62 ANIMAL LIFE 
enemies. Inthe next year another brood is carefully reared, 
and so on for the few years of the robin’s life. 
Under normal conditions in any given locality the num- 
ber of individuals of a certain species of animal remains 
about the same. The fish which produces tens of thousands 
of eggs and the bird which reproduces half a dozen eggs a 
year maintain equally well their numbers. In one case a 
few survive of many born; in the other many (relatively) 
survive of the few born ; in both cases the species is effect- 
ively maintained. In general, no agency for the perpetua- 
tion of the species is so effective as that of care for the 
young. 
