How did Darwin and Lamarck differ in there thinking about change in species

Let's use an example of a giraffe okay?

Lamark thought giraffes stretches it's neck to reach food (leaves in this case), then as time went on, it got longer and longer because of the constant use of it and then the giragges offspring uses it. Basically, he thought it happened over time

However, Darwin thought this ...

The giraffes neck was already tall to high branches, this meant the giraffe would have a larger chance of surviving and reproducing and the offspring inherits the same advantage. So, darwin thought it was kind of natural and probably part of evolution