Final answer:
The most important immediate outcome of Wallace's 1858 letter to Darwin, which included Wallace's ideas of natural selection, was that it pushed Darwin to finish his book 'On the Origin of Species'. This publication changed the field of science forever.
Step-by-step explanation:
The most important immediate outcome of Wallace's 1858 letter to Darwin, which included Wallace's ideas of natural selection, was that it pushed Darwin to finish his book On the Origin of Species. Published in 1859, this book spelled out Darwin's theory of evolution by natural selection and provided convincing arguments and evidence to support it. This publication changed the field of science forever.