ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 301 



central type, in which he assumes opposability of the first digit in both 

 maims and pes, and an arboreal habit. The earliest divergence from the 

 type is placed in the middle or upper Cretaceous. The various modern 

 arboreal groups (monkeys, squirrels, arboreal insectivores, opossums) are 

 the least altered in structure, while the amount of structural change in 

 other groups, as shown by their known palaeontology, is proportioned to 

 the change in their mode of life, the Ungulate exhibiting the greatest 

 changes. 



Orkney Vole.* — C. J. Forsyth Major points out that Microtus 

 orcadensis discovered by Mr. Millais in the Orkneys and certain parts of 

 Shetland most closely approaches 31. arvalis, one of the field-voles of 

 Continental Europe and Northern Asia. It certainly belongs to the 

 31. arvalis group, and that group (if not the species 31. arvalis) was 

 represented in Britain during Pleistocene times. This last circumstance 

 will in due time presumably help to explain satisfactorily the present 

 existence of a member of the 31. arvalis group in the Orkney and 

 Shetland Islands. 



Cave Faunas. f — Armand Vire has made a special study of this 

 subject, and finds, amongst other things, that the fauna of caves is 

 essentially a mixture of two types. One is manifestly representative of 

 the existing fauna outside ; the other has no relations in existing fresh 

 waters, but seems to have arisen from species now everywhere else 

 extinct. 



Tube Plan of Structure of the Animal Body.J — J. S. Foote, in an 

 extremely interesting paper, demonstrates the tube character of most of 

 the organs of the body, classifying them as four-, three-, two-, single- 

 coated, and one-layered tubes. Four-coated tubes, e.g. alimentary tract, 

 are adapted to the progressive motion of their contents and to the 

 application of their epithelial structures to the contents. Three-coated 

 tubes are adapted to the progressive motion of their contents when 

 necessary, e.g. fallopian tube. "Two-coated are adapted to conditions re- 

 quiring an open tube, as trachea and large bronchi. Single-coated 

 tubes are adapted to functions of secretion and special sense, while one- 

 layered tubes are adapted to osmotic conditions. By a simple method of 

 construction explained in the paper, different organs can be built up and 

 their nature and functions deduced from the various layers and 

 combinations employed. 



The Thermocline and its Biological Significance^— E. A. Birge 

 reviews a number of facts bearing on the biological significance of the 

 thermocline or Sprung schicht. This is defined as the comparatively thin 

 stratum in the water of a lake, situated below the surface, in which the 

 temperature falls rapidly— much more rapidly than in strata of similar 

 thickness above or below it. It is to be fairly inferred that the thermo- 

 cline constitutes a critical point in the distribution of the plankton in 

 the water below the surface. No single factor within the water itself 



• Ann. Nat. Hist., xv. (1905) pp. 323-4. 



t Comptes Rendus, cxxxix. (1904) pp. 992-5. 



\ Trans. Amer. Micr. Soc, xxv. (1904) pp. 65-86 (6 pis.). 



§ Tom. cit., pp. 5-33. 



