}}</ref> Another evolutionist, [[Alfred Russel Wallace|A.R. Wallace]], after reading Darwin’s statement asserting that useful variations may ''“occur in the course of thousands of generations”'', warned him that such expression may give his opponents the advantage to consider the favorable variations a rare accidents or even never occurring at all.{{#tag:ref|cf. Professor Spetner states that the beneficial mutations required by the theory of evolution have never been observed.<ref name="Spetner">{{Cite book
+
}}</ref> Another evolutionist, [[Alfred Russel Wallace|A.R. Wallace]], after reading Darwin's statement asserting that useful variations may ''“occur in the course of thousands of generations”'', warned him that such expression may give his opponents the advantage to consider the favorable variations a rare accidents or even never occurring at all.{{#tag:ref|cf. Professor Spetner states that the beneficial mutations required by the theory of evolution have never been observed.<ref name="Spetner">{{Cite book