Fish and frogs lay hundreds or thousands of eggs because fertilisation and early development occur in water, where eggs and young offspring face extremely high risks from predators, environmental hazards, and lack of parental protection.

The chapter explains that in fish and frogs, 'male and female fish or frogs eject sperm and eggs, respectively, into the water where they combine to form the zygote' and 'development of the zygote into an embryo also takes place in water.'

Advantages of laying many eggs include:

1. Higher probability that at least some offspring will survive despite high mortality rates.
2. Greater genetic variation among offspring, increasing chances that some will be adapted to survive in changing conditions.
3. No need for parental care, allowing parents to conserve energy.

Disadvantages of this strategy include:

1. Enormous energy expenditure on producing eggs that mostly will not survive.
2. No parental protection means most eggs are eaten by predators or destroyed by environmental factors.
3. Offspring are left to fend for themselves from the earliest stages.

In contrast, animals like birds and mammals lay fewer eggs or give birth to fewer young but provide parental care—building nests, protecting eggs, feeding young ones—which significantly increases survival rates of each individual offspring.