Dr Lees uses a mix of religious, scientific, and philosophical arguments for the idea that primitive man were vegetarians.
An Argument on Behalf of the Primitive Diet of Man
Frederic R. Lees
That the aboriginal diet of mankind was fruit, and that amongst persons and tribes of any degree of sensibility and refinement, butchering has been regarded as offensive, disgusting, and barbarous, are facts that indicate beyond controversy, on which side pure Nature and our moral Instincts range themselves in this discussion.
“And God said, Behold, I have given you every herb bearing seed, which is upon the face of all the earth, and every tree in which is the fruit of a tree yielding seed, to you it shall be for food.” (Gen. i. 29.)
Justifications of the slaughter and consumption of animals founded on the permissions of Scripture, prove far too-much. They would not only justify slavery, divorce, and polygamy,—which were equally departures and descents from the original and highest order of social-life,—but they would destroy all faith in scripture revelation itself. Christ, with a Divine indignation, has for ever rebuked and repudiated this shocking style of inference.