DIRECT AND DEFINITE ACTION OF THE EXTERNAL CONDITIONS OF LIFE.
SLIGHT MODIFICATIONS IN PLANTS FROM THE DEFINITE ACTION OF CHANGED CONDITIONS,
IN SIZE, COLOUR, CHEMICAL PROPERTIES, AND IN THE STATE OF THE TISSUES.
LOCAL DISEASES.
CONSPICUOUS MODIFICATIONS FROM CHANGED CLIMATE OR FOOD, ETC.
PLUMAGE OF BIRDS AFFECTED BY PECULIAR NUTRIMENT, AND BY THE INOCULATION OF
POISON.
LAND-SHELLS.
MODIFICATIONS OF ORGANIC BEINGS IN A STATE OF NATURE THROUGH THE DEFINITE
ACTION OF EXTERNAL CONDITIONS.
COMPARISON OF AMERICAN AND EUROPEAN TREES.
GALLS.
EFFECTS OF PARASITIC FUNGI.
CONSIDERATIONS OPPOSED TO THE BELIEF IN THE POTENT INFLUENCE OF CHANGED
EXTERNAL CONDITIONS.
PARALLEL SERIES OF VARIETIES.
AMOUNT OF VARIATION DOES NOT CORRESPOND WITH THE DEGREE OF CHANGE IN THE
CONDITIONS.
BUD-VARIATION.
MONSTROSITIES PRODUCED BY UNNATURAL TREATMENT.
SUMMARY.
If we ask ourselves why this or that character has been modified under
domestication, we are, in most cases, lost in utter darkness. Many
naturalists, especially of the French school, attribute every modification to
the "monde ambiant," that is, to changed climate, with all its diversities of
heat and cold, dampness and dryness, light and electricity, to the nature of
the soil, and to varied kinds and amount of food. By the term definite action,
as used in this chapter, I mean an action of such a nature that, when many
individuals of the same variety are exposed during several generations to any
particular change in their conditions of life, all, or nearly all the
individuals, are modified in the same manner. The effects of habit, or of the
increased use and disuse of various organs, might have been included under
this head; but it will be convenient to discuss this subject in a separate
chapter. By the term indefinite action I mean an action which causes one
individual to vary in one way and another individual in another way, as we
often see with plants and animals after they have been subjected for some
generations to changed conditions of life. But we know far too little of the
causes and laws of variation to make a sound classification. The action of
changed conditions, whether leading to definite or indefinite results, is a
totally distinct consideration from the effects of selection; for selection
depends on the preservation by man of certain individuals, or on their
survival under various and complex natural circumstances, and has no relation
whatever to the primary cause of each particular variation.
I will first give in detail all the facts which I have been able to collect,
rendering it probable that climate, food, etc., have acted so definitely and
powerfully on the organisation of our domesticated productions, that new sub-
varieties or races have been thus formed without the aid of selection by man
or nature. I will then give the facts and considerations opposed to this
conclusion, and finally we will weigh, as fairly as we can, the evidence on
both sides.
When we reflect that distinct races of almost all our domesticated animals
exist in each kingdom of Europe, and formerly even in each district of
England, we are at first strongly inclined to attribute their origin to the
definite action of the physical conditions of each country; and this has been
the conclusion of many authors. But we should bear in mind that man annually
has to choose which animals shall be preserved for breeding, and which shall
be slaughtered. We have also seen that both methodical and unconscious
selection were formerly practised, and are now occasionally practised by the
most barbarous races, to a much greater extent than might have been
anticipated. Hence it is difficult to judge how far differences in the
conditions between, for instance, the several districts in England, have
sufficed to modify the breeds which have been reared in each. It may be argued
that, as numerous wild animals and plants have ranged during many ages
throughout Great Britain, and still retain the same character, the difference
in conditions between the several districts could not have modified in a
marked manner the various native races of cattle, sheep, pigs, and horses. The
same difficulty of distinguishing between the effects of natural selection and
the definite action of external conditions is encountered in a still higher
degree when we compare closely allied species inhabiting two countries, such
as North America and Europe, which do not differ greatly in climate, nature of
soil, etc., for in this case natural selection will inevitably and rigorously
have acted during a long succession of ages.
Prof. Weismann has suggested (23/1. 'Ueber den Einfluss der Isolirung auf die
Artbildung' 1872.) that when a variable species enters a new and isolated
country, although the variations may be of the same general nature as before,
yet it is improbable that they should occur in the same proportional numbers.
After a longer or shorter period, the species will tend to become nearly
uniform in character from the incessant crossing of the varying individuals;
but owing to the proportion of the individuals varying in different ways not
being the same in the two cases, the final result will be the production of
two forms somewhat different from one another. In cases of this kind it would
falsely appear as if the conditions had induced certain definite
modifications, whereas they had only excited indefinite variability, but with
the variations in slightly different proportional numbers. This view may throw
some light on the fact that the domestic animals which formerly inhabited the
several districts in Great Britain, and the half wild cattle lately kept in
several British parks, differed slightly from one another; for these animals
were prevented from wandering over the whole country and intercrossing, but
would have crossed freely within each district or park.
[From the difficulty of judging how far changed conditions have caused
definite modifications of structure, it will be advisable to give as large a
body of facts as possible, showing that extremely slight differences within
the same country, or during different seasons, certainly produce an
appreciable effect, at least on varieties which are already in an unstable
condition. Ornamental flowers are good for this purpose, as they are highly
variable, and are carefully observed. All floriculturists are unanimous that
certain varieties are affected by very slight differences in the nature of the
artificial compost in which they are grown, and by the natural soil of the
district, as well as by the season. Thus, a skilful judge, in writing on
Carnations and Picotees (23/2. 'Gardener's Chronicle' 1853 page 183.) asks
"where can Admiral Curzon be seen possessing the colour, size, and strength
which it has in Derbyshire? Where can Flora's Garland be found equal to those
at Slough? Where do high-coloured flowers revel better than at Woolwich and
Birmingham? Yet in no two of these districts do the same varieties attain an
equal degree of excellence, although each may be receiving the attention of
the most skilful cultivators." The same writer then recommends every
cultivator to keep five different kinds of soil and manure, "and to endeavour
to suit the respective appetites of the plants you are dealing with, for
without such attention all hope of general success will be vain." So it is
with the Dahlia (23/3. Mr. Wildman 'Floricultural Soc.' February 7, 1843
reported in 'Gardener's Chronicle' 1843 page 86.): the Lady Cooper rarely
succeeds near London, but does admirably in other districts; the reverse holds
good with other varieties; and again, there are others which succeed equally
well in various situations. A skilful gardener (23/4. Mr. Robson in 'Journal
of Horticulture' February 13, 1866 page 122.) states that he procured cuttings
of an old and well-known variety (pulchella) of Verbena, which from having
been propagated in a different situation presented a slightly different shade
of colour; the two varieties were afterwards multiplied by cuttings, being
carefully kept distinct; but in the second year they could hardly be
distinguished, and in the third year no one could distinguish them.
