Chapter X: , this paper proceeds, with the exception of a few verbal

alterations, as follows.

[140] Vol. xli. p. 438.

"There is, however, one respect in which Professor Weismann's
statement of the principle of panmixia differs from that which was
considered by Mr. Darwin; and it is this difference of
statement--which amounts to an important difference of theory--that
I now wish to discuss.

"The difference in question is, that while Professor Weismann
believes the cessation of selection to be capable of inducing
degeneration down to the almost complete disappearance of a
rudimentary organ, I have argued that, _unless assisted by some
other principle_, it can at most only reduce the degenerating organ
to considerably above one-half its original size--or probably not
through so much as one-quarter. The ground of this argument (which
is given in detail in the _Nature_ articles of 1873-1874) is, that
panmixia depends for its action upon fortuitous variations round an
ever-diminishing average--the average thus diminishing because it
is no longer _sustained_ by natural selection. But although no
longer sustained by _natural selection_, it does continue to be
sustained by _heredity_; and therefore, as long as the force of
heredity persists unimpaired, fortuitous variations alone--or
variation which is no longer controlled by natural
selection--cannot reduce the dwindling organ to so much as one-half
of its original size; indeed, as above foreshadowed, the balance
between the positive force of heredity and the negative effects of
promiscuous variability will most likely be arrived at above the
middle line thus indicated. Only if for any reason the force of
heredity begins to fail can the average round which the cessation
of selection works become a progressively diminishing average. In
other words, so long as the original force of heredity as regards
the useless organ remains unimpaired, the mere withdrawal of
selection cannot reduce the organ much below the level of
efficiency above which it was previously _maintained_ by the
_presence_ of selection. If we take this level to be 80 or 90 per
cent. of the original size, cessation of selection will reduce the
organ through the 10 or 20 per cent., and there leave it
fluctuating about this average, unless for any reason the force of
heredity begins to fail--in which case, of course, the average will
progressively fall in proportion to the progressive weakening of
this force.

"Now, according to my views, the force of heredity under such
circumstances is always bound to fail, and this for two reasons. In
the first place, it must usually happen that when an organ becomes
useless, natural selection as regards that organ will not only
_cease_, but become _reversed_. For the organ is now absorbing
nutriment, causing weight, occupying space, and so on, _uselessly_.
Hence, even if it be not also a source of actual danger, 'economy
of growth' will determine a reversal of selection against an organ
which is now not merely useless, but deleterious. And this
degenerating influence of the reversal of selection will throughout
be assisted by the cessation of selection, which will now be always
acting round a continuously sinking average. Nevertheless, a point
of balance will eventually be reached in this case, just as it was
in the previous case where the cessation of selection was supposed
to be working alone. For, where the reversal of selection has
reduced the diminishing organ to so minute a size that its presence
is no longer a source of detriment to the organism, the cessation
of selection will carry the reduction a small degree further; and
then the organ will remain as a 'rudiment.' And so it will remain
permanently, unless there be some further reason why the still
remaining force of heredity should be abolished. This further (or
second) reason I found in the consideration that, however enduring
we may suppose the force of heredity to be, we cannot suppose that
it is actually everlasting; and, therefore, that we may reasonably
attribute the eventual disappearance of rudimentary organs to the
eventual failure of heredity itself. In support of this view there
is the fact that rudimentary organs, although very persistent, are
not everlasting. That they should be very persistent is what we
should expect, if the hold which heredity has upon them is great in
proportion to the time during which they were originally useful,
and thus firmly stamped upon the organization by natural selection
causing them to be strongly inherited in the first instance. For
example, we might expect that it would be more difficult finally to
eradicate the rudiment of a wing than the rudiment of a feather;
and accordingly we find it a general rule that long-enduring
rudiments are rudiments of organs distinctive of the higher
taxonomic divisions--i.e. of organs which were longest in building
up, and therefore longest sustained in a state of working
efficiency.

