YOUTH IN BIRDS AND LOWER ANIMALS 51 



which they are surrounded and are thus almost at the mercy of 

 the elements. If they become too hot or too cold they first get torpid, 

 and if the conditions continue they die. Reptiles will not feed or 

 grow unless they are kept warm. In the Reptile House of the 

 London Zoological Gardens the heating apparatus was greatly 

 improved in the year 191 1 ; the reptiles were much more lively and 

 active, and the rather unexpected result occurred that the food 

 bill was nearly doubled. In the varying conditions of nature, a 

 succession of warm seasons or of cold seasons must affect the rate 

 of growth of reptiles to a very large degree, and it is not surprising 

 that we can tell little of the age of any individual from its size. Very 

 few reptiles breed in captivity, whilst in the wild condition their 

 shy habits make it difficult to observe them closely. There is the 

 further difficulty that young reptiles from the first are remarkably 

 like their parents. And so it happens that we have practically no 

 information regarding the duration of youth in reptiles. 



The sizes to which the different species of frogs, toads and newts 

 may reach vary within wider Umits than those of birds and mammals, 

 but it is curious that the range is narrower, especially in the case 

 of the tailless land forms, than occurs with reptiles. Batrachians 

 are less shy in their breeding habits than are reptiles, and many 

 of them have been bred and reared in captivity. In the case of 

 those that breed in water and pass through a metamorphosis, the 

 spawn is usually laid very early in the year, but this depends 

 partly on temperature. In cold seasons it may be delayed for some 

 weeks, and in England, except at considerable levels above the 

 sea, it usually occurs early in March. I have found the spawn of 

 the common grass frog in mountain bogs in Scotland late in June. 

 The tadpoles of the common frog begin to leave the eggs in about 

 five days, and in about two months the legs have appeared, whilst 

 the metamorphosis is complete and the frogs leave the water in 

 nearly three months. The development of the common toad is 

 not quite so rapid. The tadpoles leave the eggs in about ten days, 

 but the two pairs of hmbs are not fully formed for about eighty 

 days, whilst the young toads leave the water relatively smaller than 

 frogs, when they are a little more than three months old. They 

 may begin to breed long before they are full grown, but they take 

 from three to five years to reach the normal size. The possible 

 duration of their hfe is unknown, but they have so many enemies 

 that probably few have the luck to reach old age. 



Fish, Hke reptiles, grow slowly and may live to great ages. In 



