01 - Variations Under Domestication

01-03 - correlation of Growth

Professor Wyman has recently communicated to me a good illustration of this fact; on asking some farmers in Virginia how it was that all their pigs were black, they informed him that the pigs ate the paint-root (Lachnanthes), which coloured their bones pink, and which caused the hoofs of all but the black varieties to drop off; and one of the "crackers" (i.e. Virginia squatters) added,
"we select the black members of a litter for raising, as they alone have a good chance of living."

Jeffries Wyman

pig

Hairless dogs have imperfect teeth; long-haired and coarse-haired animals are apt to have, as is asserted, long or many horns;

dog

pigeons with feathered feet have skin between their outer toes; pigeons with short beaks have small feet, and those with long beaks large feet.

Hence if man goes on selecting, and thus augmenting, any peculiarity, he will almost certainly modify unintentionally other parts of the structure, owing to the mysterious laws of correlation.

pigeon

05 - Laws of Variation

05-04 - Correlation of Growth

Homologous parts, as has been remarked by some authors, tend to cohere; this is often seen in monstrous plants: and nothing is more common than the union of homologous parts in normal structures, as in the union of the petals into a tube.

petal

Hard parts seem to affect the form of adjoining soft parts; it is believed by some authors that with birds the diversity in the shape of the pelvis causes the remarkable diversity in the shape of their kidneys.

bird

pelvis

kidney

Others believe that the shape of the pelvis in the human mother influences by pressure the shape of the head of the child.

In snakes, according to Schlegel, the form of the body and the manner of swallowing determine the position and form of several of the most important viscera.

snake

Hermann Schlegel

05 - Laws of Variation

05-04 - Correlation of Growth

I know of no case better adapted to show the importance of the laws of correlation and variation, independently of utility and therefore of natural selection, than that of the difference between the outer and inner flowers in some compositous and timbelliferous plants.

Every one is familiar with the difference between the ray and central florets of, for instance, the daisy, and this difference is often accompanied with the partial or complete abortion of the reproductive organs.

daisy

But in some of these plants, the seeds also differ in shape and sculpture. These differences have sometimes been attributed to the pressure of the involuera on the florets, or to their mutual pressure, and the shape of the seeds in the ray-florets of some Compositae countenances this idea; but with the Umbelliferae, it is by no means, as Dr. Hooker informs me, the species with the densest heads which most frequently differ in their inner and outer flowers.

seeds

umbelliferae

It might have been thought that the development of the ray-petals by drawing nourishment from the reproductive organs causes their abortion; but this can hardly be the sole cause, for in some Compositae the seeds of the outer and inner florets differ, without any difference in the corolla.

petal

Possibly these several differences may be connected with the different flow of nutriment towards the central and external flowers: we know, at least, that with irregular flowers, those nearest to the axis are most subject to peloria, that is to become abnormally symmetrical.

I may add, as an instance of this fact, and as a striking case of correlation, that in many pelargoniums, the two upper petals in the central flower of the truss often lose their patches of darker colour; and when this occurs, the adherent nectary is quite aborted; the central flower thus becoming peloric or regular.

When the colour is absent from only one of the two upper petals, the nectary is not quite aborted but is much shortened.