76 PERIOD III. 



instinct by showing that natural selection can operate 

 on the subtlest modifications. It can discriminate 

 shades of hardiness to climate, shades of intellectual 

 acuteness, or shades of courage. It can intensify 

 qualities which appear only in adults past bearing or 

 in individuals congenitally incapable of propagation. 

 Human selection, though a blunt tool in comparison 

 with natural selection, can originate a bold and hardy 

 race of dogs, or showy double flowers incapable of pro- 

 ducing seed. In the second case fertile single flowers 

 continue the race, as in the garden Stock. Darwin 

 pointed out that the barren double flowers of the Stock 

 answer to the workers of social bees and ants, the 

 fertile single flowers to the functional males and females. 

 Every modification that works to the advantage or dis- 

 advantage of the race, whether we classify it as physical, 

 intellectual, or moral, gives scope for the operation of 

 natural selection. 



The comparative psychology of small invertebrates, 

 such as insects, is impeded by our imperfect knowledge 

 of their nervous physiology. Introspection is here 

 impossible; experimental physiology and pathology, 

 which have done so much for the psychology of the 

 higher vertebrates, almost impossible ; analogy is a 

 treacherous guide where the structures involved differ 

 conspicuously. We have little to guide us in the 

 psychology of insects except their behaviour, and that 

 is often capable of a variety of interpretations. The 

 only course is to adopt Pasteur's watchword, "Travail- 

 Ions ! " the difficulties will diminish with time and 

 labour. 



The Food of Green Plants. 



Common observation taught men in very early times 

 that green plants draw nourishment from the soil, and 



