PROGRESS SHOWN IN EVOLUTION 



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directions by a number of separate evolutionary lines. The 



improvement of each line means improvement of efficiency 

 at one particular job, such as improvement in one way of 

 getting a livelihood or in planting its offspring securely in the 

 world. For instance, the early mammals of the Secondary 

 period (Mesozoic age) were small land animals whose 

 fore and hind limbs were not very different from each other 

 — four or five toes on each foot, teeth of a primitive type like 

 those of a shrew or a hedgehog, and small brains. During 

 the Tertiary period these creatures gave rise to a whole set of 

 types. Some of these types, like the whale and the dolphin, 

 are adapted to life in water; others, like the horse and the 

 deer, are adapted to herbivorous diet and rapid running; the 

 higher carnivores, like the lion and the wolf, are adapted to 

 a flesh diet, which is captured by speed, strength, and skill; 

 the bat is adapted to a life in air; the elephant survives by 

 virtue of its huge size and formidable tusks; the mole lives 

 by its adaptation to a burrowing life below ground; the 

 sloth, by its adaptation to life in trees ; armadilloes survive by 

 their protective armour. 



Each of these creatures represents the finished product of 

 a line of improvement in one particular tool or method. On 

 the other hand, each individual improvement has been made 

 at the expense of other possible improvements. The original 

 type was primitive but plastic ; it was capable of being altered 

 in many possible ways. But the whale, in becoming an effi- 

 cient swimmer and diver, has lost any possibility of ever being 

 able to run or to fly; the horse, by gaining its efficient run- 

 ning organ in the shape of a long leg with but one toe, has 

 lost the possibility of acquiring a hand for grasping or a 

 clawed foot for catching prey; the elephant's bulk precludes 

 agility; the mole, though good at burrowing, is worse off as 

 regards the possibility of climbing trees. Such improvement 



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