The idea that Lamarck and Darwin shared was the concept of evolution, the idea that life on Earth has changed over time.

However, their mechanisms for how this change happened were drastically different:

* Lamarck believed in the inheritance of acquired characteristics. This meant that traits an organism developed during its lifetime, such as bigger muscles from exercise, could be passed down to its offspring. This idea is now widely considered incorrect.

* Darwin proposed natural selection as the driving force of evolution. This theory states that individuals with traits better suited to their environment are more likely to survive and reproduce, passing on those advantageous traits to their offspring. This leads to gradual changes in populations over time.

So, while both Lamarck and Darwin believed in evolution, their explanations for how it happened were quite different. Darwin's theory of natural selection is now the widely accepted scientific explanation for evolution.