word usage - Can/could vs. may/might (expressing typical occurrences)

The following parts are taken from PEU:¹

122.2 common or typical

We often use can to say what is common or typical.

Scotland can be very warm in September.

Ann can really get on your nerves sometimes.

345.2 'general' possibility: can/could, not may/might

We normally use can/could to say that things are possible in general: people are able to do them, the situation makes them possible, or there is nothing to stop them. May/might are not used in this way.

These roses can grow anywhere. (NOT ••• These roses may grow anywhere.)

Can gases freeze? (NOT ••• May gases freeze?)

In those days, everybody could find a job. (NOT ••• In those days, everybody might find a job.)

339.10 another use of may/might: typical occurrences

In scientific and academic language, may is often used to talk about typical occurrences - things that can happen in certain situations.

A female crocodile may lay 30-40 eggs.

The flowers may have five or six petals, pink or red in colour.

Children of divorced parents may have difficulty with relationships.

With this meaning, might can be used to talk about the past.

In those days, a man might be hanged for stealing a sheep.

These explanations made me puzzled. Since they can both relate to typical occurrences, is it possible to substitute can/could for may/might in PEU 339.10 if the context is not written in scientific and academic language?

^{1. PEU = Michael Swan's, Practical English Usage.}

Answer

The word typical is, I think, ill-chosen here. What the writers are saying at 122.2 might be better expressed thus;

We often use can to signify that an eventuality does occur—not always, not necessarily even usually, but from time to time.

At 339.10 there are really two different uses which should be distinguished:

May is used, particularly in academic and scientific contexts, to signify that an outcome is possible but not necessary:
Children of divorced parents may have difficulty with relationships.

May is also used in academic and scientific contexts to describe a range within which possible outcomes lie:
A female crocodile may lay 30-40 eggs.
The flowers may have five or six petals, pink or red in colour.

Either of these may uses can be expressed with can in colloquial registers. Colloquial can has largely (but not entirely) replaced may.

And (as you doubtless know) these do not exhaust the possible uses of either may or can.