I had read about it a decade ago, but it looks like now I have forgotten his point. Can I know what was his argument against Darwin?
Nietzsche apparently believed that evolution by natural selection accounted for species diversity, but he also believed that the quality of various lifeforms was differentiated by the degree to which they possess a 'vitalistic force'.
He maintained that life was not only impelled by a struggle for survival, but by a drive towards 'ever-greater complexity, diversity, multiplicity and creativity'.
I can offer only a derivative response.
On a side note:
How much did Nietzsche know?
Nietzsche, all agree, is unlikely to have read either Darwin’s The Origin of Species (1859) or his Descent of Man (1871). However, he had a basic if not entirely accurate grasp of Darwinism derived from popularizers and synthesizers, and his direct engagement with the evolutionary theory of [Herbert] Spencer, in the latter’s three-volume System of Synthetic Philosophy (1862–67), is evident. He mentions Darwin, Darwinism, and Darwinians in works ranging from his early Untimely Meditations (1873) on the rationalist critic of Christianity, David Strauss, to the notes of the late 1880s that were assembled into a book by his sister and later published under the title Der Wille zur Macht. (Catherine Wilson, 'Darwin and Nietzsche: Selection, Evolution, and Morality', Journal of Nietzsche Studies, Vol. 44, No. 2 (Summer 2013), pp. 354-370: 355.)
The clash of viewpoints
To summarize, Nietzsche was engaged in a three-cornered argument. In one corner stood the traditional Christians with their commitment to the repression of sensuality as evil and sin, and their supernaturalism. In another corner were the agnostic or atheistic philosophers who wanted to assert, preserve, and even extend Christian values in the face of science, including Kant, Schopenhauer, Darwin, Strauss, and Spencer. In the third corner, there was Nietzsche himself, aesthetically repelled by the masses and their unhealthy propensities, distressed by the suffering of superior individuals, and hostile to traces of asceticism and Orientalism in his contemporaries.
In some respects, then, Nietzsche is a Darwinian. He believes that the history of humanity is part of natural history and that there is no sacred history as opposed to a history of sacerdotalism, and he believes (except in his idealist moments) that mental powers supervene on physical organization. Like Darwin, he believes that individuals vary in their intrinsic quality: some are superior to others. He is also much closer to the view of the modern, neo-Darwinian biologist than were the nineteenth-century Darwinians. It is the individual that matters to itself, regardless of the fate of the “species” to which it belongs. But he rejects the basic Darwinian picture, according to which all differences between members of the same species are necessarily minute and often invisible and at the same time cumulatively significant in the production of new “races” and ultimately, new species. He rejects as well the Darwinian postulate that altruism and self-abnegation are human traits acquired in the course of evolution and further refined in the highest forms of civilization, identifying instead an unnatural and sinister origin for them in archaic resentment [ressentiment: GT]. (Wilson: 366-7.)
It doesn't affect the substance of her statement of Nietzsche's views about Darwin but the blithe reference to 'the agnostic or atheistic philosophers who wanted to assert, preserve, and even extend Christian values in the face of science, including Kant, Schopenhauer, Darwin, Strauss, and Spencer' lacks historical nuance.