284 " NATURE STUDY AND LIFE 



proportion of animal life as well. Toad tadpoles are espe- 

 cially good for this work and will generally keep the water 

 in the aquarium as clear as crystal. An instructive experi- 

 ment may be arranged by setting up two small aquaria just 

 alike. Put in plants and everything else, but leave all 

 the tadpoles out of one and observe differences in cleanli- 

 ness of the water. Unite with this study observations as 

 to feeding habits of tadpoles in the ponds. They may be 

 seen swarming around dead fishes, frogs, or other matter 

 that would otherwise pollute the water. Apply the 

 knowledge gained to the problem of keeping the ponds 

 and park waters in the neighborhood clear and sweet.^ 



Natural enemies of the toad form the next series of 

 topics for study. Why is it that, laying from five to ten 

 thousand eggs a year, toads do not become more numer- 

 ous .? No one knows the natural length of a toad's life. 

 It is claimed that one lived for thirty-six years in a garden 

 in England and was then killed by a tame raven. They 

 probably mature and begin to lay eggs when about four 

 years old. Suppose the females continue laying for ten 

 years, the fact that the species does not increase in num- 

 bers means that of the possible 100,000 eggs only two 

 survive to take the place of their parents. How are all 

 the rest killed off ? 



Practically every egg in a laying hatches, and I have 

 not discovered that any fishes, newts, tadpoles, or aquatic 

 insects eat the eggs. But the tadpoles from the time 



*i As filth is washed into the ponds by the freshets of early spring, no 

 small service may be rendered in cleaning surface waters at this season. 1 

 have seen pools even among city dumpage perfectly clean while the tad- 

 poles were in them during the spring. 



