The object of this communication is to try to give a theory of what takes place when one species ischanged into another. When the species A is converted into the species B there must be an intermediatetransitional stage in which the collection of individuals is neither the species A nor the species B. I have already defined a species as a centre round which individuals are thickly clustered, and the spaces between these centres may be either devoid of individuals or contain here and there an abnormal form (Davidson's “ Suppt. Jurassic Brachiopoda,”Pal. Soc, 1876, p. 122).