The Three-coloured Mannikin (Munia malacca) of South India is probably simply a white-bellied form of the widely-ranging Black-headed Mannikin (M. atricapilla), which has the abdomen chestnut like the back. Intermediate wild-caught forms have been recorded.
The African Cordon-bleu (Estrelda phœnicotis) and Blue-bellied Waxbill (E. cyanogastra) would also seem to be mutations, as almost the only difference between them lies in the fact that the male of the former has a crimson cheek-patch, which is wanting in the latter.
The Ringed Finch (Stictoptera annulosa) of Java, and Bicheno’s Finch (S. bichenovii) of Australia, only differ in the former having the rump black, while in the latter it is white, and this difference appears to be of the nature of a mutation.
So, it might be urged, is the pure white breast of the male Upland Goose (Chloëphaga magellanica), which part, in the very similar C. dispar, is barred as in the females, the latter form being probably the ancestor.
The differences between the silver-grey-necked Crowned Crane of the Cape (Balearica chrysopelargus) and the dark-necked species of West Africa (B. regulorum) seem also to be not more than could be accounted for by mutation.
Peculiar forms, such as a rabbit with a convoluted brain or a mouse with a peculiar pattern of molar teeth, have been come upon by anatomists.
The above-cited mutations are all very considerable ones, and we do not profess to have mentioned a tenth part of those which have actually been recorded.
We trust that we have collected and set forth sufficient evidence to show that the phenomenon of discontinuous variation is a very general one, and this would seem to tell against the hypothesis of De Vries that species pass through alternate periods of comparative stability and periods when swarms of mutations appear. We think it more probable that all species throw off at greater or less intervals discontinuous variations, and that it is upon these that natural selection acts.
We further hope that we have succeeded in making clear what we believe to be the very sharp distinction between continuous and discontinuous variations, even when the latter are inconsiderable, as frequently happens.