Continuation of my earlier reply.
Darwin faced big headwinds in trying to establish that natural selection is the main process for evolutionary change. In this quote he is debunking the charge that he said it was the exclusive mode. Yet, he always considered it the primary mode. This was not accepted by other biologists till about 1910 when elucidation of Mendelian genetics proved him right.
See link below,
Darwin's Influence on Modern Thought
For 80 years after 1859, bitter controversy raged as to which of four competing evolutionary theories was valid. “Transmutation” was the establishment of a new species or new type through a single mutation, or saltation. “Orthogenesis” held that intrinsic teleological tendencies led to transformation. Lamarckian evolution relied on the inheritance of acquired characteristics. And now there was Darwin’s variational evolution, through natural selection. Darwin’s theory clearly emerged as the victor during the evolutionary synthesis of the 1940s, when the new discoveries in genetics were married with taxonomic observations concerning systematics, the classification of organisms by their relationships. Darwinism is now almost unanimously accepted by knowledgeable evolutionists. In addition, it has become the basic component of the new philosophy of biology.
All current research is adding extra dimensions to the bedrock of Darwin's theory of variational evolution through natural selection.
In the Darwin’s own statement that is in OP, he says that natural selection was the primary process but not the only one. But TOE does not also propose that sentience arose naturally.
I will read the link at leisure.
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