"Thus, upon the whole, my view of the facts of degeneration remains
the same as it was when first published in these columns seventeen
years ago, and may be summarized as follows.

"The cessation of selection when working alone (as it probably does
during the first centuries of its action upon structures or colours
which do not entail any danger to, or perceptible drain upon, the
nutritive resources of the organism) cannot cause degeneration
below, probably, some 10 to 20 per cent. But if from the first the
cessation of selection has been assisted by the _reversal_ of
selection (on account of the degenerating structure having
originally been of a size sufficient to entail a perceptible drain
on the nutritive resources of the organism, having now become a
source of danger, and so forth), the two principles acting together
will continue to reduce the ever-diminishing structure down to the
point at which its presence is no longer a perceptible disadvantage
to the species. When that point is reached, the reversal of
selection will terminate, and the cessation of selection will not
then be able of itself to reduce the organ through more than at
most a very few further percentages of its original size. But,
after this point has been reached, the now total absence of
selection, either for or against the organ, will sooner or later
entail this further and most important consequence, a failure of
heredity as regards the organ. So long as the organ was of use, its
efficiency was constantly _maintained_ by the _presence_ of
selection--which is merely another way of saying that selection was
constantly maintaining the force of heredity as regards that organ.
But as soon as the organ ceased to be of use, selection ceased to
maintain the force of heredity; and thus, sooner or later, that
force began to waver or fade. Now it is this wavering or fading of
the force of heredity, thus originally due to the cessation of
selection, that in turn co-operates with the still continued
cessation of selection in reducing the structure below the level
where its reduction was left by the actual reversal of selection.
So that from that level downwards the cessation of selection, and
the consequent failing of heredity, act and react in their common
work of causing obsolescence. In the case of newly added
characters, the force of heredity will be less than in that of more
anciently added characters; and thus we can understand the long
endurance of 'vestiges' characteristic of the higher taxonomic
divisions, as compared with those characteristic of the lower. But
in all cases, if time enough be allowed under the cessation of
selection, the force of heredity will eventually fall to zero, when
the hitherto obsolescent structure will finally become obsolete. In
cases of newly added and comparatively trivial characters, with
regard to which reversal of selection is not likely to take place
(e.g. slight differences of colour between allied species),
cessation of selection is likely to be very soon assisted by a
failure in the force of heredity; seeing that such newly added
characters will not be so strongly inherited as are the more
ancient characters distinctive of higher taxonomic groups.

"Let us now turn to Weismann's view of degeneration. First of all,
he has omitted to perceive that 'panmixia' alone (if unassisted
either by reversed selection or an inherent diminishing of the
force of heredity) cannot reduce a functionless organ to the
condition of a _rudiment_. Therefore he everywhere represents
panmixia (or the mere _cessation_ of selection) as of itself
sufficient to cause degeneration, say from 100 to 5, instead of
from 100 to 90 or 80, which, for the reasons above given, appeared
(and still appears) to me about the most that this principle can
accomplish, so long as the original force of heredity continues
unimpaired. No doubt we have here what must be regarded as a mere
oversight on the part of Professor Weismann; but the oversight is
rendered remarkable by the fact that he _does_ invoke the aid of
reversed selection _in order to explain the final disappearance of
a rudiment_. Yet it is self-evident that the reversal of selection
must be much more active during the initial than during the final
stages of degeneration, seeing that, _ex hypothesi_, the greater
the degree of reduction which has been attained the less must be
the detriment arising from any useless expenditure of nutrition,
&c.