The nature of the season has an especial influence on certain varieties of the
Dahlia: in 1841 two varieties were pre-eminently good, and the next year these
same two were pre-eminently bad. A famous amateur (23/5. 'Journal of
Horticulture' 1861 page 24.) asserts that in 1861 many varieties of the Rose
came so untrue in character, "that it was hardly possible to recognise them,
and the thought was not seldom entertained that the grower had lost his
tally." The same amateur (23/6. Ibid 1862 page 83.) states that in 1862 two-
thirds of his Auriculas produced central trusses of flowers, and such trusses
are liable not to keep true; and he adds that in some seasons certain
varieties of this plant all prove good, and the next season all prove bad;
whilst exactly the reverse happens with other varieties. In 1845 the editor of
the 'Gardener's Chronicle' (23/7. 'Gardener's Chronicle' 1845 page 660.)
remarked how singular it was that this year many Calceolarias tended to assume
a tubular form. With Heartsease (23/8. Ibid 1863 page 628.) the blotched sorts
do not acquire their proper character until hot weather sets in; whilst other
varieties lose their beautiful marks as soon as this occurs.
Analogous facts have been observed with leaves: Mr. Beaton asserts (23/9.
'Journal of Hort.' 1861 pages 64, 309.) that he raised at Shrubland, during
six years, twenty thousand seedlings from the Punch Pelargonium, and not one
had variegated leaves; but at Surbiton, in Surrey, one-third, or even a
greater proportion, of the seedlings from this same variety were more or less
variegated. The soil of another district in Surrey has a strong tendency to
cause variegation, as appears from information given me by Sir F. Pollock.
Verlot (23/10. 'Des Varietes' etc. page 76.) states that the variegated
strawberry retains its character as long as grown in a dryish soil, but soon
loses it when planted in fresh and humid soil. Mr. Salter, who is well known
for his success in cultivating variegated plants, informs me that rows of
strawberries were planted in his garden in 1859, in the usual way; and at
various distances in one row, several plants simultaneously became variegated;
and what made the case more extraordinary, all were variegated in precisely
the same manner. These plants were removed, but during the three succeeding
years other plants in the same row became variegated, and in no instance were
the plants in any adjoining row affected.
The chemical qualities, odours, and tissues of plants are often modified by a
change which seems to us slight. The Hemlock is said not to yield conicine in
Scotland. The root of the Aconitum napellus becomes innocuous in frigid
climates. The medicinal properties of the Digitalis are easily affected by
culture. As the Pistacia lentiscus grows abundantly in the South of France,
the climate must suit it, but it yields no mastic. The Laurus sassafras in
Europe loses the odour proper to it in North America. (23/11. Engel 'Sur les
Prop. Medicales des Plantes' 1860 pages 10, 25. On changes in the odours of
plants see Dalibert's Experiments quoted by Beckman 'Inventions' volume 2 page
344; and Nees in Ferussac 'Bull. des Sc. Nat.' 1824 tome 1 page 60. With
respect to the rhubarb etc. see also 'Gardener's Chronicle' 1849 page 355;
1862 page 1123.) Many similar facts could be given, and they are remarkable
because it might have been thought that definite chemical compounds would have
been little liable to change either in quality or quantity.
The wood of the American Locust-tree (Robinia) when grown in England is nearly
worthless, as is that of the Oak-tree when grown at the Cape of Good Hope.
(23/12. Hooker 'Flora Indica' page 32.) Hemp and flax, as I hear from Dr.
Falconer, flourish and yield plenty of seed on the plains of India, but their
fibres are brittle and useless. Hemp, on the other hand, fails to produce in
England that resinous matter which is so largely used in India as an
intoxicating drug.
The fruit of the Melon is greatly influenced by slight differences in culture
and climate. Hence it is generally a better plan, according to Naudin, to
improve an old kind than to introduce a new one into any locality. The seed of
the Persian Melon produces near Paris fruit inferior to the poorest market
kinds, but at Bordeaux yields delicious fruit. (23/13. Naudin 'Annales des Sc.
Nat.' 4th series, Bot. tome 11 1859 page 81. 'Gardener's Chronicle' 1859 page
464.) Seed is annually brought from Thibet to Kashmir (23/14. Moorcroft
'Travels' etc. volume 2 page 143.) and produces fruit weighing from four to
ten pounds, but plants raised next year from seed saved in Kashmir give fruit
weighing only from two to three pounds. It is well known that American
varieties of the Apple produce in their native land magnificent and brightly-
coloured fruit, but these in England are of poor quality and a dull colour. In
Hungary there are many varieties of the kidney-bean, remarkable for the beauty
of their seeds, but the Rev. M.J. Berkeley (23/15. 'Gardener's Chronicle' 1861
page 1113.) found that their beauty could hardly ever be preserved in England,
and in some cases the colour was greatly changed. We have seen in the ninth
chapter, with respect to wheat, what a remarkable effect transportal from the
north to the south of France, and conversely, produced on the weight of the
grain.]
When man can perceive no change in plants or animals which have been exposed
to a new climate or to different treatment, insects can sometimes perceive a
marked change. A cactus has been imported into India from Canton, Manilla
Mauritius, and from the hot-houses of Kew, and there is likewise a so-called
native kind which was formerly introduced from South America; all these plants
belong to the same species and are alike in appearance, but the cochineal
insect flourishes only on the native kind, on which it thrives prodigiously.
(23/16. Royle 'Productive Resources of India' page 59.) Humboldt remarks
(23/17. 'Personal Narrative' English translation volume 5 page 101. This
statement has been confirmed by Karsten 'Beitrag zur Kenntniss der
Rhynchoprion' Moscow 1864 s. 39 and by others.) that white men "born in the
torrid zone walk barefoot with impunity in the same apartment where a
European, recently landed, is exposed to the attacks of the Pulex penetrans."
