The Darwinian foundation of individual moral agency finds slightly different foundations in Nietzsche and James. The former identifies that fuel as the will-to-power, an intrinsic power which seeks to expand itself and incorporate the world into itself. All individual agencies express this will-to-power, but there is a qualitative difference between its expression among those agencies for Nietzsche. The traditional Christian "slave morality", for instance, expresses a 'weak' will-to-power. This weakness is not social or political, because that moral outlook actually came to dominate Europe for centuries. In many ways, it is still dominant in the West today, although less explicitly religious.
Instead, the 'slave morality' is weak in a metaphysical sense, i.e. it convinces those spiritual individuals who adopt it to deny their instinct to prioritize virtuous and exemplary qualities in their inner value hierarchy. It convinces them to expand their sphere of power through superficial asceticism and moralizing rather than authentic self-transformation. It is not a coincidence that the most apologetically religious commentators are also generally skeptical of Darwinian science and the evolutionary narrative. They view the evolved drives of humanity as products of "original sin" which must be resisted at every turn. That is their path towards the restoration of a pre-Fall paradisiacal Eden.
D&P wrote:...will-to-power for Nietzsche is not a drive towards a particular kind of power (such as economic or political) but rather a general tendency toward expansion and incorporation. The will-to-power is thus not defined negatively as an emptiness that seeks power. It is already intrinsically power, and it seeks to grow. Nietzsche's concerns about will-to-power are more qualitative than quantitative. All agency expresses will-to-power in some way, but some forms of will-to-power seem relatively devious or perverse. In particular, Nietzsche argues that the 'slave morality' of Christianity expresses a 'weak' will-to-power. This will-to-power is not weak in the sense of being easily cowed or dominated. On the contrary, it seems to have taken over Europe. It is weak in that it convinces the spiritually exemplary to deny their instincts for enforcing certain kinds of hierarchy and in this way protects the mediocre from being subordinated.
James similarly finds the fuel of individual moral agency to be rooted in 'energy'. He posits that all individuals have evolved 'layers' of unrealized energy stored up in their physiology and psychology, normally blocked by inveterate habits of mind. Through various 'dynamogenic' practices, such as yoga, meditation, self-restraint, etc., these energies can be cultivated and used to resist sub-optimal habits. James references the 'second wind' phenomenon as an example of that latent energy in human individuals. It is that experience of feeling so utterly exhausted you may disintegrate at any given moment, but, through mental perseverance and forging ahead with whatever task was being undertaken, you are suddenly revitalized and make further progress which just seemed unimaginable a few minutes earlier.
Both Nietzsche and James consider the individual to be an internally constructed 'society' of living and dynamic sub-personalities who are hierarchically organized. Nietzsche proposes that all of moral philosophy is an expression of the actual and desired relations among the individual sub-personalities. He even gives this internal society an aesthetic imperative of 'style': "
to give style to one's character - a great and rare art! It is practiced by those who survey all the strengths and weaknesses of their nature and then fit them into an artistic plan until every one of them appears as art and reason and even weaknesses delight the eye." The self-styling of individual sub-personalities is akin to the Darwinian concept of
exaptation, where extent structures gain new evolutionary functions over time.
While Nietzsche is somewhat reductionist in his view of the individual as a swarm of 'under-souls' struggling for existence and power, James actually runs this conception of the individual upwards to the level of family, community and societies. These are all self-similar dynamic organizations nested within each other. The fundamental task and 'purpose' of Nature is to find an alignment across all these layers simultaneously. It is very similar to what Carl Jung termed the process of "individuation", to which he also factors in the dimension of time. The evolutionary process is 'searching' for an equilibrium across all temporal iterations of all organizations, and individual human agency has become its primary tool in that search.
Yet, for both Nietzsche and James, that individual agency is by no means a guarantee the search will be successful. In fact, the agency would be quite meaningless if there was not the possibility of abject failure. Freedom, then, plays a vital role in their Darwinian metaphysics and ethics. Blind obedience to 'moral codes' or Zen-like apathy to individual ambitions have no place in their philosophical systems. We are not unique or spiritual agents because we can follow objective moral values 'out there', rather we are moral agents because we are unique, spiritual and freely capable of discovering that essential nature from within.