Question

Certain land mammals migrated to an aquatic environment, adapted, and evolved into today's water mammals such as whales and dolphins. What would be the explanation for this, according to Lamarck's theory?

Answer 1

Uninteresting on account of it being wrong. Presumably (and actually nicking a bit of inspiration from something Darwin briefly mused on for the bear mention), an animal something or other like a bear took to also exploiting marine resources. This habit encouraged a tolerance of saltier conditions which, by a process akin to magic, the salt-licking parent was able to pass on to its descendants. The acquisation of this tolerance of salt resulted, over the generations, in a sort of salt love. In addition, the swimming prowess of the parent, enhanced by plentiful practise, was also inherited via magical processes by its grateful progeny.

This is the inheritance of acquired characteristics, not genetic inheritance (which happens to happen).

Anyway, the enhanced swimming abilities prompted changes to sympathetically arise from the habit, and these in turn reinforced. The lack of walking practise across the generations led to the redundancy of waling muscles, and the musculation was reenigineered through different usage into much better swim-swimminess.

Quite possibly shaving gave rise to a stark restriction in the hairiness.

Should this scenario appear to make some kind of sense, then this a likely sign that you've misunderstood evolutionary realities. It's actually fairy-dust based twaddle.