54 CHILDHOOD OF ANIMALS 



close between us and the higher monkeys and gets more and more 

 remote as we pass through the various orders of mammals and 

 descend through birds to reptiles, and from them to batrachians 

 and fishes. Fear and anger, cowardice and bravery, dislike and 

 affection, the relations of individuals to individuals and of species 

 to species, may differ in quality and degree, but appear to be 

 essentially similar in kind in all these different sets of animals. 

 They are all in mental touch with their environment in the same 

 sort of fashion. I think that we must be right in interpreting the 

 phases of their life by the same kind of standards that we can apply 

 to our own case. The duration of youth in all is settled by no 

 invariable chain of organic necessity. It has no relation to the 

 duration of the complete cycle of life from birth to death. It is 

 linked with size, but only in an indirect fashion, most apparent in 

 animals most akin. It is linked much more closely with complexity 

 of organisation, so that the higher forms usually take longer to 

 mature than their near but lower relations. It is linked most 

 closely with intelligence, the more intelligent animals having 

 relatively longer youth. And as we pass downwards from intelli- 

 gence to instinct we find that the duration of youth shortens. 



The case of the eel, where the adult life is only a very small 

 portion of the total length of life, is not so curious as the cases of 

 many insects. Among insects there are all gradations between 

 creatures which live only a few weeks and creatures which enjoy 

 life for many years. Insects, however, are very closely dependent 

 on temperature, partly indirectly because their food-supply often 

 ceases in cold weather, and partly directly because they become 

 torpid and die when their bodies are subjected to cold. The dura- 

 tion of life of most insects is limited to less than a year. The eggs 

 hatch out when the temperature has become sufficiently high, the 

 larvae grow bigger, pass through their metamorphoses and become 

 transformed to the adult in the same season. The life of most of 

 the adults ceases when the cold of winter comes on, if it lias not 

 been arrested sooner ; but the species maintains existence, either 

 because the eggs are laid in a position where they may lie dormant 

 until next spring, or because a few of the adults hibernate in some 

 sheltered place. Sometimes the total life is limited to a very short 

 part of a single season. In many of the plant-lice, for instance, the 

 little green flies which plague the gardener, the total life lasts only 

 two or three weeks. The eggs are laid, the larvae are hatched, 

 mature, become adult, and die all within a month. The total life 