"And this leads me to a second oversight in Professor Weismann's
statement, which is of more importance than the first. For the
place at which he does invoke the assistance of reversed selection
is exactly the place at which reversed selection must necessarily
have ceased to act. This place, as already explained, is where an
obsolescent organ has become rudimentary, or, as above supposed,
reduced to 5 per cent. of its original size; and the reason why he
invokes the aid of reversed selection at this place is in order to
save his doctrine of 'the stability of germ-plasm.' That the force
of heredity should finally become exhausted if no longer
_maintained_ by the _presence_ of selection, is what Darwin's
theory of perishable gemmules would lead us to expect, while such a
fact would be fatal to Weismann's theory of an imperishable
germ-plasm. Therefore he seeks to explain the eventual failure of
heredity (which is certainly a fact) by supposing that after the
point at which the cessation of selection alone can no longer act
(and which his first oversight has placed some 80 per cent. too
low), the reversal of selection will begin to act directly against
the force of heredity as regards the diminishing organ, until such
direct action of reversed selection will have removed the organ
altogether. Or, in his own words, 'The complete disappearance of a
rudimentary organ can only take place by the operation of natural
selection; this principle will lead to its diminution, inasmuch as
the disappearing structure takes the place and the nutriment of
other useful and important organs.' That is to say, the
rudimentary organ finally disappears, not because the force of
heredity is finally exhausted, but because natural selection has
begun to utilize this force against the continuance of the
organ--always picking out those congenital variations of the organ
which are of smallest size, and thus, by its now _reversed_ action,
_reversing_ the force of heredity as regards the organ.

"Now the oversight here is in not perceiving that the smaller the
disappearing structure becomes, the less hold must 'this principle'
of reversed selection retain upon it. As above observed, during the
earlier stages of reduction (or while co-operating with the
cessation of selection) the reversal of selection will be at its
_maximum_ of efficiency; and, as the process of diminution
continues, a point must eventually be reached at which the reversal
of selection can no longer act. Take the original mass of a now
obsolescent organ in relation to that of the entire organism of
which it then formed a part to be represented by the ratio 1:100.
For the sake of argument we may assume that the mass of the
organism has throughout remained constant, and that by 'mass' in
both cases is meant capacity for absorbing nutriment, causing
weight, occupying space, and so forth. Now, we may further assume
that when the mass of the organ stood to that of its organism in
the ratio of 1:100, natural selection was strongly reversed with
respect to the organ. But when this ratio fell to 1:1000, the
activity of such reversal must have become enormously diminished,
even if it still continued to exercise any influence at all. For we
must remember, on the one hand, that the reversal of selection can
only act as long as the presence of a diminishing organ continues
to be so injurious that variations in its size are matters of life
and death in the struggle for existence; and, on the other hand,
that natural selection in the case of the diminishing organ does
not have reference to the presence and the absence of the organ,
but only to such variations in its mass as any given generation may
supply. Now, the process of reduction does not end even at 1:1000.
It goes on to 1:10,000, and eventually 1:[infinity]. Consequently,
however great our faith in natural selection may be, a point must
eventually come for all of us at which we can no longer believe
that the reduction of an obsolescent organ is due to reversed
selection. And I cannot doubt that if Professor Weismann had
sufficiently considered the matter, he would not have committed
himself to the statement that 'the complete disappearance of a
rudimentary organ can only take place by the operation of natural
selection.'

"According to my view, the complete disappearance of a rudimentary
organ can only take place by the _cessation_ of natural selection,
which permits the eventual exhaustion of heredity, when heredity is
thus simply left to itself. During all the earlier stages of
reduction, the cessation of selection was assisted in its work by
the reversal of selection; but when the rudiment became too small
for such assistance any longer to be supplied, the rudiment
persisted in that greatly reduced condition until the force of
heredity with regard to it was eventually worn out. This appears to
me, as it appeared in 1873, the only reasonable conclusion that can
be drawn from the facts. And it is because this conclusion is fatal
to Professor Weismann's doctrine of the permanent 'stability' of
germ-plasm, while quite in accordance with all theories which
belong to the family of pangenesis, that I deem the facts of
degeneration of great importance as tests between these rival
interpretations of the facts of heredity. It is on this account
that I have occupied so much space with the foregoing discussion;
and I shall be glad to ascertain whether any of the followers of
Professor Weismann are able to controvert these views.

