8o8 LIFE : OUTLINES OF GENERAL BIOLOGY 



and perfectly to recuperate their wear and tear. They multiply so 

 inexpensively that there is no Nemesis on their reproduction. As 

 Weismann long ago pointed out, they enjoy a protoplasmic immor- 

 tality. Natural death was the price paid for having a body. 



PLAY IN ANIMALS 



Play is illustrated by kittens with their ball, puppies and their 

 sham-hunt, lambs and their races and game of "King o' the 

 castle", monkeys and their "follow my leader". There are 

 often sham-fights among birds which seem to be entirely playful, 

 besides exhibitions of flying powers that have no direct usefulness. 

 For that is one of the criteria of play, that it is not directly useful. 

 Play is not work, though it may be as strenuous and may lead to 

 exhaustion; it is not mere exercise, though, perhaps, it exercises 

 best ; it has no deliberate (or, in animals, perceived) end, for the sake 

 of which it is played; and yet it may be almost indispensable if 

 the animal is to attain to the full use of its powers. Play is not 

 necessarily social, for many a kitten plays alone ; and it is not neces- 

 sarily competitive, though rivalry may give it zest. Its keynote is 

 its anticipation of modes of activity which are characteristic of 

 adult life. 



Play is well illustrated by many young Carnivores, such as cats, 

 dogs, foxes, otters, and bears; by many young Ungulates, such as 

 lambs, kids, calves, and foals; by most monkeys, and by less familiar 

 cases like young squirrels and water-shrews. Yet it cannot be said 

 to be a general feature in the youthful life of mammals. It is not 

 common among birds; it is only hinted at in reptiles, amphibians, 

 and fishes; and it is at most rare among backboneless animals. This 

 raises the question why a playing period should be interpolated in 

 the life-history of only a small minority. There must be some 

 particular biological advantage in play, yet one which only certain 

 t5rpes have been able to secure. 



(i) The poet Schiller suggested that animal play is an expression 

 of overflowing energy. It is the byplay of exuberant vigour and 

 animal spirits. But while this theory has its grain of truth, it is far 

 too simple. Thus there are many yoimg animals with abimdant 

 vigour that never play ; and it is well known that a thoroughly tired 

 animal, such as a dog, may turn in a moment from fatigue to play, 

 as children often do. Moreover, half of the problem is that different 

 types of mammals play in characteristic or specific ways. 



(2) Schiller's theory of play was re-expressed by Herbert Spencer, 

 with the important additional suggestion that imitation accounts 

 for the particular form that the playing takes. The physiological 

 condition of play is superfluous energy, but imitation defines the 