This insect, the too well-known chigoe, must therefore be able to perceive
what the most delicate chemical analysis fails to discover, namely, a
difference between the blood or tissues of a European and those of a white man
born in the tropics. But the discernment of the chigoe is not so surprising as
it at first appears; for according to Liebig (23/18. 'Organic Chemistry'
English translation 1st edition page 369.) the blood of men with different
complexions, though inhabiting the same country, emits a different odour.
[Diseases peculiar to certain localities, heights, or climates, may be here
briefly noticed, as showing the influence of external circumstances on the
human body. Diseases confined to certain races of man do not concern us, for
the constitution of the race may play the more important part, and this may
have been determined by unknown causes. The Plica Polonica stands, in this
respect, in a nearly intermediate position; for it rarely affects Germans, who
inhabit the neighbourhood of the Vistula, where so many Poles are grievously
affected; neither does it affect Russians, who are said to belong to the same
original stock as the Poles. (23/19. Prichard 'Phys. Hist. of Mankind' 1851
volume 1 page 155.) The elevation of a district often governs the appearance
of diseases; in Mexico the yellow fever does not extend above 924 metres; and
in Peru, people are affected with the verugas only between 600 and 1600 metres
above the sea; many other such cases could be given. A peculiar cutaneous
complaint, called the Bouton d'Alep, affects in Aleppo and some neighbouring
districts almost every native infant, and some few strangers; and it seems
fairly well established that this singular complaint depends on drinking
certain waters. In the healthy little island of St. Helena the scarlet-fever
is dreaded like the Plague; analogous facts have been observed in Chili and
Mexico. (23/20. Darwin 'Journal of Researches' 1845 page 434.) Even in the
different departments of France it is found that the various infirmities which
render the conscript unfit for serving in the army, prevail with remarkable
inequality, revealing, as Boudin observes, that many of them are endemic,
which otherwise would never have been suspected. (23/21. These statements on
disease are taken from Dr. Boudin 'Geographie et Statistique Medicale' 1857
tome 1 pages 44 and 52; tome 2 page 315.) Any one who will study the
distribution of disease will be struck with surprise at what slight
differences in the surrounding circumstances govern the nature and severity of
the complaints by which man is at least temporarily affected.
The modifications as yet referred to are extremely slight, and in most cases
have been caused, as far as we can judge, by equally slight differences in the
conditions. But such conditions acting during a series of generations would
perhaps produce a marked effect.
With plants, a considerable change of climate sometimes produces a conspicuous
result. I have given in the ninth chapter the most remarkable case known to
me, namely, that of varieties of maize, which were greatly modified in the
course of only two or three generations when taken from a tropical country to
a cooler one, or conversely. Dr. Falconer informs me that he has seen the
English Ribston-pippin apple, a Himalayan oak, Prunus and Pyrus, all assume in
the hotter parts of India a fastigiate or pyramidal habit; and this fact is
the more interesting, as a Chinese tropical species of Pyrus naturally grows
thus. Although in these cases the changed manner of growth seems to have been
directly caused by the great heat, we know that many fastigiate trees have
originated in their temperate homes. In the Botanic Gardens of Ceylon the
apple-tree (23/22. 'Ceylon' by Sir J.E. Tennent volume 1 1859 page 89.) "sends
out numerous runners under ground, which continually rise into small stems,
and form a growth around the parent-tree.) The varieties of the cabbage which
produce heads in Europe fail to do so in certain tropical countries (23/23.
Godron 'De l'Espece' tome 2 page 52.) The Rhododendron ciliatum produced at
Kew flowers so much larger and paler-coloured than those which it bears on its
native Himalayan mountain, that Dr. Hooker (23/24. 'Journal of Horticultural
Soc.' volume 7 1852 page 117.) would hardly have recognised the species by the
flowers alone. Many similar facts with respect to the colour and size of
flowers could be given.
The experiments of Vilmorin and Buckman on carrots and parsnips prove that
abundant nutriment produces a definite and inheritable effect on the roots,
with scarcely any change in other parts of the plant. Alum directly influences
the colour of the flowers of the Hydrangea. (23/25. 'Journal of Hort. Soc.'
volume 1 page 160.) Dryness seems generally to favour the hairiness or
villosity of plants. Gartner found that hybrid Verbascums became extremely
woolly when grown in pots. Mr. Masters, on the other hand, states that the
Opuntia leucotricha "is well clothed with beautiful white hairs when grown in
a damp heat, but in a dry heat exhibits none of this peculiarity." (23/26. See
Lecoq on the Villosity of Plants 'Geograph. Bot.' tome 3 pages 287, 291;
Gartner 'Bastarderz.' s. 261; Mr. Masters on the Opuntia in 'Gardener's
Chronicle' 1846 page 444.) Slight variations of many kinds, not worth
specifying in detail, are retained only as long as plants are grown in certain
soils, of which Sageret (23/27. 'Pom. Phys.' page 136.) gives some instances
from his own experience. Odart, who insists strongly on the permanence of the
varieties of the grape, admits (23/28. 'Ampelographie' 1849 page 19.) that
some varieties, when grown under a different climate or treated differently,
vary in a slight degree, as in the tint of the fruit and in the period of
ripening. Some authors have denied that grafting causes even the slightest
difference in the scion; but there is sufficient evidence that the fruit is
sometimes slightly affected in size and flavour, the leaves in duration, and
the flowers in appearance. (23/29. Gartner 'Bastarderz.' s. 606, has collected
nearly all recorded facts. Andrew Knight in 'Transact. Hort. Soc.' volume 2
page 160, goes so far as to maintain that few varieties are absolutely
permanent in character when propagated by buds or grafts.)
There can be no doubt, from the facts given in the first chapter, that
European dogs deteriorate in India, not only in their instincts but in
structure; but the changes which they undergo are of such a nature, that they
may be partly due to reversion to a primitive form, as in the case of feral
animals. In parts of India the turkey becomes reduced in size, "with the
pendulous appendage over the beak enormously developed." (23/30. Mr. Blyth
'Annals and Mag of Nat. Hist.' volume 20 1847 page 391.) We have seen how soon
the wild duck, when domesticated, loses its true character, from the effects
of abundant or changed food, or from taking little exercise. From the direct
action of a humid climate and poor pasture the horse rapidly decreases in size
in the Falkland Islands. From information which I have received, this seems
likewise to be the case to a certain extent with sheep in Australia.