"GEORGE J. ROMANES."

"P.S.--Since the above article was sent in, Professor Weismann has
published in these columns (February 6) his reply to a criticism by
Professor Vines (October 24, 1889). In this reply he appears to
have considerably modified his views on the theory of degeneration;
for while in his Essays he says (as in the passage above quoted)
that 'the complete disappearance of a rudimentary organ can only
take place by the operation of natural selection'--i.e. only by the
_reversal_ of selection,--in his reply to Professor Vines he says,
'I believe that I have proved that organs no longer in use become
rudimentary, and must finally disappear, solely by 'panmixia'; not
through the direct action of disuse, but because natural selection
no longer sustains their standard structure'--i.e. solely by the
_cessation_ of selection. Obviously, there is here a flat
contradiction. If Professor Weismann now believes that a
rudimentary organ 'must finally disappear _solely_' through the
_withdrawal_ of selection, he has abandoned his previous belief
that 'the complete disappearance of a rudimentary organ can _only_
take place by the _operation_ of selection.' And this change of
belief on his part is a matter of the highest importance to his
system of theories as a whole, since it betokens a surrender of his
doctrine of the 'stability' of germ-plasm--or of the virtually
everlasting persistence of the force of heredity, and the
consequent necessity for a reversal of this force itself (by
natural selection placing its premium on _minus_ instead of on
_plus_ variations), in order that a rudimentary organ should
finally disappear. In other words, it now seems he no longer
believes that the force of heredity in one direction (that of
sustaining a rudimentary organ) can only be abolished by the active
influence of natural selection determining this force in the
opposite direction (that of removing a rudimentary organ). It seems
he now believes that the force of heredity, if merely left to
itself by the withdrawal of natural selection altogether, will
sooner or later become exhausted through the mere lapse of time.
This, of course, is my own theory of the matter as originally
published in these columns; but I do not see how it is to be
reconciled with Professor Weismann's doctrine of so high a degree
of stability on the part of germ-plasm, that we must look to the
Protozoa and the Protophyta for the original source of congenital
variations as now exhibited by the Metazoa and Metaphyta.
Nevertheless, and so far as the philosophy of degeneration is
concerned, I shall be very glad if (as it now appears) Professor
Weismann's more recent contemplation has brought his principle of
panmixia into exact coincidence with that of my cessation of
selection."

Before passing on it may here be noted that, to any one who believes in the inheritance of acquired characters, there is open yet another hypothetical cause of degeneration, and one to which the final disappearance of vestigial organs may be attributed. Roux has shown in his work on _The Struggle for Existence between Parts of an Organism_ that the principle of selection must operate in every constituent tissue, and as between every constituent cell of which an organism is composed. Now, if an organ falls into disuse, its constituent cells become worsted in their struggles with other cells in the organism. Hence, degeneration of the disused organ may progressively increase, quite independently of any struggle for existence on the part of the organism as a whole. Consequently, degeneration may proceed without any reference to the principle of "economized nutrition"; and, if it does so, and if the effects of its doing so are transmitted from generation to generation, the disused organ will finally disappear by means of Roux's principle.

The long communication above quoted led to a still longer correspondence in the pages of _Nature_. For Professor Ray Lankester wrote[141] to impugn the doctrine of panmixia, or cessation of selection, _in toto_, arguing with much insistence that "cessation of selection must be supplemented by economy of growth in order to produce the results attributed to panmixia." In other words, he denied that panmixia alone can cause degeneration in any degree at all; at most, he said, it can be but "a condition," or "a state," which occurs when an organ or part ceases to be useful, and therefore falls under the degenerating influence of active causes, such as economy of nutrition. Or, in yet other words, he refused to recognize that any degenerative process can be due to natural selection as merely withdrawn: only when, besides being _withdrawn_, natural selection is _reversed_, did he regard a degenerative process as possible. As a result of the correspondence, however, he eventually[142] agreed that, if the "birth-mean" of an organ, in respect either of size or complexity of structure, be lower than the "selection-mean" while the organ is useful (a fact which he does not dispute); then, if the organ ceases to be useful, it will degenerate by the withdrawal of selection alone. Which, of course, is merely a re-statement of the doctrine of panmixia, or cessation of selection, in somewhat varied terminology--provided that the birth-mean be taken over a number of generations, or not only over a few following the selection-mean of the structure while still in its highest state of efficiency. For the sake of brevity I will hereafter speak of these "few following" generations by the term of "first generations."

