440 PHYSICAL DETERIORATION & MICROBIC DISEASE 



beings because, owing to more intimate knowledge, we are better 

 able to analyse our own characters than those of other types. As 

 far as I am able to judge, the apparently fundamental divergencies 

 of opinion which separate various classes of scientific workers 

 (biometricians, Mendelians, and those who endeavour to take other 

 classes of facts into account), social reformers, educationalists, 

 and the like, are due mainly to this neglect. Every section 

 appears to the others to be building on a basis of unproved 

 assumption. A common platform is lacking. In attempting the 

 task of analysis it is only too probable that like most other pioneers 

 I shall often be in error. Nevertheless, however wrong I may be 

 in the conclusions I reach, yet at least the reader will be placed in 

 possession of a good deal of material evidence which he will be 

 able to link up with that already set forth in this volume, and apply 

 to the problem as to whether it is possible to obtain improvement 

 in this or that character (e.g. intelligence) by altering the conditions 

 under which individuals develop, or whether improvement is 

 possible only through selective breeding. It should be borne in 

 mind constantly that selection alters only capacity for develop- 

 ment, whereas improved surroundings alter only the stimuli which 

 awaken capacity. Apart from selection, then, improved human 

 development may be achieved by altering the supply of nutriment, 

 by removing causes of injury, and by altering the training which 

 the body and mind receive. 



722. All human diets contain certain necessary constituents, 

 water, proteids, carbohydrates, various salts, and the like. Almost 

 all diets, whether vegetable, animal, or mixed, contain in excess 

 one or more of these constituents, and a deficiency of others. If in 

 any diet the quantity of a necessary constituent is deficient, the 

 instinct of the individual prompts him to eat till the deficiency is 

 supplied, the excess of the other constituents being wasted, or 

 within limits which vary with the individual, stored in the tissues. 1 

 For this reason a mixed or varied diet is usually the most 

 economical, though the variations of the individual and the kind 

 of life he leads must be taken into account. We tend instinctively 

 to grow tired of a diet which contains this or that constituent in 

 excess, and to turn with avidity to one which has less of it, but 

 more of a constituent in which the first was deficient. Under 

 1 This power of storing excess of nutriment in the tissues, especially as fat 

 against a period of scarcity is, speaking relatively, of little use to the average 

 civilized man whose supply of food is regular and secure ; but it must have been 

 very useful to his remote ancestors, as it now is to many lower animals for ex- 

 ample, bears during hybernation or insects when undergoing metamorphosis. 



