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In 1967,Damon Knight criticized the novel's scientific incoherence...

The seed pods, says Finney, drifted across interstellar space to Earth, propelled by light pressure. This echoes a familiar notion, the spore theory of Arrhenius. But the spores referred to are among the smallest living things – small enough to be knocked around by hydrogen molecules... In confusing these minute particles with three-foot seed pods, Finney invalidates his whole argument – and makes ludicrous nonsense of the final scene in which the pods, defeated, float up into the sky to hunt another planet.

...and its crude plot development:

Almost from the beginning, the characters follow the author's logic rather than their own. Bennell and his friends, intelligent and capable people, exhibit an invincible stupidity whenever normal intelligence would allow them to get ahead with the mystery too fast. When they have four undeveloped seed pods on their hands, for instance, they do none of the obvious things – make no tests, take no photographs, display the objects to no witnesses. Bennell, a practicing physician, never thinks of X-raying the pods.