Variation Due to Change in the Individual Gene

THE present paper will be concerned rather with problems, and the possible means of attacking them, than with the details of cases and data. The opening up of these new problems is dne to the fundamental conitribution which genetics has made to cell physiology witlhin the last decade. This contribution, wliclh has! so far scarcely been assimilated by the general physiologists theemselves, consists in tlhe demonstration that, besides the ordinary proteins, carbohydrates, lipoids, and extractives;, of their several types, there are present within the cell thotusands of distinct substances--the " genes "; these genes exist a.s ultra-microscopic particles; their influences nevertlheless perme-ate the entire cell, and th-ey play a fundamental role in determining the nature of all cell substa.nces, cell structures, and cell activities. Through these cell effects, in turn, the genes affect the entire organism. It is not mere guesswork to say that the genes' are ultra-microscopic bodies. For thie work on D-osophila has not only proved that the genes are in the chromosomes, in definite positions, but it has shown that there mnst be, hundreds of such genes within ea,ch of the la.rger chromosomues, although the length of tlhese clhromosomes is not over a, few microns. If, then, we divide the size of the chromnosome by the minimum number of its genes, we find that the latter are particles too small to give a visible image. The chemical composition of the genes, and the formulm of their reactiois, remilaimi a.s vet quite unknown. We do know, for example, tlhat in certain ca.ses a given