Origin of Species Variorum

↑ 1 blocks not present in 1872; present in 1859 1860 1861 1866 1869

An indefinite repetition of the same part or organ is the common characteristic (as Owen has observed) of all low or little-modified forms; therefore we may readily believe that the unknown progenitor of the vertebrata possessed many vertebræ; the unknown progenitor of the articulata, many segments; and the unknown progenitor of flowering plants, many spiral whorls of leaves.

→ not only in number, but in form. 1872

in number and structure; consequently it is quite probable that natural selection, during a long-continued course of modification, should have seized on a certain number of the primordially similar elements, many times repeated, and have adapted them to the most diverse purposes. 1859 1860 1861 1866

in number and structure. 1869

→ Consequently 1869 1872

And as the whole amount of modification will have been effected by slight successive steps, we need not wonder at discovering in 1859 1860 1861

And as the whole amount of modification will have been effected by successive slight steps, we need not wonder at discovering in 1866

or organs, a certain degree of fundamental resemblance, retained by the strong principle 1859 1860

or organs a certain degree of fundamental resemblance, retained by the strong principle 1861 1866

being already present, and being highly variable, would afford the materials for adaptation to the most different purposes; and they would generally retain through the force 1869

→ inheritance, plain traces of their original or fundamental resemblance. 1872

inheritance. 1859 1860 1861 1866

inheritance plain traces of their original or fundamental resemblance. 1869

→ OMIT 1872

we can homologise 1859 1860 1866

we can homo- logise 1861

it can easily be shown that 1869

→ in 1869 1872

of one species with those of another and 1859

of one species with those of other and 1860 1861 1866

→ can be shown to be homologous, only a 1872

we can indicate but 1859 1860 1861 1866

are homologous, but 1869

→ such as the valves of Chitons, can be indicated; that 1872

that 1859 1860 1861 1866

can be indicated; that 1869

plication of the parts developed from such cells. It must suffice for our purpose to bear in mind that an indefinite repetition of the same part or organ is the common characteristic, as Owen has remarked, of all low or little specialised forms; therefore the unknown progenitor of the Vertebrata probably possessed many vertebræ; the unknown progenitor of the Articulata, many segments; and the unknown progenitor of flowering plants, many leaves arranged in one or more spires. ↑ We have

formerly

also formerly

seen that parts many times repeated are eminently liable to

vary

vary,

→not only in number, but in form. →Consequently such

parts

parts,

→being already present in considerable numbers, and being highly variable, would naturally afford the materials for adaptation to the most different purposes; yet they would generally retain, through the force of →inheritance, plain traces of their original or fundamental resemblance. They would retain this resemblance all the more, as the variations, which afforded the basis for their subsequent modification through natural selection, would tend from the first to be similar; the parts being at an early stage of growth alike, and being subjected to nearly the same conditions. Such parts, whether more or less modified, unless their common origin became wholly obscure, would be serially homologous.

In the great class of molluscs, though →OMIT the parts →in distinct

species,

species

→can be shown to be homologous, only a few serial

homologies;

homologies

homologies,

→such as the valves of Chitons, can be indicated; that is, we are seldom enabled to say that one part

or organ

....

is homologous with another

part in

the same individual. And we can understand this fact; for in molluscs, even in the lowest members of the class, we do not find nearly so much indefinite repetition of any one

part,

part

as we find in the other great classes of the animal and vegetable kingdoms.

But morphology is a much more complex subject than it at first appears, as has lately been well shown in a remarkable paper by Mr. E. Ray Lankester, who has drawn an important distinction between certain classes of cases which have all been equally ranked by naturalists as homologous. He proposes to call the structures which resemble each other in distinct animals, owing to their descent from a common progenitor with subsequent modification, homogenous; and the resemblances which cannot thus be accounted for, he proposes to call homoplastic .

For instance, he believes that the hearts of birds and mammals are as a whole homogenous,— that is, have been derived from a common progenitor; but that the four cavities of the heart in the two classes are homoplastic,— that is, have been independently developed. Mr. Lankester also