THE CYCLE OF LIFE 405 



capacity of enregistering within itself its experiences. In 

 many cases we do not know enough to read the diary ; in 

 many cases the creature destroys its own records in the 

 continuous process of self-repair or of replacement of old 

 cells by new. Here we feel the extraordinary importance 

 of the fact that in higher animals there is no replacement 

 of nerve-cells. We cannot add to them after we are born. 



Young Animals. In his remarkable book on The 

 Childhood of Animals, which can hardly be over-praised, 

 Dr. Chalmers Mitchell suggests a threefold classification. 

 There are young animals which are in a general way like 

 miniature editions of the parents, or which, at any rate, 

 very soon become like their parents, as is the case with 

 reptiles, birds and mammals, and with a great variety of 

 backboneless animals, such as cuttlefishes, snails, spiders, 

 and earwigs. There is a distinct youthful period, with 

 interesting growth-changes, but young and old are of the 

 same type. 



In the second place, there are animals whose young stages 

 are very unlike the parents, with a different kind of bodily 

 structure sometimes on a quite different plan and with 

 well-finished adaptations to a mode of life very different 

 from that of the adults. Tadpoles are very different from 

 frogs, and caterpillars from butterflies. The young of the 

 sedentary water-bag-like sea-squirts are free-swimming 

 creatures like miniature transparent tadpoles, and no one 

 who did not know could guess that the ' Glass-crab ' 

 larva would become a rock-lobster. These larval forms 

 are of great biological interest, and their marked 

 unlikeness to their parents reaches a climax in most of the 

 Echinoderms (starfishes, brittle-stars, sea-urchins, sea- 

 cucumbers, and feather-stars), where the free-swimming 



