- Every species creates so many descendants that in the course of time there will be a shortage of food and space.
- As a consequence there comes a struggle for life. This needs not to be a fight, but rather a competition in bringing up offspring. Left picture: as long as there is enough food, there is no problem, but in case of shortage of food there comes trouble
- Inside species of plants and animals is much variation (look around to members of your own species or look at the marguerites on the photo on the right). Some of the variations give better possibilities than others in the struggle for life. The blackbird with the toughest bill for example will have advantage of this trait. He will have the most offspring because he can obtain food for his nestlings.
- If such a change in heritable qualities continues during many generations, the species will change and it is even possible that a new species comes into existence.
Thus Darwin had discovered the mechanism which he had been looking for so long. He called it natural selection, because nature selects out the plants and animals which are best adapted to the (difficult) circumstances.