How struggle for existence drove evolution of organism

In On the Origin of Species, Darwin claimed that there was a continual 'struggle for existence' in nature, in which only the fittest would survive. This theory came partly from his reading of Thomas Malthus's Essay on the Principle of Population.

Explanation:

Generally, we can say that we have to kind of cells based on reproduction:
Somatic and germinal cells.
Germ cells come from a division of stem cells (meiosis). They are at the origin of the formation of the reproductive cells, or gametes (spermatozoids in the man and the oocytes at the woman) The DNA of these cells affected by mutations can be transmitted to the offspring (which is not the case for somatic cell mutations).

Somatic cells represent all the cells of the body, except germ cells and embryonic cells. It could be bone cells, liver cells or blood cells... Theoretically, since these cells are not germ cells, they will never transmit to their descendants the mutations they may have undergone. But it could lead to cancers if they are mutated (skin, liver, or lung cancer).