arck, whose disciple Mr. Spencer would
appear to be,[345] admits "a certain peculiar, but limited power of
response and adaptation in each animal and plant"--to the conditions of
their existence. "Such theories," says Mr. Mivart, "have not to contend
against the difficulty proposed, and it has been urged that even very
complex extremely similar structures have again and again been developed
quite independently one of the other, and this because the process has
taken place not by merely haphazard, indefinite variations in all
directions, but by the concurrence of some other internal natural law or
laws co-operating with external influences and with Natural Selection in
the evolution of organic forms.

"_It must never be forgotten that to admit any such constant operation
of any such unknown natural cause is to deny the purely Darwinian theory
which relies upon the survival of the fittest by means of minute
fortuitous indefinite variations._

"Among many other obligations which the author has to acknowledge to
Professor Huxley, are the pointing out of this very difficulty, and the
calling his attention to the striking resemblance between certain teeth
of the dog, and of the thylacine, as one instance, and certain ornithic
peculiarities of pterodactyles as another."[346]

In brief then, changed distribution of use and disuse in consequence of
changed conditions of the environment is with Lamarck the main cause of
modification. According to Mr. Darwin natural selection, or the survival
of favourable but accidental variations, is the most important means of
modification. In a word, with Lamarck the variations are definite; with
Mr. Darwin indefinite.

FOOTNOTES:

[333] Vol. ii. chap. i.

[334] Vol. ii. chap, xxxiv., ed. 1872.

[335] 'Philosophie Zoologique,' ed. M. Martins, Paris, Lyons, 1873, tom.
i. p. 24.

[336] 'Philosophie Zoologique,' tom. i. p. 72.

[337] 'Origin of Species,' p. 3.

[338] Ibid. p. 5.

[339] 'Origin of Species,' p. 8.

[340] Ibid. p. 9.

[341] Ibid. p. 158.

[342] Ibid. p. 159.

[343] 'Origin of Species,' p. 4.

[344] 'Genesis of Species,' p. 74, 1871.

[345] See _ante_, p. 330, line 1 after heading.

[346] 'Genesis of Species,' p. 76, ed. 1871.

CHAPTER XX.

NATURAL SELECTION CONSIDERED AS A MEANS OF MODIFICATION. THE CONFUSION
WHICH THIS EXPRESSION OCCASIONS.

When Mr. Darwin says that natural selection is the most important
"means" of modification, I am not sure