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?

Land mammals progressively modified their organs to survive in water. These features were passed on to the next generation, and the mammal gradually evolved into today's aquatic mammals.

Some land mammals adapted to the environment. The organisms which were best adapted were selected by nature, and they then evolved into today's mammals.

Land mammals underwent immediate mutations to survive in the aquatic environment and were selected by nature to evolve into today's aquatic mammals.

The correct answer, according to Lamarck's theory of evolution, is that A) "Land mammals progressively modified their organs to survive in water. These features were passed on to the next generation, and the mammal gradually evolved into today's aquatic mammals." Lamarck believed that animals could (a) acquire the traits that they needed and (b) pass those acquired traits on to their offspring. According to part (a) of Lamarck's idea, if I needed to be a faster swimmer (if I were an athlete), I could modify my body to have webbing between my toes, which would allow me to beat the competition! Also, according to part (b) of Lamarck's idea, if I were a body builder and developed (acquired) very large muscles through my training, then my children should be born with very large muscles. Based on the these examples, you can see why Lamarck's theory of evolution is not the one that is generally accepted by scientists. Rather, Darwin's theory of evolution, in which the traits that we are born with confer either an advantage or disadvantage to us in our particular environment, which determine whether or not we are successful (ie, survive and reproduce). New traits come about randomly by mutation, not by our need for them.