tHE RISE OF LAND LIFE 37 



The feathers of birds are modified scales; the hairs of 

 mammals originated as sensory organs, somewhat Hke the 

 " whiskers " of a cat.* There were perhaps three of them 

 protruding from beneath each scale. These sensory hairs in- 

 creased in importance, perhaps under the stimulus of a cool- 

 ing climate, multiplied and became a coat, while the scales 

 gradually disappeared. The original use of these hairs is 

 hard to determine. They may have been of use in the crannies 

 and dark corners whither the mammals took refuge when 

 hunted by the reptiles. 



The next group of mammals after the egg-laying mono- 

 tremes, like platypus, was that of the marsupials. Here the 

 female carries the eggs and embryos in the uterus- without any 

 placenta or organic connection between mother and young. 

 This embryonic and foetal period is very brief.. The young are 

 born in an immature condition and are carried and suckled 

 in a marsupium or pouch until able to care for them- 

 selves. 



Placental mammals seem to have quickly outgrown the mar- 

 supial stage. In them a foetal membrane has put out pro- 

 jections which interlock with similar projections from the 

 lining of the uterus of the mother forming the placenta. Hence 

 oxygen and nutriment can diffuse through the thin membranes 

 separating the blood-vessels of the mother from those of the 

 young. Its foetal stage can now be* greatly lengthened and 

 birth delayed. 



The Eocene, the earliest period of Cenozoic or Tertiary 

 time, shows us placental mammals already distributed in their 

 chief orders. It appears to have been preceded by a colder 

 epoch at the close of Mesozoic, and this may have contributed ^^ 

 to the downfall of reptiles whose most highly developed forms ^ 

 have now disappeared. From this time on placental mam- 

 mals multiply and become more diversified. Insectivorous 

 forms lead over to true carnivora; rodents, represented to-day 

 by rabbits, squirrels and mice, have appeared. We find un- 

 gulates, like cattle, deer and horses, with hoofs, and with 



*V. 85 



