Natural History 457 



in her health and enable her to make the most of her food. 

 Before the young one is born, chemical messengers have been 

 carried by the blood to the mother's mammary glands, so 

 that they are stimulated to begin the production of milk. 

 There is much of this physiological telegraphy in the business 

 of living. 



It is probable that the long-drawn-out ante-natal develop- 

 ment has greatly favoured the improvement of the brain. Thus 

 everyone knows how wide-awake a foal is after its long sleep 

 of eleven months within its mother's womb. But it must be 

 added that the structure of the brain in placental mammals 

 had got on to lines much more promising than in marsupials. 

 Granting this, we seem justified in saying that the prolonged 

 gestation, plainly adapted to the exigencies of terrestrial 

 life, opened up the possibility of being born with an advanced 

 brain equipment. In the same way, the prolonged infancy, 

 familiar in mankind, has its great rewards as well as its great 

 risks. 



It is interesting that mammals should bear a name that 

 emphasises the mother's breasts, and this strikes a true biological 

 note. For the success of mammals is wrapped up with their 

 maternal care, taken in conjunction with improved brains. To 

 the difficulties and limitations implied in the struggle for ex- 

 istence, some mammals have answered back by evolving teeth 

 and horns, others by evolving swiftness, others by evolving 

 armour, others by evolving wings but the answer back that is 

 common to them all is the maternal sacrifice and devotion. 



4 

 Many Habitats 



Like the reptiles before them, mammals have sought out 

 many habitats, and have become adapted to as many modes of 

 life. Perhaps it was in the trees that they served their apprentice- 

 ship; in any case they have tried all possible haunts, entering 