[141] _Nature_, vol. xli. p. 486.

[142] _Ibid._ vol. xlii. p. 52.

It remains to consider the views of Professor Lloyd Morgan upon the subject. In my opinion he is the shrewdest, as well as the most logical critic that we have in the field of Darwinian speculation; therefore, if possible, I should like to arrive at a full agreement with him upon this matter. His latest utterance with regard to it is as follows:--

"To account for the diminution of organs or structures no longer of
use, apart from any inherited effects of disuse, Mr. Romanes has
invoked the Cessation of Selection; and Mr. Francis Galton has, in
another connexion, summarized the effects of this cessation of
selection in the convenient phrase 'Regression to Mediocrity.' This
is the Panmixia of Professor Weismann and his followers; but the
phrase regression to mediocrity through the cessation of selection
appears to me preferable. It is clear that so long as any organ or
structure is subject to natural selection through elimination, it
is, if not actually undergoing improvement, kept at a high standard
of efficiency through the elimination of all those individuals in
which the organ in question falls below the required standard. But
if, from change in the environment or any other cause, the
character in question ceases to be subject to selection,
elimination no longer takes place, and the high standard will no
longer be maintained. There will be reversion to mediocrity. The
probable amount of this reversion is at present a matter under
discussion[143]."

[143] _Presidential Address to the Bristol Naturalists' Society_,
1891.

So far, then, Professor Lloyd Morgan is in complete agreement with previous writers upon the subject. He does not doubt that the cessation of selection must always be a cause of degeneration: the only question is as to the _potency_ of this cause, or the _amount_ of degeneration which it is capable of effecting.

Taking, first, the case of bulk or size of an organ, as distinguished from its organization or complexity, we have seen that Weismann represents the cessation of selection--even if working quite alone, or without any assistance from the reversal of selection--to be capable of reducing a fully developed organ to the state of a rudiment, or even, if we take his most recent view, of abolishing the organ _in toto_.

Professor Lloyd Morgan, on the other hand, does not think that the cessation of selection alone can cause reduction further than the level of "mediocrity" in the first generations--or, which is much the same thing, further than the difference between the "birth-mean" and the "selection-mean" of the first generations. This amount of reduction he puts at 5 per cent., as "a very liberal estimate."

Here, then, we have three estimates of the amount of degeneration which can be produced by panmixia alone, where mere size or bulk of an organ is concerned--say, 3 to 5 per cent., 10 to 20 per cent., and 95 per cent. to 0. At first sight, these differences appear simply ludicrous; but on seeking for the reasons of them, we find that they are due to different views touching the manner in which panmixia operates. The oversights which have led to Weismann's extremely high estimate have already been stated. The reason of the difference between the extremely low estimate of Professor Lloyd Morgan, as compared with my own intermediate one, is, that he supposes the power of panmixia to become exhausted as soon as the level of mediocrity of the first generations has become the general level in succeeding generations. In my view, however, the level of mediocrity is itself a sinking level in successive generations, with the result that there is no reason why the reducing power of panmixia should ever become exhausted, save that the more reduction it effects the greater is the force of heredity which remains to be overcome, as previously explained. Thus the only question between Professor Lloyd Morgan and myself is--Does the level of mediocrity fall in successive generations under the cessation of selection, or does it remain permanently where it used to be under the presence of selection? Does the "birth-mean" remain constant throughout any number of generations, notwithstanding that the sustaining influence of selection has been withdrawn; or does it progressively sink as a consequence of such withdrawal?

