The Principles of Biology, Volume 1 (of 2)

by Herbert Spencer

§ 85. Equally conspicuous with the truth that every organism bears a general likeness to its parents, is the truth that no organism is exactly like either parent. Though similar to both in generic and specific traits, and usually, too, in those traits which distinguish the variety, it diverges in numerous traits of minor importance. No two plants are indistinguishable; and no two animals are without differences. Variation is co-extensive with Heredity.

The degrees of variation have a wide range. There are deviations so small as to be not easily detected; and there are deviations great enough to be called monstrosities. In plants we may pass from cases of slight alteration in the shape of a leaf, to cases where, instead of a flower with its calyx above the seed-vessel, there is produced a flower with its calyx below the seed-vessel; and while in one animal there arises a scarcely noticeable unlikeness in the length or colour of the hair, in another an organ is absent or a supernumerary organ appears. Though small variations are by far the most general, yet variations of considerable magnitude are not uncommon; and even those variations constituted by additions or suppressions of parts, are not so rare as to be excluded from the list of causes by which organic forms are changed. Cattle without horns are frequent. Of sheep there are horned breeds and breeds that have lost their horns. At one time there existed in Scotland a race of pigs with solid feet instead of cleft feet. In pigeons, according to Mr. Darwin, "the number of the caudal and sacral vertebræ vary; as does the number of the ribs, together with their relative breadth and the presence of processes."

That variations, both small and large, which arise without any specific assignable cause, tend to become hereditary, was shown in the last chapter. Indeed the evidence which proves Heredity in its smaller manifestations is the same evidence which proves Variation; since it is only when there occur variations that the inheritance of anything beyond the structural peculiarities of the species can be proved. It remains here, however, to be observed that the transmission of variations is itself variable; and that it varies both in the direction of decrease and in the direction of increase. An individual trait of one parent may be so counteracted by the influence of the other parent, that it may not appear in the offspring; or, not being so counteracted, the offspring may possess it, perhaps in an equal degree or perhaps in a less degree; or the offspring may exhibit the trait in even a still higher degree. Among illustrations of this, one must suffice. I quote it from the essay by Sir J. Struthers referred to in the last chapter.

"The great-great-grandmother, Esther P—— (who married A—— L——), had a sixth little finger on one hand. Of their eighteen children (twelve daughters and six sons), only one (Charles) is known to have had digital variety. We have the history of the descendants of three of the sons, Andrew, Charles, and James.

"(1.) Andrew L—— had two sons, Thomas and Andrew; and Thomas had two sons all without digital variety. Here we have three successive generations without the variety possessed by the great-grandmother showing itself.

"(2.) James L——, who was normal, had two sons and seven daughters, also normal. One of the daughters became Mrs. J—— (one of the informants), and had three daughters and five sons, all normal except one of the sons, James J——, now æt. 17, who had six fingers on each hand....

"In this branch of the descendants of Esther, we see it passing over two generations and reappearing in one member of the third generation, and now on both hands.

"(3.) Charles L——, the only child of Esther who had digital variety, had six fingers on each hand. He had three sons, James, Thomas, and John, all of whom were born with six fingers on each hand, while John has also a sixth toe on one foot. He had also five other sons and four daughters, all of whom were normal.

"(a.) Of the normal children of this, the third generation, the five sons had twelve sons and twelve daughters, and the four daughters have had four sons and four daughters, being the fourth generation, all of whom were normal. A fifth generation in this sub-group consists as yet of only two boys and two girls who are also normal.

"In this sub-branch, we see the variety of the first generation present in the second, passing over the third and fourth, and also the fifth as far as it has yet gone.