438 PHYSICAL DETERIORATION & MICROBIC DISEASE 



The present progressive evolution of man, at any rate of 

 civilized man, is chiefly, if not exclusively, against disease which 

 is, apparently, the only selective agency acting on him sufficiently 

 stringent to do more than merely maintain characters previously 

 evolved. Human beings differ from lower animals mainly in that 

 they owe their development, especially their mental development, 

 to a much greater extent to the stimulus of use and experience. 



718. The evidence on which these conclusions are based is very 

 massive and appears to me conclusive. But the reader is now in a 

 position to judge for himself. Some evidence remains for con- 

 sideration, but it can be dealt with conveniently as we apply our 

 'laws 1 to the practical problems of human life. The following 

 are the factors in all development: (i) capacity for growth in 

 directions more or less fixed in every species, but so differing with 

 different species that the latter differ in their characteristics ; (2) 

 stimulus which awakens the capacity; and (3) nutriment which 

 supplies the material for all growth (as well as the stimulus for 

 much of it). Capacity for growing physically and mentally, for 

 responding in such and such a way, in such and such a degree, to 

 such and such a stimulus, depends on the antecedent evolution of 

 the race and the variations of the individual. A variation, indeed, 

 is nothing other than an alteration of capacity to grow, an altera- 

 tion which is founded on an alteration of the germ-plasm. Capacity 

 arises in the race through slow processes of selection ; and can be 

 altered in it only through selection or cessation of selection. It is 

 that with which every individual begins life, and is wholly an 'inborn ' 

 or nutritional character which cannot be increased in the individual 

 by use though it may be diminished or lost in him through injury. 1 

 It must be borne in mind, however, that the effect produced by 

 such alteration of stimulus is limited by the capacity of the indi- 

 vidual ; thus, no matter how a given man be fed or trained, he cannot 

 achieve more than a certain degree of height, strength, or capacity. 



719. If, then, we desire to improve a human race, two ways of 

 attaining our aim are conceivable, (i) We may follow the plan 

 of Nature and of plant and animal breeders and alter by selection 

 the racial CAPACITY for growth in this or that direction ; or (2), by 

 altering the conditions under which the individuals of the race 



1 If anything may be rightly described as innate it is capacity. In a sense all 

 capacities for growth lie latent in the fertilized ovum, and, with variations, are 

 passed on to the descendent germ-cells. Thus mathematical knowledge is a pure 

 ' acquirement ' in every sense of the term ; but that which enables it to arise under 

 the stimulus of experience has its roots in the ovum. 



