306 SUMMARY OF CUERENT RESEARCHES RELATING TO 



offspring to resemble the parent. It is obvious that in some wav this 

 force must be modified as time progresses, otherwise evolution could not 

 take place, and the manner and means of this modification is just what 

 we mean by the phrase "method of evolution." Darwin and Wallace 

 assumed that small variations are heritable, and farther that the force 

 which produced a deviation of heredity would continue to act in the 

 same direction in succeeding generations. " Pure line " investigations 

 are against the theory that progressive results can l)e attained by selec- 

 tion of these small quantitative variations. Thus emphasis has been 

 laid on the importance of sports or mutants. But mutations tend to be 

 of the nature of " cripples " — deviations from the normal which are 

 notoriously unlike the differentiating characters which distinguish allied 

 species from one another. If we have to reject small individual differ- 

 ences and larger occasional mutations as the raw material of evolution, 

 there remains only a third alternative— that evolutionary change is due 

 to the inheritance of the effects of use and disuse. If it be said that 

 the experimental evidence is against this alternative, there is the work 

 of Kammerer, which MacBride finds convincing, and there is transmission 

 of acquired characters in bacteria (where, however, there is no " body " 

 in the strict sense). But according to MacBride tbe distinction between 

 somatoplasm and germ-plasm is a "Weismaunian nightmare." The 

 inheritance of the effects of use and disuse is the method of evolution, 

 "the dominating influence which has moulded the animal world from 

 simple beginnings into the great fabric of varied life which we see 

 around us." J. A. T. 



Mutational and " Recapitulatory " Characters. — R. Ruggles 

 Gates {Rep. Brit. Assoc, 1919, 87, H40). A mutation is due to a 

 chemical or ^)hysical change in a chromosome of a germ-cell, and is 

 continued through the ontogeny by the equal splitting of the chromo- 

 somes in mitosis. A^'hat are called, not very happily, " recapitulatory " 

 characters " ari^e througli the impress of the environment, usually involve 

 adaptation to new conditions, are gradually developed, and in becoming 

 permanent involve the principal of inheritance of acquired characters. 

 In the lengthening out of a life-cycle by the addition of adaptive lai-val 

 stages there are good instances of recapitulatory characters. Such 

 characters could not have arisen through a mutation, for that would 

 modify every stage instead of adding certain stages as it does. Thus 

 both mutational and recapitulatory characters are necessary for the 

 phenomena of evolution. The one is nuclear in origin and centrifugal 

 in effect ; the other extrinsic in origin and ultimately centripetal in its 

 effect in the organism." (But the transmission of an exogenous somatic 

 modification as such or in any representative degree has not vet been 

 proved.) J. A. T. 



h. Histologry. 



Blood Platelets in Mammals. — A. Cesahis-Demel {Atii. Soc. 

 Toscaiia Sci. N((t., 1915, 30, lUO-lU, 2 pis.). Blood platelets are due 

 to megakaryocytes in the s})leen which peiutrate into the veins. They 

 may also arise from megakaryocytes in the marrow. The megakaryocytes 