Climate definitely influences the hairy covering of animals; in the West
Indies a great change is produced in the fleece of sheep, in about three
generations. Dr. Falconer states (23/31. 'Natural History Review' 1862 page
113.) that the Thibet mastiff and goat, when brought down from the Himalaya to
Kashmir, lose their fine wool. At Angora not only goats, but shepherd-dogs and
cats, have fine fleecy hair, and Mr. Ainsworth (23/32. 'Journal of Roy.
Geographical Soc.' volume 9 1839 page 275.) attributes the thickness of the
fleece to the severe winters, and its silky lustre to the hot summers. Burnes
states positively (23/33. 'Travels in Bokhara' volume 3 page 151.) that the
Karakool sheep lose their peculiar black curled fleeces when removed into any
other country. Even within the limits of England, I have been assured that the
wool of two breeds of sheep was slightly changed by the flocks being pastured
in different localities. (23/34. See also on the influence of marshy pastures
on the wool Godron 'L'Espece' tome 2 page 22.) It has been asserted on good
authority (23/35. Isidore Geoffroy Saint-Hilaire 'Hist. Nat. Gen.' tome 3 page
438.) that horses kept during several years in the deep coal-mines of Belgium
become covered with velvety hair, almost like that on the mole. These cases
probably stand in close relation to the natural change of coat in winter and
summer. Naked varieties of several domestic animals have occasionally
appeared; but there is no reason to believe that this is in any way related to
the nature of the climate to which they have been exposed. (23/36. Azara has
made some good remarks on this subject 'Quadrupedes du Paraguay' tome 2 page
337. See an account of a family of naked mice produced in England 'Proc.
Zoolog. Soc.' 1856 page 38.)
It appears at first sight probable that the increased size, the tendency to
fatten, the early maturity and altered forms of our improved cattle, sheep,
and pigs, have directly resulted from their abundant supply of food. This is
the opinion of many competent judges, and probably is to a great extent true.
But as far as form is concerned, we must not overlook the more potent
influence of lessened use on the limbs and lungs. We see, moreover, as far as
size is concerned, that selection is apparently a more powerful agent than a
large supply of food, for we can thus only account for the existence, as
remarked to me by Mr. Blyth, of the largest and smallest breeds of sheep in
the same country, of Cochin-China fowls and Bantams, of small Tumbler and
large Runt pigeons, all kept together and supplied with abundant nourishment.
Nevertheless there can be little doubt that our domesticated animals have been
modified, independently of the increased or lessened use of parts, by the
conditions to which they have been subjected, without the aid of selection.
For instance, Prof. Rutimeyer (23/37. 'Die Fauna der Pfahlbauten' 1861 s. 15.)
shows that the bones of domesticated quadrupeds can be distinguished from
those of wild animals by the state of their surface and general appearance. It
is scarcely possible to read Nathusius's excellent 'Vorstudien' (23/38.
'Schweineschadel' 1864 s. 99.) and doubt that, with the highly improved races
of the pig, abundant food has produced a conspicuous effect on the general
form of the body, on the breadth of the head and face, and even on the teeth.
Nathusius rests much on the case of a purely bred Berkshire pig, which when
two months old became diseased in its digestive organs, and was preserved for
observation until nineteen months old; at this age it had lost several
characteristic features of the breed, and had acquired a long, narrow head, of
large size relatively to its small body, and elongated legs. But in this case
and in some others we ought not to assume that, because certain characters are
lost, perhaps through reversion, under one course of treatment, therefore that
they were at first directly produced by an opposite treatment.
In the case of the rabbit, which has become feral on the island of Porto
Santo, we are at first strongly tempted to attribute the whole change--the
greatly reduced size, the altered tints of the fur, and the loss of certain
characteristic marks--to the definite action of the new conditions to which it
has been exposed. But in all such cases we have to consider in addition the
tendency to reversion to progenitors more or less remote, and the natural
selection of the finest shades of difference.
The nature of the food sometimes either definitely induces certain
peculiarities, or stands in some close relation with them. Pallas long ago
asserted that the fat-tailed sheep of Siberia degenerate and lose their
enormous tails when removed from certain saline pastures; and recently Erman
(23/39. 'Travels in Siberia' English translation volume 1 page 228.) states
that this occurs with the Kirgisian sheep when brought to Orenburgh.
It is well known that hemp-seed causes bullfinches and certain other birds to
become black. Mr. Wallace has communicated to me some much more remarkable
facts of the same nature. The natives of the Amazonian region feed the common
green parrot (Chrysotis festiva, Linn.) with the fat of large Siluroid fishes,
and the birds thus treated become beautifully variegated with red and yellow
feathers. In the Malayan archipelago, the natives of Gilolo alter in an
analogous manner the colours of another parrot, namely, the Lorius garrulus,
Linn., and thus produce the Lori rajah or King-Lory. These parrots in the
Malay Islands and South America, when fed by the natives on natural vegetable
food, such as rice and plaintains, retain their proper colours. Mr. Wallace
has, also, recorded (23/40. A.R. Wallace 'Travels on the Amazon and Rio Negro'
page 294.) a still more singular fact. "The Indians (of S. America) have a
curious art by which they change the colours of the feathers of many birds.
They pluck out those from the part they wish to paint, and inoculate the fresh
wound with the milky secretion from the skin of a small toad. The feathers
grow of a brilliant yellow colour, and on being plucked out, it is said, grow
again of the same colour without any fresh operation."
Bechstein (23/41. 'Naturgeschichte der Stubenvogel' 1840 s. 262, 308.) does
not entertain any doubt that seclusion from light affects, at least
temporarily, the colours of cage-birds.
It is well known that the shells of land-mollusca are affected by the
abundance of lime in different districts. Isidore Geoffroy Saint-Hilaire
(23/42. 'Hist. Nat Gen.' tome 3 page 402.) gives the case of Helix lactea,
which has recently been carried from Spain to the South of France and to the
Rio Plata, and in both countries now presents a distinct appearance, but
whether this has resulted from food or climate is not known. With respect to
the common oyster, Mr. F. Buckland informs me that he can generally
distinguish the shells from different districts; young oysters brought from
Wales and laid down in beds where "natives" are indigenous, in the short space
of two months begin to assume the "native" character. M. Costa (23/43. 'Bull.
de La Soc. Imp. d'Acclimat.' tome 8 page 351.) has recorded a much more
remarkable case of the same nature, namely, that young shells taken from the
shores of England and placed in the Mediterranean, at once altered their
manner of growth and formed prominent diverging rays, like those on the shells
of the proper Mediterranean oyster. The same individual shell, showing both
forms of growth, was exhibited before a society in Paris. Lastly, it is well
known that caterpillars fed on different food sometimes either themselves
acquire a different colour or produce moths differing in colour. (23/44. See
an account of Mr. Gregson's experiments on the Abraxus grossulariata 'Proc.
