ZO©LOGY AND BOTANY, MICROSCOPY, ETC. 185 



C. General. 



Myelin and Advancing Age." — Henry H. Donaldson has inquired 

 into the fact that, starting from birth, the water-content of the mam- 

 raalian body as a whole, and in certain systems, diminishes with age. In 

 the albino rat he finds that there is a progressive loss in the percentage 

 of water in the brain and in the spinal cord. This proceeds thirty times 

 faster than in man. Donaldson's observations show further that the 

 diminution is not in the cell-bodies and their unsheathed axons, but is 

 mainly due to the accumulation of myelin — with a water-content of 

 about 60 p.c. The myelin must be regarded as a more or less extraneous 

 substance, having but little significance for the characteristic activities 

 of the neurons. Myelin formation is a function of age. 



Theory of Nerve Conduction.t — A. G. Mayer has been led by experi- 

 ments on the jelly-fish, Cassiopea xamacliana, to the theory that 

 adsorption may play a fundamental role in nerve-conduction, and that 

 the only cations which are necessary to the reaction are the adsorbed 

 sodium, calcium, and potassium ions, the rate of nerve-conduction being 

 proportional to the concentration of these adsorbed ions. There is a 

 change in the rate of nerve-conduction in Cassiopea in successive dilutions 

 of sea-water. Nerve-conduction is probably a phenomenon of adsorption 

 combined with an ordinary chemical reaction. 



Origin of Air-breathing Vertebrates. % — Joseph Barrell accepts 

 Chamberlin's theory that fishes arose in land-waters, and constituted 

 primarily a river fauna. The lung-fishes arose under semi-arid climates 

 and in seasonal waters. The evidence is strong that the air-bladder was 

 originally a supplemental breathing-organ, although in modern fishes it 

 has been mostly diverted to other uses. "Among certain Devonian 

 fishes, living under more and more strenuous climatic conditions of 

 seasonal dryness, the use of the air-bladder for respiration became essential, 

 and with the diminishing availability of the waters of certain regions 

 the gills in those species became correspondingly atrophied. The 

 amphibians thus arose under the compulsion of seasonal dryness." 

 " Climatic oscillation is a major ulterior factor in evolution." 



Rats and Evolution.§— A. G. and A. L. Hagedoorn, dealing 

 particularly with rats, define a species as " a group of individuals which 

 is so constituted genotypically, and which is so situated, that it auto- 

 matically tends to restrict its total potential variabiUty." The " total 

 potential variability" is "the quantity of genes which not all the 

 members of a group have in common, or for which they are not pure 

 (homozygous), and the variabihty which this impurity makes possible in 



* Proc. Nat. Acad. Sci. U.S.A., ii. (1916) pp. 350-6. 

 t Proc. Nat. Acad. Sci. U.S.A., ii. (1916) pp. 37-42. 



X Bull. Geol. Soc. America, xxvii. (1916) pp. 345-436. See also Proc. Nat Acad 

 Sci. U.S.A., ii. (1916) pp. 499-504. 



§ Amer. Nat., Ii. (1917) pp. 385-418. 







