Chapter IV: Variation and Natural Selection (3)

[Q] We are anxious to extend our observations and to compare series of
bats from different localities. If any of my readers should feel
disposed to help us, by sending specimens (_with the locality duly
indicated_) to Mr. H. Charbonnier, 7, The Triangle South, Clifton,
Bristol, we shall be grateful.

[R] _Nature_, vol. xli. p. 393. The variation in molluscs is often
considerable. In one of the bays in the basement hall of the
Natural History Museum is a series showing the variation in size,
form, and sculpturing of _Paludomus loricatus_, which is found in
the streams of Ceylon. These varieties have in former times been
named as ten distinct species!

[S] More observations and fuller knowledge on this latter point and on
the relative numbers of the sexes in different species are much to
be desired. It is clear that the number of offspring mainly
depends upon the number of females. But if it be true that good
times and favourable conditions lead to an increased production of
females, while hard times and unfavourable conditions lead to a
relative increase of males, then it is evident that good times
will lead to a more rapid increase and hard times to a less rapid
increase of the species. Suppose, for example, in a particular
district food and other conditions were especially favourable for
frogs. Among the well-nourished tadpoles there would be a
preponderance of females. In the next generation the many females
would produce abundant offspring (for one male may fertilize the
ova laid by several females). There would be a greater number of
tadpoles to compete for the same amount of nutriment. They would
be less nourished. There would be less females; and in the
succeeding generation a diminished number of tadpoles. Thus to
some extent a balance between the number of tadpoles and the
amount of available nutrition would be maintained. These
conclusions are, perhaps, too theoretical to be of much value,
while the tendency here indicated would be but one factor among
many.

[T] "Origin of Species," pp. 62, 63.

[U] "Animals and Plants under Domestication," vol. ii. p. 177.

[V] I may here draw attention to the fact that the bats whose wing-bone
measurements were given above are those which have so far survived
and escaped such elimination as is now in progress.

[W] "Origin of Species," p. 109.

[X] "Darwinism," p. 106.

[Y] Ibid. p. 106.

[Z] Proceedings Liverpool Biological Society, 1889.

[AA] Since this chapter was written, Mr. Poulton has published his
interesting and valuable work on "The Colours of Animals," from
which I have contrived to insert one or two additional examples.

[AB] _Ann. and Mag. Nat. Hist._, September, 1889, p. 209, quoted by
Poulton, "Colours of Animals," p. 55.

[AC] Nature, vol. xxxv. p. 77.

[AD] Many other instances might be added. The hornet clear-wing moth
(_Sphecia apiformis_) mimics the hornet or wasp; the
narrow-bordered bee-hawk moth (_Sesia bombyliformis_) mimics a
bumble-bee. These insects may be seen in the lepidoptera drawers
in the Natural History Museum. But perhaps the most wonderful
instance of insect-mimicry is that observed by Mr. W. L. Sclater,
and given by Mr. E. B. Poulton, in his "Colours of Animals" (p.
252), where a (probably) homopterous insect mimics a leaf-cutting
ant, _together with its leafy burden_--a membranous expansion in
the mimic closely resembling the piece of leaf carried by the
particular kind of ant he resembles.

[AE] The late Mr. H. W. Oakley first drew my attention to this snake.
Since then Mr. Hammond Tooke has described the facts in _Nature_,
vol. xxxiv. p. 547.

[AF] _Nature_, vol. xlii. p. 115.

[AG] Since the above was written and sent to press, there has been
added, at the Natural History Museum, in the basement hall, a case
illustrating the adaptation of external colouring to the
conditions of life. All the animals, birds, etc., there grouped
were collected in the Egyptian desert, whence also the rocks,
stones, and sand on which they are placed were brought. Though
somewhat crowded, they exemplify protective resemblance very well.

[AH] I have to thank Mr. H. A. Francis for drawing my attention to
this, and showing me the insects in his cabinet.

[AI] "Colours of Animals," p. 73.

[AJ] "Origin of Species," p. 161.

[AK] "Descent of Man," summary of chap. xvi. pt. ii.

[AL] Ibid. chap. xiv.

[AM] "Darwinism," p. 108.

[AN] Its importance in artificial selection was emphasized by Darwin:
"The prevention of free crossing, and the intentional matching of
individual animals, are the corner-stones of the breeder's art"
("Animals and Plants under Domestication," ii. 62).

[AO] From the absence of interblending in some cases (to be considered
shortly), both brown _and_ green forms may be produced; and under
certain circumstances, even a power of becoming either brown _or_
green in the presence of appropriate stimuli.

[AP] Wallace, "Darwinism," p. 172, where other examples are cited.

[AQ] Ibid. pp. 217, _et seq._

[AR] _Journal of the Linnæan Society_, vol. xix. No. 115: "Zoology."

[AS] "Animals and Plants under Domestication," p. 145.

[AT] Ibid. chap. xvii.

[AU] "Darwinism," p. 326.

[AV] "Animals and Plants under Domestication," vol. ii. p. 65. For
Darwin's general conclusions on hybridism, see vol. ii. p. 162 of
the same work.

[AW] "In every case there are two factors, namely, the nature of the
organism and the nature of the conditions. The former seems to be
much the more important; for nearly similar variations sometimes
arise under, as far as we can judge dissimilar conditions; and, on
the other hand, dissimilar variations arise under conditions which
appear to be nearly uniform" ("Origin of Species," p. 6).

[AX] See "Evolution without Natural Selection," by Charles Dixon. This
author's facts are valuable; his theories are ill digested.

[AY] _Nature_, vol. xlii. p. 136.

[AZ] We may here note, in passing, the fact that the changes of
life-forms in a succession of beds points in nine cases out of ten
rather to substitution through migration than to transmutation.
Still, there are notable cases of transmutation, as in the
fresh-water _Planorbes_ of Steinhem, in Wittenberg (described,
after Hilgendorf, by O. Schmidt, "The Doctrine of Descent," p.
96).

[BA] I would ask historians whether there have not been, in English
history, good times of free and beneficial divergence exemplified
in diverse intellectual activity, hard times of rigorous
elimination, and intermediate times of placid, somewhat humdrum
conservatism.

[BB] Two more technical examples may be noticed in a note. (1)
Professor Haeckel has recently (_Challenger_ Reports, vol.
xxviii.) shown that the Siphonophora include two groups, closely
resembling each other, but of different ancestry: (_a_) The
Disconanthæ, traceable to trachomedusoid ancestors; (_b_) the
Siphonanthæ, traceable to anthomedusoid ancestors like Sarsia. (2)
M. Paul Pelseneer has been led to the conclusion that the pteropod
molluscs also include two groups resembling each other, but of
different ancestry: (_a_) The Thecosomes, traceable to tornatellid
ancestors; (_b_) the Gymnosomes, traceable to aphysiid ancestors.
In each case, the ancestral sea-slug has been modified for a
free-swimming life.

[BC] For evidence in copious abundance, see Nicholson's "Manual of
Palæontology," new edition, vol. i.: "Vertebrata," by R. Lydekker.