Entomolog. Soc.' January 6, 1862: these experiments have been confirmed by Mr.
Greening in 'Proc. of the Northern Entomolog. Soc.' July 28, 1862. For the
effects of food on caterpillars see a curious account by M. Michely in 'Bull.
De La Soc. Imp. d'Acclimat.' tome 8 page 563. For analogous facts from Dahlbom
on Hymenoptera see Westwood 'Modern Class. of Insects' volume 2 page 98. See
also Dr. L. Moller 'Die Abhangigkeit der Insecten' 1867 s. 70.)
It would be travelling beyond my proper limits here to discuss how far organic
beings in a state of nature are definitely modified by changed conditions. In
my 'Origin of Species' I have given a brief abstract of the facts bearing on
this point, and have shown the influence of light on the colours of birds, and
of residence near the sea on the lurid tints of insects, and on the succulency
of plants. Mr. Herbert Spencer (23/45. 'The Principles of Biology' volume 2
1866. The present chapters were written before I had read Mr. Herbert
Spencer's work, so that I have not been able to make so much use of it as I
should otherwise probably have done.) has recently discussed with much ability
this whole subject on general grounds. He argues, for instance, that with all
animals the external and internal tissues are differently acted on by the
surrounding conditions, and they invariably differ in intimate structure. So
again the upper and lower surfaces of true leaves, as well as of stems and
petioles, when these assume the function and occupy the position of leaves,
are differently circumstanced with respect to light, etc., and apparently in
consequence differ in structure. But, as Mr. Herbert Spencer admits, it is
most difficult in all such cases to distinguish between the effects of the
definite action of physical conditions and the accumulation through natural
selection of inherited variations which are serviceable to the organism, and
which have arisen independently of the definite action of these conditions.]
Although we are not here concerned with the definite action of the conditions
of life on organisms in a state of nature, I may state that much evidence has
been gained during the last few years on this subject. In the United States,
for instance, it has been clearly proved, more especially by Mr. J.A. Allen,
that, with birds, many species differ in tint, size of body and of beak, and
in length of tail, in proceeding from the North to the South; and it appears
that these differences must be attributed to the direct action of temperature.
(23/46. Professor Weismann comes to the same conclusion with respect to
certain European butterflies in his valuable essay 'Ueber den Saison-
Dimorphismus' 1875. I might also refer to the recent works of several other
authors on the present subject; for instance to Kerner's 'Gute und schlechte
Arten' 1866.) With respect to plants I will give a somewhat analogous case:
Mr. Meehan (23/47. 'Proc. Acad. Nat. Soc. of Philadelphia' January 28, 1862.),
has compared twenty-nine kinds of American trees with their nearest European
allies, all grown in close proximity and under as nearly as possible the same
conditions. In the American species he finds, with the rarest exceptions, that
the leaves fall earlier in the season, and assume before their fall a brighter
tint; that they are less deeply toothed or serrated; that the buds are
smaller; that the trees are more diffuse in growth and have fewer branchlets;
and, lastly, that the seeds are smaller--all in comparison with the
corresponding European species. Now considering that these corresponding trees
belong to several distinct orders, and that they are adapted to widely
different stations, it can hardly be supposed that their differences are of
any special service to them in the New and Old worlds; and if so such
differences cannot have been gained through natural selection, and must be
attributed to the long continued action of a different climate.
GALLS.
Another class of facts, not relating to cultivated plants, deserves attention.
I allude to the production of galls. Every one knows the curious, bright-red,
hairy productions on the wild rose-tree, and the various different galls
produced by the oak. Some of the latter resemble fruit, with one face as rosy
as the rosiest apple. These bright colours can be of no service either to the
gall-forming insect or to the tree, and probably are the direct result of the
action of the light, in the same manner as the apples of Nova Scotia or Canada
are brighter coloured than English apples. According to Osten Sacken's latest
revision, no less than fifty-eight kinds of galls are produced on the several
species of oak, by Cynips with its sub-genera; and Mr. B.D. Walsh (23/48. See
Mr. B.D. Walsh's excellent papers in 'Proc. Entomolog. Soc. Philadelphia'
December 1866 page 284. With respect to the willow see ibid 1864 page 546.)
states that he can add many others to the list. One American species of
willow, the Salix humilis, bears ten distinct kinds of galls. The leaves which
spring from the galls of various English willows differ completely in shape
from the natural leaves. The young shoots of junipers and firs, when punctured
by certain insects, yield monstrous growths resembling flowers and fir-cones;
and the flowers of some plants become from the same cause wholly changed in
appearance. Galls are produced in every quarter of the world; of several sent
to me by Mr. Thwaites from Ceylon, some were as symmetrical as a composite
flower when in bud, others smooth and spherical like a berry; some protected
by long spines, others clothed with yellow wool formed of long cellular hairs,
others with regularly tufted hairs. In some galls the internal structure is
simple, but in others it is highly complex; thus M. Lacaze-Duthiers (23/49.
See his admirable 'Histoire des Galles' in 'Annal. des Sc. Nat. Bot.' 3rd
series tome 19 1853 page 273.) has figured in the common ink-gall no less than
seven concentric layers, composed of distinct tissue, namely, the epidermic,
sub-epidermic, spongy, intermediate, and the hard protective layer formed of
curiously thickened woody cells, and, lastly, the central mass, abounding with
starch-granules on which the larvae feed.
Galls are produced by insects of various orders, but the greater number by
species of Cynips. It is impossible to read M. Lacaze-Duthiers' discussion and
doubt that the poisonous secretion of the insect causes the growth of the
gall; and every one knows how virulent is the poison secreted by wasps and
bees, which belong to the same group with Cynips. Galls grow with
extraordinary rapidity, and it is said that they attain their full size in a
few days (23/50. Kirby and Spence 'Entomology' 1818 volume 1 page 450; Lacaze-
Duthiers ibid page 284.); it is certain that they are almost completely
developed before the larvae are hatched. Considering that many gall-insects
are extremely small, the drop of secreted poison must be excessively minute;
it probably acts on one or two cells alone, which, being abnormally
stimulated, rapidly increase by a process of self-division. Galls, as Mr.
