CHAPTER XXIII 

 PHYSICAL DETERIORATION AND MICROBIC DISEASE 



Summary The factors in development Methods of improving development 

 Human selective breeding The effects of diet Of exercise Of mental training 

 Physical deterioration The opinions of biometricians Of medical men The 

 right method of improving the physique of urban populations Microbic diseases 

 External and internal sanitation Contagious, water-borne, insect-borne, earth- 

 borne, and air-borne diseases. 



o 



717. /^^\UR study of heredity from the purely scientific stand- 

 point is ended. We have reached certain conclusions, 

 certain broad principles, ' laws,' or, ' brief, simple, and 

 comprehensive classification of facts,' from which necessary con- 

 sequences may be deduced. 1 No characters are really hereditary; 

 or, if the term be admitted, all characters are equally hereditary. 

 Characters differ, not because some are inborn and inheritable while 

 others are acquired and not inheritable, but because they arise 

 under different classes of stimuli. Evolution is only another name 

 for adaptation, and in the last analysis all adaptation results from 

 the Natural Selection of favourable variations. A variation is due 

 to an alteration in the germ-plasm. Variations are either ' spon- 

 taneous' or due to the direct action of the environment on the 

 germ-plasm. The latter class of variations are relatively rare, and 

 can very seldom, if ever, be other than unfavourable. Therefore 

 Natural Selection builds solely, or almost solely, on spontaneous 

 variations, and a main part of its work is, on the one hand, to 

 provide for the occurrence of spontaneous variations and regulate 

 their magnitude, and, on the other, to render the germ-plasm in- 

 susceptible to the direct action of the environment. It follows 

 that the variability of species and characters is not a fixed quantity ; 

 on the contrary, each species and each of its characters tends to be 

 variable, on the average, in a degree best suited to secure adapta- 

 tion. In other words, the tendency to vary, displayed by all 

 living beings, is itself an adaptation which is subject to variations, 

 by means of which Natural Selection regulates the amount of 

 variability and provides for the occurrence of variations all round 

 1 See 76 (footnote), 590, 819 et seq. 



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