Darwin and Lamarck were both scientists who tried to understand evolution.
Lamarck’s theory of evolution was based around how organisms (e.g. animals, plants) change during their lifetime, and then pass these changes onto their offspring. For example, Lamarck believes that the giraffe had a long neck because its neck grew longer during its lifetime, as it stretched to reach leaves in high-up trees, meaning that each generation of giraffe had a longer neck than previous generations.
Darwin’s theory, known as natural selection, believed that organisms possessed variation (each individual was slightly different from one another), and these variations led to some being more likely to survive and reproduce than others. Features that made an organism more likely to survive or reproduce are therefore more likely to appear to each generation. In terms of the giraffe, Darwin’s theory would state that longer necked giraffes were more likely to survive, because they could eat leaves from taller trees, and therefore more long-necked giraffes will be born, which eventually caused all giraffes to have longer necks. Longer necked giraffes survived because they were a better fit for their environment. It is from this idea that we get the phrase ‘survival of the fittest’.
Darwin’s theory became accepted because it had more evidence that supported it. Lamarck’s theory suggest that all organisms become more complicated over time, and therefore doesn’t account for simple organisms, such as single-cell organisms. We also know through observation that characteristics inherited during an individual’s lifetime do not get passed onto their offspring; for example, if someone pierces their ear, it doesn’t mean that their children will be born with pierced ears. Darwin’s theory became even stronger when, many years after his death, the study of genetics emerged. Genetics ultimately showed that all inherited traits were passed on through genes, which are unaffected by outside world, as Lamarck would predict, and are instead naturally varied, as Darwin predicted. Though Darwin didn’t know what genes were, he could see their effect on natural selection.