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small mutations which affect: the colour of the petals; the number 
of petals, sepals and bracts; the presence or absence of external 
hairs on leaves and stem, of awns on the glumes of grasses; the 
shape of the leaf; the amount of anthocyanin pigment present; and 
many presumptive wild mutations of a similar kind which have been 
held to account for varietal differences. In animals may be mentioned 
mutations which affect: the colour of the hair and the colour-pattern; 
the colour and shape of the feathers in birds; the shape of the comb 
in domestic fowls; the size and proportion of the skeleton; the 
presence or absence of horns in cattle; eye and body colour in 
Drosophila; the colour and size of the shells of gasteropods, and the 
occasional appearance, of sinistral varieties in normally dextral 
forms; the artificially induced mutations in the colour markings of 
Chrysomelid beetles observed by Tower. There are many presumptive 
mutations of the same kind, such as the enormous variety in the 
spicules of sponges and in the tests of Radiolaria. In some cases, 
variations of this class may affect several different superficial 
characters simultaneously. 
In the case of domesticated plants and animals indeed, the 
physiologically less important characters become apparently plastic 
and may vary enormously in even the same species. On the other 
hand, those structures and functions which are important proximate 
factors in somatic equilibrium, such as the mechanism of the nervous, 
circulatory and digestive systems, generally have a long phylogenetic 
history, and by no process of artificial selection can they be changed 
appreciably 1 . 
There do, however, occasionally occur marked single mutations 
in the stable and physiologically important characters and these are 
nearly always harmful, if not actually non-viable or incapable of 
hereditary transmission. In plants may be mentioned: sterile anthers; 
petaloidy in flowers; the absence of chlorophyll; also many of the 
(Enothera mutations which are associated with changes in the 
number of chromosomes. In animals: inherited sterility; albinism; 
the absence of the coagulating function of the blood; congenital 
structural defects in the eye, the heart, the thyroid gland; brachy- 
dactyly and other inherited deformities; congenital mental diseases. 
1 I.e. rendered more efficient or changed in type. Pathological variations of 
the kind next considered may of course be selected. There is also the possibility 
that the prolonged exposure of a species to domesticated conditions by 
wholly altering the conditions of energy-assimilation (like parasitism) may 
itself result in a new position of somatic equilibrium in the manner subsequently 
discussed. 
