A strain of rabbit fibroma virus (changed virus) producing partly fibromatous, partly inflammatory, lesions is believed to represent a mixture of the original virus with a strain (inflammatory virus) causing only necrotic and inflammatory lesions. The inflammatory virus does not represent a contamination from without, but probably arose as a mutant from the original strain. The occurrence of mutation amongst viruses and the propriety of using the word in this field are briefly discussed. Consideration is also given to the nature of the change in the virus which leads to a tissue reaction so widely different from that produced by the original strain.

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