Wallace: "....My dear Darwin,— I have been so repeatedly struck by the utter inability of number of intelligent persons to see clearly, or at all, the self-acting and necessary effects of Natural Selection, that I am led to conclude that the term itself, and your mode of illustrating it however beautiful to many of us, are yet not the best adapted to impress it on the general naturalistic public...I think [the difficulty in understanding] arises almost entirely from your choice of the term Natural Selection, and so constantly comparing it in its effects to man's selection, and also to your so frequently personifying nature as 'selecting', as 'preferring', 'as seeking only the good of the species', etc., etc. To the few this is as clear as daylight, and beautifully suggestive, but to many it is evidently a stumbling block....I wish, therefore to suggest to you the possibility of entirely avoiding this source of misconception in your great work (if now not too late)...by adopting Spencer's term viz. 'Survival of the Fittest'. This term is a plain expression of the fact; 'Natural Selection' is a metaphorical expression of it, and to a certain degree indirect and incorrect, since, even personifying Nature, she does not so much select special variations as exterminate the most unfavourable ones. ..." "....even personifying Nature, she does not so much select special variations as exterminate the most unfavourable ones. ..."
Other than noting that certain variations were exterminated how was their unfavorability measured by Wallace ?