December 2, 1921] 



SCIENCE 



533 



endocrinal glands, the action of which has 

 heen so successfully studied, among others, 

 by Sir Sharpey Schaffer in this University. 

 Thus, as is well shown in man, the higher ani- 

 mals acquire considerable independence, and 

 are little affected in their development by mi- 

 nor changes of environment. Inheritance is 

 thus made secure by ensuring that the neces- 

 sary conditions are always present. 



We may seem to have wandered far from 

 our original question; but the answer now ap- 

 pears to be that only those characters can be 

 regularly inherited which depend for their 

 appearance on conditions always fulfilled in 

 the normal environment (external or inter- 

 nal) ; and those characters will not be regu- 

 larly inherited which depend on stimuli that 

 may or may not be present. Thus, while the 

 offspring of a dark-skinned race will be dark 

 in whatever climate they are born, those of a 

 fair-skinned race will be born fair, but may be 

 darkened by sun-burn, if they spend their 

 holiday in the open. 



ISTow it will be said, and not without some 

 truth, that all this is mere commonplace ad- 

 mitted by all; but, if so, it is, I think, often 

 ignored or misunderstood in discussions on 

 heredity, more especially in semi-popular writ- 

 ings, and sometimes even in scientific works. 

 However, I quite willingly admit that the 

 real problems Darwin left to be solved by the 

 evolutionist are the nature of the germinal 

 factors themselves, and more especially the 

 origin of the differences between them, the 

 origin of those changes which give rise to 

 mutations. 



That these factors^ must at least be self- 



7 Herbert Spencer 's ' ' physiological units, ' ' Dar- 

 win 's " pangens, " Weismann's " determinants," 

 are all terms denoting faotors, but with somewhat 

 different meanings. More recently Professor W. 

 Johannsen (" Elemente der exakten Erblichkeits- 

 lehre, " 1909) has proposed the term " gene " for 

 a factor, " genotype " for the whole assemblage 

 of factors transmitted by a species, and " pheno- 

 type " for the characters developed from them. 

 This clear system of nomenclature, although much 

 used in America, has not been generally adopted 

 in this country. 



propagating substances, subsidiary vortices in 

 the main stream of metabolizing living proto- 

 plasm, is certain, since they grow and multiply 

 repeatedly, to be distributed to new generations 

 of germ-cells. That they may be relatively 

 constant and remain unaltered for generations 

 seems also certain, since organisms or their 

 parts can continue almost unchanged for un- 

 told ages. That they can act independently, 

 can be separately distributed into different 

 germ-cells, and can be re-combined seems like- 

 wise to have been proved by the brilliant work 

 of Mendel and his followers. So independent 

 and constant do they appear to be that mod- 

 ern students of heredity tend to treat them 

 as so many beads in a row, as separate par- 

 ticles themselves endowed with all the proper- 

 ties of independent living organisms, the very 

 properties we wish to explain. While not pre- 

 pared to accept these views without qualifica- 

 tion, it seems to me that it can scarcely be 

 doubted that some such units must exist 

 whether in the form of discrete particles or 

 merely of separable substances. But not until 

 these factors have been brought into relation 

 with the general metabolism of the organism, 

 as links in the chain of processes, will the 

 problem of inheritance approach solution. If 

 the theory is to be completed it must at- 

 tempt to explain how they come to differ, how 

 their orderly behavior is regulated, in what 

 functional relation they stand to each other, 

 what is the metabolic bond between them. 

 That harmonious processes may be carried out 

 by discrete elements in cooperation is shown 

 in cases of symbiotic combinations such as 

 the lichens, or the green algse in such ani- 

 mals as Hydra and Convoluta. Here an 

 originally independent organism takes its 

 place and does its work regularly in another 

 organism, and may even be propagated and 

 transmitted from one generation to the next 

 in the germ-cell! Most instructive, also, are 

 the recently studied cases of bacteria and 

 yeasts living regularly in certain special tis- 

 sues of various species of insects, where they 

 exert a definite influence on the metabolism 

 (see the works of Pierantoni, Buchner, Gla- 