Walsh (23/51. 'Proc. Entomolog. Soc. Philadelphia' 1864 page 558.) remarks,
afford good, constant, and definite characters, each kind keeping as true to
form as does any independent organic being. This fact becomes still more
remarkable when we hear that, for instance, seven out of the ten different
kinds of galls produced on Salix humilis are formed by gall-gnats
(Cecidomyidae) which "though essentially distinct species, yet resemble one
another so closely that in almost all cases it is difficult, and in most cases
impossible, to distinguish the full-grown insects one from the other." (23/52.
Mr. B.D. Walsh ibid page 633 and December 1866 page 275.) For in accordance
with a wide-spread analogy we may safely infer that the poison secreted by
insects so closely allied would not differ much in nature; yet this slight
difference is sufficient to induce widely different results. In some few cases
the same species of gall-gnat produces on distinct species of willows galls
which cannot be distinguished; the Cynips fecundatrix, also, has been known to
produce on the Turkish oak, to which it is not properly attached, exactly the
same kind of gall as on the European oak. (23/53. Mr. B.D. Walsh ibid 1864
pages 545, 411, 495; and December 1866 page 278. See also Lacaze-Duthiers.)
These latter facts apparently prove that the nature of the poison is a more
powerful agent in determining the form of the gall than the specific character
of the tree which is acted on.
As the poisonous secretion of insects belonging to various orders has the
special power of affecting the growth of various plants; as a slight
difference in the nature of the poison suffices to produce widely different
results; and lastly, as we know that the chemical compounds secreted by plants
are eminently liable to be modified by changed conditions of life, we may
believe it possible that various parts of a plant might be modified through
the agency of its own altered secretions. Compare, for instance, the mossy and
viscid calyx of a moss-rose, which suddenly appears through bud-variation on a
Provence-rose, with the gall of red moss growing from the inoculated leaf of a
wild rose, with each filament symmetrically branched like a microscopical
spruce-fir, bearing a glandular tip and secreting odoriferous gummy matter.
(23/54. Lacaze-Duthiers ibid pages 325, 328.) Or compare, on the one hand, the
fruit of the peach, with its hairy skin, fleshy covering, hard shell and
kernel, and on the other hand one of the more complex galls with its
epidermic, spongy, and woody layers, surrounding tissue loaded with starch
granules. These normal and abnormal structures manifestly present a certain
degree of resemblance. Or, again, reflect on the cases above given of parrots
which have had their plumage brightly decorated through some change in their
blood, caused by having been fed on certain fishes, or locally inoculated with
the poison of a toad. I am far from wishing to maintain that the moss-rose or
the hard shell of the peach-stone or the bright colours of birds are actually
due to any chemical change in the sap or blood; but these cases of galls and
of parrots are excellently adapted to show us how powerfully and singularly
external agencies may affect structure. With such facts before us, we need
feel no surprise at the appearance of any modification in any organic being.
[I may, also, here allude to the remarkable effects which parasitic fungi
sometimes produce on plants. Reissek (23/55. 'Linnaea' volume 17 1843; quoted
by Dr. M.T. Masters, Royal Institution, March 16, 1860.) has described a
Thesium, affected by an Oecidium, which was greatly modified, and assumed some
of the characteristic features of certain allied species, or even genera.
Suppose, says Reissek, "the condition originally caused by the fungus to
become constant in the course of time, the plant would, if found growing wild,
be considered as a distinct species or even as belonging to a new genus." I
quote this remark to show how profoundly, yet in how natural a manner, this
plant must have been modified by the parasitic fungus. Mr. Meehan (23/56.
'Proc. Acad. Nat. Sc., Philadelphia' June 16, 1874 and July 23, 1875.) also
states that three species of Euphorbia and Portulaca olereacea, which
naturally grow prostrate, become erect when they are attacked by the Oecidium.
Euphorbia maculata in this case also becomes nodose, with the branchlets
comparatively smooth and the leaves modified in shape, approaching in these
respects to a distinct species, namely, the E. hypericifolia.]
FACTS AND CONSIDERATIONS OPPOSED TO THE BELIEF THAT THE CONDITIONS OF LIFE ACT
IN A POTENT MANNER IN CAUSING DEFINITE MODIFICATIONS OF STRUCTURE.
I have alluded to the slight differences in species naturally living in
distinct countries under different conditions; and such differences we feel at
first inclined to attribute, probably often with justice, to the definite
action of the surrounding conditions. But it must be borne in mind that there
exist many animals and plants which range widely and have been exposed to
great diversities of climate, yet remain uniform in character. Some authors,
as previously remarked, account for the varieties of our culinary and
agricultural plants by the definite action of the conditions to which they
have been exposed in the different parts of Great Britain; but there are about
200 plants (23/57. Hewett C. Watson 'Cybele Britannica' volume 1 1847 page
11.) which are found in every single English county; and these plants must
have been exposed for an immense period to considerable differences of climate
and soil, yet do not differ. So, again,, some animals and plants range over a
large portion of the world, yet retain the same character.
[Notwithstanding the facts previously given on the occurrence of highly
peculiar local diseases and on the strange modifications of structure in
plants caused by the inoculated poison of insects, and other analogous cases;
still there are a multitude of variations--such as the modified skull of the
niata ox and bulldog, the long horns of Caffre cattle, the conjoined toes of
the solid-hoofed swine, the immense crest and protuberant skull of Polish
fowls, the crop of the pouter-pigeon, and a host of other such cases--which we
can hardly attribute to the definite action, in the sense before specified, of
the external conditions of life. No doubt in every case there must have been
some exciting cause; but as we see innumerable individuals exposed to nearly
the same conditions, and one alone is affected, we may conclude that the
constitution of the individual is of far higher importance than the conditions
to which it has been exposed. It seems, indeed, to be a general rule that
conspicuous variations occur rarely, and in one individual alone out of
millions, though all may have been exposed, as far as we can judge, to nearly
the same conditions. As the most strongly marked variations graduate
insensibly into the most trifling, we are led by the same train of thought to
attribute each slight variation much more to innate differences of
constitution, however caused, than to the definite action of the surrounding
conditions.