In order to answer this question we had better begin by considering now the case of organization of structure, as distinguished from mere size of structure. Take any case where a complex organ--such as a compound eye--has been slowly elaborated by natural selection, and is it not self-evident that, when natural selection is withdrawn, the complex structure will deteriorate? In other words, the level of mediocrity, say in the hundred thousandth generation after the sustaining influence of natural selection has been withdrawn, will not be so high as it was in the first generations. For, by hypothesis, there is now no longer any elimination of unfavourable variations, which may therefore perpetuate themselves as regards any of the parts of this highly complex mechanism; so that it is only a matter of time when the mechanism must become disintegrated. I can scarcely suppose that any one who considers the subject will question this statement, and therefore I will not say anything that might be said in the way of substantiating it. But, if the statement be assented to, it follows that there is no need to look for any cause of deterioration, further than the withdrawal of selection--or cessation of the principle which (as we are supposing) had hitherto been the sole means of maintaining efficient harmony among all the independently variable parts of the highly complex structure.

Now, I hold that the same thing is true, though in a lesser degree, as regards degeneration of size. That there is no difference _in kind_ between the two cases, Professor Lloyd Morgan implicitly allows; for what he says is--

"In any long-established character, such as wing-power in birds,
brain-development, the eyes of crustacea, &c., no shortcomer in
these respects would have been permitted by natural selection to
transmit his shortcomings for hundreds of generations. All tendency
to such shortcomings would, one would suppose, have been bred out
of the race. If after this long process of selection there still
remains a strong tendency to deterioration, this tendency demands
an explanation[144]."

[144] _Presidential Address to the Bristol Naturalists' Society_,
1891.

Here, then, deterioration as to size of structure (wings of birds), and deterioration as to complexity of structure (brain and eyes) are expressly put upon the same footing. Therefore, if in the latter case the "tendency to deterioration" does not "demand an explanation," beyond the fact that the hitherto maintaining influence has been withdrawn, neither is any such further explanation demanded in the former case. Which is exactly my own view of the matter. It is also Mr. Galton's view. For although, in the passage formerly quoted, Professor Lloyd Morgan appears to think that by the phrase "Regression to Mediocrity" Mr. Galton means to indicate that panmixia can cause degeneration only as far as the mediocrity level of the first generations, this, in point of fact, is not what Galton means, nor is it what he says. The phrase in question occurs "in another connexion," and, indeed, in a different publication. But where he expressly alludes to the cessation of selection, this is what he says. The italics are mine.

"A special cause may be assigned for the effects of use in causing
hereditary _atrophy_ of disused parts. It has already been shown
that all exceptionally developed organs tend to deteriorate:
consequently, those that are not _protected_ by selection will
_dwindle_. The level of muscular efficiency in the wing of a
strongly flying bird [curiously enough, the same case that is
chosen by Professor Lloyd Morgan to illustrate his opposite view],
is like the level of water in the leaky vessel of a Danaid, only
secured to the race by _constant effort_, so to speak. _Let the
effort be relaxed ever so little, and the level immediately
falls[145]._"

[145] _A Theory of Heredity_, Journal of Anthropological Institute,
1875. Vol. v. p. 345.

I take it, then, that the burden of proof lies with Professor Lloyd Morgan to show why the withdrawal of selection is _not_ sufficient to account for degeneration any further than the mediocrity-level in the former presence of selection. Why does "the strong tendency[146] to deterioration demand an explanation," further than the fact that when all variations below the average in every generation are allowed to survive, they must gradually lower the average itself through a series of generations? To answer that any such tendency "would have been bred out of the race" by the previous action of selection, is to suppose that the function of selection is at an end when once it has built up a structure to the highest point of working efficiency,--that the presence of selection is no longer required to _maintain_ the structure at that point. But it is enough to ask in reply--Why, under the cessation of selection, does _complexity_ of structure degenerate so much more rapidly than _size_ of structure? Why is it, for instance, that "the eyes of crustacea" in dark caves have entirely disappeared, while their foot-stalks (when originally present) still remain? Can it be maintained that "for hundreds of generations" natural selection was more intent on developing the foot-stalks than the eyes which were mounted upon them--so that while the latter were left by selection with "a strong tendency to deterioration," the former have had this tendency "bred out in the race"[147]?

