Today | News | Books | Recipes Adventure | Science Fiction | Ghost stories | Poetry | Children | History Booky made complete by Natural
Selection, whether the sterile varieties originate together in a
definite portion of the area occupied by the two species, or occur
scattered over the whole area. (211/4. The first part of this discussion
should be considered alone, as it is both more simple and more
important. I now believe that the utility, and therefore the cause of
sterility between species, is during the process of differentiation.
When species are fully formed, the occasional occurrence of hybrids
is of comparatively small importance, and can never be a danger to the
existence of the species. A.R.W. (1899).)
P.S.--In answer to the objection as to the unequal sterility of
reciprocal crosses ("Variation, etc." Volume II., page 186) I
reply that, as far as it went, the sterility of one cross would be
advantageous even if the other cross was fertile: and just as characters
now co-ordinated may have been separately accumulated by Natural
Selection, so the reciprocal crosses may have become sterile one at a
time.
LETTER 212. TO A.R. WALLACE. 4, Chester Place, March 17th, 1868.
(212/1. Mr. Darwin had already written a short note to Mr. Wallace
expressing a general dissent from his view.)
I do not feel that I shall grapple with the sterility argument till my
return home; I have tried once or twice, and it has made my stomach feel
as if it had been placed in a vice. Your paper has driven three of my
children half mad--one sat up till 12 o'clock over it. My second son,
the mathematician, thinks that you have omitted one almost inevitable
deduction which apparently would modify the result. He has written out
what he thinks, but I have not tried fully to understand him. I suppose
that you do not care enough about the subject to like to see what he has
written.
LETTER 212A. A.R. WALLACE TO CHARLES DARWIN. Hurstpierpoint, March, 24th
[1868].
I return your son's notes with my notes on them. Without going into any
details, is not this a strong general argument?
1. A species varies occasionally in two directions, but owing to their
free intercrossing the varieties never increase.
2. A change of conditions occurs which threatens the existence of the
species; but the two varieties are adapted to the changing conditions,
and if accumulated will form two new species adapted to the new
conditions.
3. Free crossing, however, renders this impossible, and so the species
is in danger of extinction.
4. If sterility would be induced, then the pure races would increase
more rapidly, and replace the old species.
5. It is admitted that partial sterility between varieties does
occasionally occur. It is admitted [that] the degree of this sterility
varies; is it not probable that Natural Selection can accumulate these
variations, and thus save the species? If Natural Selection can NOT do
this, how do species ever arise, except when a variety is isolated?
Closely allied species in distinct countries being sterile is no
difficulty; for either they diverged from a common ancestor in contact,
and Natural Selection increased the sterility, or they were isolated,
and have varied since: in which case they have been for ages influenced
by distinct conditions which may well produce sterility.
If the difficulty of grafting was as great as the difficulty of
crossing, and as regular, I admit it would be a most serious objection.
But it is not. I believe many distinct species can be grafted, while
others less distinct cannot. The regularity with which natural species
are sterile together, even when very much alike, I think is an argument
in favour of the sterility having been generally produced by Natural
Selection for the good of the species.
The other difficulty, of unequal sterility of reciprocal crosses, seems
none to me; for it is a step to more complete sterility, and as such
would be increased by selection.
LETTER 213. TO A.R. WALLACE. Down, April 6th [1868].
I have been considering the terrible problem. Let me first say that
no man could have more earnestly wished for the success of Natural
Selection in regard to sterility than I did; and when I considered a
general statement (as in your last note) I always felt sure it could be
worked out, but always failed in detail. The cause being, as I
believe, that Natural Selection cannot effect what is not good for the
individual, including in this term a social community. It would take a
volume to discuss all the points, and nothing is so humiliating to me
as to agree with a man like you (or Hooker) on the premises and disagree
about the result.
I agree with my son's argument and not with the rejoinder. The cause of
our difference, I think, is that I look at the number of offspring as an
important element (all circumstances remaining the same) in keeping up
the average number of individuals within any area. I do not believe that
the amount of food by any means is the sole determining cause of number.
Lessened fertility is equivalent to a new source of destruction. I
believe if in one district a species produced from any cause fewer
young, the deficiency would be supplied from surrounding districts. This
applies to your Paragraph 5. (213/1. See Letter 211.) If the species
produced fewer young from any cause in every district, it would become
extinct unless its fertility were augmented through Natural Selection
(see H. Spencer).
I demur to probability and almost to possibility of Paragraph 1., as
you start with two forms within the same area, which are not mutually
sterile, and which yet have supplanted the parent-form.
(Paragraph 6.) I know of no ghost of a fact supporting belief that
disinclination to cross accompanies sterility. It cannot hold with
plants, or the lower fixed aquatic animals. I saw clearly what an
immense aid this would be, but gave it up. Disinclination to cross seems
to have been independently acquired, probably by Natural Selection; and
I do not see why it would not have sufficed to have prevented incipient
species from blending to have simply increased sexual disinclination to
cross.
(Paragraph 11.) I demur to a certain extent to amount of sterility and
structural dissimilarity necessarily going together, except indirectly
and by no means strictly. Look at vars. of pigeons, fowls, and cabbages.
I overlooked the advantage of the half-sterility of reciprocal crosses;
yet, perhaps from novelty, I do not feel inclined to admit probability
of Natural Selection having done its work so queerly.
I will not discuss the second case of utter sterility, but your
assumptions in Paragraph 13 seem to me much too complicated. I cannot
believe so universal an attribute as utter sterility between remote
species was acquired in so complex a manner. I do not agree with
your rejoinder on grafting: I fully admit that it is not so closely
restricted as crossing, but this does not seem to me to weaken the case
as one of analogy. The incapacity of grafting is likewise an invariable
attribute of plants sufficiently remote from each other, and somet |