You will find many answers to this question because Darwin held many fundamental points in his thesis, what he termed as "one long argument." I would say that one of the primary elements here in this is the idea that species' reproduction demands that the adaptation to changes in the environment help to determine the success of those organisms' ability to live. It is adaptation that Darwin sees as the fundamental element in the "struggle" to survive. It is not that there is a constant state of aggressive conflict or predatory nature in the state of being for Darwin. Rather, that the competition for resources in the state of nature makes adaptation critical for survival. It is adaptation to changes in the environment which marks the struggle for existence, and is one where organisms who don't do so will die out. The need to adapt is something that presents itself at one moment in time and is something that fully develops over a scope of geological time. In this light, Darwin makes the critical and vital argument that the drive for survival is based off of adaptation over a prolonged period of time.