[146] No one has supposed that the tendency need be "strong": it has
only to be persistent.

[147] Of course it must be observed that degeneration of complexity
involves also degeneration of size, so that a more correct
statement of the case would be--Why, under the cessation of
selection, does an organ of extreme complexity degenerate much
more rapidly than one of much less complexity? For example,
under domestication the brains of rabbits and ducks appear to
have been reduced in some cases by as much as 50 per cent.
(Darwin, and Sir J. Crichton Browne.) But if it is possible to
attribute this effect--or part of it--to an artificial
selection of stupid animals, I give in the text an example
occurring under nature. Many other cases, however, might be
given to show the general rule, that under cessation of
selection complexity of structure degenerates more
rapidly--and also more thoroughly--than size of it. This, of
course, is what Mr. Galton and I should expect, seeing that
the more complex a structure the greater are the number of
points for deterioration to invade when the structure is no
longer "protected by selection." (On the other hand, of
course, this fact is opposed to the view that degeneration of
useless structures below the "birth-mean" of the first
generations, is exclusively due to the reversal of selection;
for economy of growth, deleterious effect of weight, and so
forth, ought to affect size of structure _much more_ than
complexity of it.) But I choose the above case, partly because
Professor Lloyd Morgan has himself alluded to "the eyes of
crustacea," and partly because Professor Ray Lankester has
maintained that the loss of these eyes in dark caves is due to
the reversal of selection, as distinguished from the cessation
of it. In view of the above parenthesis it will be seen that
the point is not of much importance in the present connexion;
but it appears to me that cessation of selection must here
have had at least the larger share in the process of atrophy.
For while the economy of nutrition ought to have removed the
relatively large _foot-stalks_ as rapidly as the _eyes_, I
cannot see that there is any advantage, other than the economy
of nutrition, to be gained by the rapid loss of hard-coated
_eyes_, even though they have ceased to be of use.

To sum up. There is now no question in any quarter touching the fact that panmixia, or the cessation of selection, is a true cause of degeneration. The only question is as to the amount of degeneration which it is able to effect when not assisted by the reversal of selection, or any other cause of degeneration. Moreover, even with regard to this question of amount, there is no doubt on any side that panmixia alone causes degeneration _more rapidly_ where it has to do with complexity of organization, than it does where it is concerned with a mere reduction of mass.

The question as to the amount of degeneration that is caused by the cessation of selection alone is without any practical importance where species in a state of nature are concerned, because here the cessation of selection is probably always associated more or less with the reversal of it; and it is as impossible as it is immaterial to determine the relative shares which these two co-operating principles take in bringing about the observed results. But where organisms in a state of domestication are concerned, the importance of the question before us is very great. For if the cessation of selection alone is capable of reducing an organ through 10 or 12 per cent. of its original size, nearly all the direct evidence on which Darwin relied in favour of use-inheritance is destroyed. On the other hand, if reduction through 5 per cent. be deemed a "very liberal estimate" of what this principle can accomplish, the whole body of Darwin's direct evidence remains as he left it. I have now given my reasons for rejecting this lower estimate on the one band, and what seems to me the extravagant estimate of Weismann on the other. But my own intermediate estimate is enough to destroy the apparent proof of use-inheritance that was given by Darwin. Therefore it remains for those who deny Lamarckian principles, either to accept some such estimate, or else to acknowledge the incompatibility of any lower one with the opinion that there is no evidence in favour of these principles.