Answer:
Darwin is very well-known for his Nature selection theory which is the differential survival and reproduction of individuals due to differences in phenotype. This is one of the key mechanism in evolution. We can easily understand why struggles are crutial for this to happen. These struggles can be the sudden change in climate, a dangerous disease or maybe a serious lack of food. This is where diversity comes in. Every individual is born a little bit different from the others, even from their siblings. And when struggles happen to them, the idividuals who fortunately have an advantage, for example sharper teeth, can survive and reproduct while others die. This process continues and to the point when the animal is perfectly adapted to the enviroment. So basiclly, without any struggle, animals won't become so diverse and obviously inevitable.
