers, from the facts of geographical zoology, that in most 

 cases the hypotheses which seek to explain the diversity of organisms 

 by the modifying influence of climate and other conditions, combined 

 with the struggle for existence and working through natural selection, 

 are insufiicient for the puri^ose. 



In putting forward some general views suggested by the study of 

 the subject, he takes occasion to define the terms species, variety, 

 and race. 



" The characters of a race" he says, " are decided by the result 

 of two forces whose directions may l)e either parallel and similar, 

 or diverging and even ojiposite. One of these forces is atavism, 

 or a tendency on the part of the being which is in course of 

 development to realize the structural forms of its ancestors, and to 

 become a faithful image of them. The other is the power of 

 biological adaptation, that is to say, the tendency of the living being 

 to become modified when the conditions to which it is subjected 

 happen to alter." He regards the fact of hereditary transmission of 

 certain powers and structural peculiarities as fully proved, and as 

 probably depending on the nature of the structure of the organs con- 

 cerned. Similarly, the descendants of common ancestors may vary 

 considerably with differences of climate, food, &c., and thus form races. 

 But the power of atavism is greater than that of these influences, and 

 it increases with the number of generations which follow each other 

 under similar external conditions. The power of adaptation does 

 not thus increase, and is very limited for single individuals ; but it 

 may, in conjunction with the working of atavism, when this coincides 

 with it, after many generations become cumulative and produce a 

 steadily increasing effect on the zoological characters of a species. 



♦ Ann. Sci. Nat. (Zool.), x. (1880) Article No. 9 (4 pis. und a map). 



