The Heart of a Dog

of the main themes in The Heart of a Dog is that it is impossible to predict the outcome of an experiment involving the human psyche

Doctor, humanity-takes care of all that for us in her own good time and according to the order of evolution, and by distinguishing from the mass of the low and the lowly, she creates a few dozen exceptional geniuses to grace this earth of ours. Now you see, Doctor, why I faulted your conclusions on the case history of Sharik. My discovery, devil take it and swallow it whole, is of as much use as a sick headache. Don't argue with me, Ivan Arnoldovich. I've understood it all now. I never talk hot air, you know that. Theoretically it's interesting. All right, then! Physiologists will be in ecstasy. Moscow will go crazy... But, practically speaking, what will happen? Who do you see before you now?" Preobrazhensky pointed in the direction of the consulting room, where Sharikov was taking his rest.

have to realise that the whole horror of the thing is that he already has not the heart of a dog but the heart of a man. And one of the most rotten in nature!"