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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 efficient swimmer and diver, has lost any possibility of ever being able to run or to fly; the horse, by gaining its efficient running 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