Origin of Species Variorum

→ however slight, in its development, 1859 1860 1861 1866 1869

OMIT 1872

→ seized on by natural selection, 1859 1860 1861 1866 1869

favoured, 1872

gains largely in size and quality. In our poultry, a large tuft of feathers on the head is generally accompanied by a diminished comb, and a large beard by diminished wattles. With species in a state of nature it can hardly be maintained that the law is of universal application; but many good observers, more especially botanists, believe in its truth. I will not, however, here give any instances, for I see hardly any way of distinguishing between the effects, on the one hand, of a part being largely developed through natural selection and another and adjoining part being reduced by this same process or by disuse, and, on the other hand, the actual withdrawal of nutriment from one part owing to the excess of growth in another and adjoining part.

I suspect, also, that some

of the

....

cases of compensation which have been advanced, and likewise some other facts, may be merged under a more general principle, namely, that natural selection is continually trying to economise

every part of the organisation. If under changed conditions of life a

structure,

structure

before

useful,

useful

becomes less useful,

its

any

diminution

diminution,

→however slight, in its development, will be →seized on by natural selection, for it will profit the individual not to have its nutriment wasted in building up an useless structure. I can thus only understand a fact with which I was much struck when examining cirripedes, and of which many

analogous

other

instances could be given: namely, that when a cirripede is parasitic within another

cirripede and

and

is thus protected, it loses more or less completely

its

own shell or carapace. This is the case with the male Ibla, and in a truly extraordinary manner with the Proteolepas: for the carapace in all other cirripedes consists of the three highly-important anterior segments of the head enormously developed, and furnished with great nerves and muscles; but in the parasitic and protected Proteolepas, the whole anterior part of the head is reduced to the merest rudiment