We are led to the same conclusion by considering the cases, formerly alluded
to, of fowls and pigeons, which have varied and will no doubt go on varying in
directly opposite ways, though kept during many generations under nearly the
same conditions. Some, for instance, are born with their beaks, wings, tails,
legs, etc., a little longer, and others with these same parts a little
shorter. By the long-continued selection of such slight individual differences
which occur in birds kept in the same aviary, widely different races could
certainly be formed; and long-continued selection, important as is the result,
does nothing but preserve the variations which arise, as it appears to us,
spontaneously.
In these cases we see that domesticated animals vary in an indefinite number
of particulars, though treated as uniformly as is possible. On the other hand,
there are instances of animals and plants, which, though they have been
exposed to very different conditions, both under nature and domestication,
have varied in nearly the same manner. Mr. Layard informs me that he has
observed amongst the Caffres of South Africa a dog singularly like an arctic
Esquimaux dog. Pigeons in India present nearly the same wide diversities of
colour as in Europe; and I have seen chequered and simply barred pigeons, and
pigeons with blue and white loins, from Sierra Leone, Madeira, England, and
India. New varieties of flowers are continually raised in different parts of
Great Britain, but many of these are found by the judges at our exhibitions to
be almost identical with old varieties. A vast number of new fruit-trees and
culinary vegetables have been produced in North America: these differ from
European varieties in the same general manner as the several varieties raised
in Europe differ from one another; and no one has ever pretended that the
climate of America has given to the many American varieties any general
character by which they can be recognised. Nevertheless, from the facts
previously advanced on the authority of Mr. Meehan with respect to American
and European forest-trees it would be rash to affirm that varieties raised in
the two countries would not in the course of ages assume a distinctive
character. Dr. M. Masters has recorded a striking fact (23/58. 'Gardener's
Chronicle' 1857 page 629.) bearing on this subject: he raised numerous plants
of Hybiscus syriacus from seed collected in South Carolina and the Holy Land,
where the parent-plants must have been exposed to considerably different
conditions; yet the seedlings from both localities broke into two similar
strains, one with obtuse leaves and purple or crimson flowers, and the other
with elongated leaves and more or less pink flowers.
We may, also, infer the prepotent influence of the constitution of the
organism over the definite action of the conditions of life, from the several
cases given in the earlier chapters of parallel series of varieties,--an
important subject, hereafter to be more fully discussed. Sub-varieties of the
several kinds of wheat, gourds, peaches, and other plants, and to a limited
extent sub-varieties of the fowl, pigeon, and dog, have been shown either to
resemble or to differ from one another in a closely corresponding or parallel
manner. In other cases, a variety of one species resembles a distinct species;
or the varieties of two distinct species resemble one another. Although these
parallel resemblances no doubt often result from reversion to the former
characters of a common progenitor; yet in other cases, when new characters
first appear, the resemblance must be attributed to the inheritance of a
similar constitution, and consequently to a tendency to vary in the same
manner. We see something of a similar kind in the same monstrosity appearing
and reappearing many times in the same species of animal, and, as Dr. Maxwell
Masters has remarked to me, in the same species of plant.]
We may at least conclude, that the amount of modification which animals and
plants have undergone under domestication does not correspond with the degree
to which they have been subjected to changed circumstances. As we know the
parentage of domesticated birds far better than of most quadrupeds, we will
glance through the list. The pigeon has varied in Europe more than almost any
other bird; yet it is a native species, and has not been exposed to any
extraordinary change of conditions. The fowl has varied equally, or almost
equally, with the pigeon, and is a native of the hot jungles of India. Neither
the peacock, a native of the same country, nor the guinea-fowl, an inhabitant
of the dry deserts of Africa, has varied at all, or only in colour. The
turkey, from Mexico, has varied but little. The duck, on the other hand, a
native of Europe, has yielded some well-marked races; and as this is an
aquatic bird, it must have been subjected to a far more serious change in its
habits than the pigeon or even the fowl, which nevertheless have varied in a
much higher degree. The goose, a native of Europe and aquatic like the duck,
has varied less than any other domesticated bird, except the peacock.
Bud-variation is, also, important under our present point of view, in some few
cases, as when all the eyes on the same tuber of the potato, or all the fruit
on the same plum-tree, or all the flowers on the same plant, have suddenly
varied in the same manner, it might be argued that the variation had been
definitely caused by some change in the conditions to which the plants had
been exposed; yet, in other cases, such an admission is extremely difficult.
As new characters sometimes appear by bud-variation, which do not occur in the
parent-species or in any allied species, we may reject, at least in these
cases, the idea that they are due to reversion. Now it is well worth while to
reflect maturely on some striking case of bud-variation, for instance that of
the peach. This tree has been cultivated by the million in various parts of
the world, has been treated differently, grown on its own roots and grafted on
various stocks, planted as a standard, trained against a wall, or under glass;
yet each bud of each sub-variety keeps true to its kind. But occasionally, at
long intervals of time, a tree in England, or under the widely different
climate of Virginia, produces a single bud, and this yields a branch which
ever afterwards bears nectarines. Nectarines differ, as every one knows, from
peaches in their smoothness, size, and flavour; and the difference is so great
that some botanists have maintained that they are specifically distinct. So
permanent are the characters thus suddenly acquired, that a nectarine produced
by bud-variation has propagated itself by seed. To guard against the
supposition that there is some fundamental distinction between bud and seminal
variation, it is well to bear in mind that nectarines have likewise been
produced from the stone of the peach; and, reversely, peaches from the stone
of the nectarine. Now is it possible to conceive external conditions more
closely alike than those to which the buds on the same tree are exposed? Yet
one bud alone, out of the many thousands borne by the same tree, has suddenly,
without any apparent cause, produced a nectarine. But the case is even
stronger than this, for the same flower-bud has yielded a fruit, one-half or
one-quarter a nectarine, and the other half or three-quarters a peach. Again,
seven or eight varieties of the peach have yielded by bud-variation
nectarines: the nectarines thus produced, no doubt, differ a little from one
another; but still they are nectarines. Of course there must be some cause,
internal or external, to excite the peach-bud to change its nature; but I
cannot imagine a class of facts better adapted to force on our minds the
conviction that what we call the external conditions of life are in many cases
quite insignificant in relation to any particular variation, in comparison
with the organisation or constitution of the being which varies.
