IMPROVING QUALITIES 501 



or with waiting for the very rare sport ; they have attempted to 

 bring about variations of a kind that were both useful and 

 permanent. But it is only since the beginning of the present 

 century that the biological principles necessary for controHing 

 plants and animals in this way have been known. These biolog- 

 ical principles have to do with heredity, or that relation between 

 parents and offspring which concerns their resemblances and 

 differences. Working on these principles, the breeders of plants 

 and animals are systematically improving domestic organisms 

 in a way that would have been considered little short of magic 

 only thirty years ago. 



IMPROVING QUALITIES IN PLANTS AND ANIMALS 



I. Variation in plants and animals 



How plants and animals differ within a species 

 Size 



Number of repeated parts (ribs on leaf ; florets on head ; sta- 

 mens ; segments in worm : hairs ; spines ; markings) 

 Dimensions of repeated part (hair ; cotton fibers ; horns ; 



leaves ; seeds ; eggs ; fingers ; vocal cords) 

 Physiological properties 



Pigmentation 



Proportion of sugar ; fat ; protein ; alkaloid 



Muscular capacity 



Speed of reaction 



Amount of milk ; amount of nicotin 



Resistance to particular poison 



Susceptibility to particular infection 



Educability in general ; special 

 Sources of variation 

 External conditions (resulting in modifications) 



Food and air 



Moisture 



Temperature 



Chemical (poisons, parasites, etc.) 



Fatigue 

 Internal conditions (derived from ancestry) 

 (Mutations) 