It is known from the labours of Geoffroy Saint-Hilaire, and recently from
those of Dareste and others, that eggs of the fowl, if shaken, placed upright,
perforated, covered in part with varnish, etc., produce monstrous chickens.
Now these monstrosities may be said to be directly caused by such unnatural
conditions, but the modifications thus induced are not of a definite nature.
An excellent observer, M. Camille Dareste (23/59. 'Memoire sur la Production
Artificielle des Monstruosites' 1862 pages 8-12; 'Recherches sur les
Conditions, etc., chez les Monstres' 1863 page 6. An abstract is given of
Geoffroy's Experiments by his son, in his 'Vie, Travaux' etc. 1847 page 290.),
remarks "that the various species of monstrosities are not determined by
specific causes; the external agencies which modify the development of the
embryo act solely in causing a perturbation--a perversion in the normal course
of development." He compares the result to what we see in illness: a sudden
chill, for instance, affects one individual alone out of many, causing either
a cold, or sore-throat, rheumatism, or inflammation of the lungs or pleura.
Contagious matter acts in an analogous manner. (23/60. Paget 'Lectures on
Surgical Pathology' 1853 volume 1 page 483.) We may take a still more specific
instance: seven pigeons were struck by rattle-snakes (23/61. 'Researches upon
the Venom of the Rattle-snake' January 1861 by Dr. Mitchell page 67.): some
suffered from convulsions; some had their blood coagulated, in others it was
perfectly fluid; some showed ecchymosed spots on the heart, others on the
intestines, etc.; others again showed no visible lesion in any organ. It is
well known that excess in drinking causes different diseases in different men;
but in the tropics the effects of intemperance differ from those caused in a
cold climate (23/62. Mr. Sedgwick 'British and Foreign Medico-Chirurg. Review'
July 1863 page 175.); and in this case we see the definite influence of
opposite conditions. The foregoing facts apparently give us as good an idea as
we are likely for a long time to obtain, how in many cases external conditions
act directly, though not definitely, in causing modifications of structure.
SUMMARY.
There can be no doubt, from the facts given in this chapter, that extremely
slight changes in the conditions of life sometimes, probably often, act in a
definite manner on our domesticated productions; and, as the action of changed
conditions in causing indefinite variability is accumulative, so it may be
with their definite action. Hence considerable and definite modifications of
structure probably follow from altered conditions acting during a long series
of generations. In some few instances a marked effect has been produced
quickly on all, or nearly all, the individuals which have been exposed to a
marked change of climate, food, or other circumstance. This has occurred with
European men in the United States, with European dogs in India, with horses in
the Falkland Islands, apparently with various animals at Angora, with foreign
oysters in the Mediterranean, and with maize transported from one climate to
another. We have seen that the chemical compounds of some plants and the state
of their tissues are readily affected by changed conditions. A relation
apparently exists between certain characters and certain conditions, so that
if the latter be changed the character is lost--as with the colours of
flowers, the state of some culinary plants, the fruit of the melon, the tail
of fat-tailed sheep, and the peculiar fleeces of other sheep.
The production of galls, and the change of plumage in parrots when fed on
peculiar food or when inoculated by the poison of a toad, prove to us what
great and mysterious changes in structure and colour, may be the definite
result of chemical changes in the nutrient fluids or tissues.
We now almost certainly know that organic beings in a state of nature may be
modified in various definite ways by the conditions to which they have been
long exposed, as in the case of the birds and other animals in the northern
and southern United States, and of American trees in comparison with their
representatives in Europe. But in many cases it is most difficult to
distinguish between the definite result of changed conditions, and the
accumulation through natural selection of indefinite variations which have
proved serviceable. If it profited a plant to inhabit a humid instead of an
arid station, a fitting change in its constitution might possibly result from
the direct action of the environment, though we have no grounds for believing
that variations of the right kind would occur more frequently with plants
inhabiting a station a little more humid than usual, than with other plants.
Whether the station was unusually dry or humid, variations adapting the plant
in a slight degree for directly opposite habits of life would occasionally
arise, as we have good reason to believe from what we actually see in other
cases.
The organisation or constitution of the being which is acted on, is generally
a much more important element than the nature of the changed conditions, in
determining the nature of the variation. We have evidence of this in the
appearance of nearly similar modifications under different conditions, and of
different modifications under apparently nearly the same conditions. We have
still better evidence of this in closely parallel varieties being frequently
produced from distinct races, or even distinct species; and in the frequent
recurrence of the same monstrosity in the same species. We have also seen that
the degree to which domesticated birds have varied, does not stand in any
close relation with the amount of change to which they have been subjected.
To recur once again to bud-variations. When we reflect on the millions of buds
which many trees have produced, before some one bud has varied, we are lost in
wonder as to what the precise cause of each variation can be. Let us recall
the case given by Andrew Knight of the forty-year-old tree of the yellow
magnum bonum plum, an old variety which has been propagated by grafts on
various stocks for a very long period throughout Europe and North America, and
on which a single bud suddenly produced the red magnum bonum. We should also
bear in mind that distinct varieties, and even distinct species,--as in the
case of peaches, nectarines, and apricots,--of certain roses and camellias,--
although separated by a vast number of generations from any progenitor in
common, and although cultivated under diversified conditions, have yielded by
bud-variation closely analogous varieties. When we reflect on these facts we
become deeply impressed with the conviction that in such cases the nature of
the variation depends but little on the conditions to which the plant has been
exposed, and not in any especial manner on its individual character, but much
more on the inherited nature or constitution of the whole group of allied
beings to which the plant in question belongs. We are thus driven to conclude
that in most cases the conditions of life play a subordinate part in causing
any particular modification; like that which a spark plays, when a mass of
combustibles bursts into flame--the nature of the flame depending on the
combustible matter, and not on the spark. (23/63. Professor Weismann argues
strongly in favour of this view in his 'Saison-Dimorphismus der
Schmetterlinge' 1875 pages 40-43.)
No doubt each slight variation must have its efficient cause; but it is as
hopeless an attempt to discover the cause of each, as to say why a chill or a
poison affects one man differently from another. Even with modifications
resulting from the definite action of the conditions of life, when all or
nearly all the individuals, which have been similarly exposed, are similarly
affected, we can rarely see the precise relation between cause and effect. In
the next chapter it will be shown that the increased use or disuse of various
organs produces an inherited effect. It will further be seen that certain
variations are bound together by correlation as well as by other laws. Beyond
this we cannot at present explain either the causes or nature of the
variability of organic beings.