WILD FORMS AND DOMESTIC VARIETIES
In discussing the phenomena of reversion we have seen that in most cases
such reversion occurs when the two varieties which are crossed each contain
certain factors lacking in the other, of which the full complement is
necessary for the production of the reversionary wild form. This at once
suggests the idea that the various domestic forms of animals and plants
have arisen by the omission from time to time of this factor or of that. In
some cases we have clear evidence that this is the most natural
interpretation of the relation between the cultivated and the wild forms.
Probably the species in which it is most evident is the sweet pea
(_Lathyrus odoratus_). We have already seen reason to suppose that as
regards certain structural features the Bush variety is a wild lacking the
factor for the procumbent habit, that the Cupid is a wild without the
factor for the long inter-node, and that the Bush Cupid is a wild minus
both these factors. Nor is the evidence less clear for the many colour
varieties. In illustration we may consider in more detail a case in which
the cross between two whites resulted {80} in a complete reversion to the
purple colour characteristic of the wild Sicilian form (Pl. IV.). In this
particular instance subsequent breeding from the purples resulted in the
production of six different colour forms in addition to whites. The
proportion of the coloured forms to the whites was 9 : 7 (cf. p. 44), but
it is with the relation of the six coloured forms that we are concerned
here. Of these six forms three were purples and three were reds. The three
purple forms were (1) the wild bicolor purple with blue wings known in
cultivation as the Purple Invincible (Pl. IV., 4); (2) a deep purple with
purple wings (Pl. IV., 5); and (3) a very dilute purple known as the
Picotee (Pl. IV., 6). Corresponding to these three purple forms were three
reds: (1) a bicolor red known as Painted Lady (Pl. IV., 7); (2) a deep red
with red wings known as Miss Hunt (Pl. IV., 8); and (3) a very pale red
which we have termed Tinged White[5] (Pl. IV., 9). In the F_2 generation
the total number of purples bore to the total number of reds the ratio
3 : 1, and this ratio was maintained for each of the corresponding classes.
Purple, therefore, is dominant to red, and each of the three classes of red
differs from its corresponding purple in not possessing the blue factor (B)
which turns it into purple.
[Illustration: PLATE IV.
1, 2, Emily Henderson; 3, F_1 reversionary Purple; 4-10, Various F_2 forms:
4, Purple; 5, Deep Purple; 6, Picotee; 7, Painted Lady; 8, Miss Hunt; 9,
Tinged White; 10, White.]
{81} Again, the proportion in which the three classes of purples appeared
was 9 bicolors, 3 deep purples, 4 picotees. We are, therefore, concerned
here with the operation of two factors: (1) a light wing factor, which
renders the bicolor dominant to the dark winged form; and (2) a factor for
intense colour, which occurs in the bicolor and in the deep purple, but is
lacking in the dilute picotee. And here it should be mentioned that these
conclusions rest upon an exhaustive set of experiments involving the
breeding of many thousands of plants. In this cross, therefore, we are
concerned with the presence or absence of five factors, which we may denote
as follows:--
A colour base, R.
A colour developer, C.
A purple factor, B.
A light wing factor, L.
A factor for intense colour, I.
On this notation our six coloured forms are:--
(1) Purple bicolor CRBLI.[6]
(2) Deep purple CRBlI.
(3) Picotee CRBLi or CRBli.
(4) Red bicolor ( = Painted Lady) CRbLI.
(5) Deep red ( = Miss Hunt) CRblI.
(6) Tinged white CRbLi or CRbli.
It will be noticed in this series that the various coloured {82} forms can
be expressed by the omission of one or more factors from the purple bicolor
of the wild type. With the complete omission of each factor a new colour
type results, and it is difficult to resist the inference that the various
cultivated forms of the sweet pea have arisen from the wild by some process
of this kind. Such a view tallies with what we know of the behaviour of the
wild form when crossed by any of the garden varieties. Wherever such
crossing has been made the form of the hybrid has been that of the wild,
thus supporting the view that the wild contains a complete set of all the
differentiating factors which are to be found in the sweet pea.
Moreover, this view is in harmony with such historical evidence as is to be
gleaned from botanical literature, and from old seedsmen's catalogues. The
wild sweet pea first reached England in 1699, having been sent from Sicily
by the monk Franciscus Cupani as a present to a certain Dr. Uvedale in the
county of Middlesex. Somewhat later we hear of two new varieties, the red
bicolor, or Painted Lady, and the white, each of which may be regarded as
having "sported" from the wild purple by the omission of the purple factor,
or of one of the two colour factors. In 1793 we find a seedsman offering
also what he called black and scarlet varieties. It is probable that these
were our deep purple and Miss Hunt varieties, and that somewhere about this
time the factor for the {83} light wing (L) was dropped out in certain
plants. In 1860 we have evidence that the pale purple or Picotee, and with
it doubtless the Tinged White, had come into existence. This time it was
the factor for intense colour which had dropped out. And so the story goes
on until the present day, and it is now possible to express by the same
simple method the relation of the modern shades, of purple and reds, of
blues and pinks, of hooded and wavy standards, to one another and to the
original wild form. The constitution of many of these has now been worked
out, and to-day it would be a simple though perhaps tedious task to denote
all the different varieties by a series of letters indicating the factors
which they contain, instead of by the present system of calling them after
kings and queens, and famous generals, and ladies more or less well known.
From what we know of the history of the various strains of sweet peas one
thing stands out clearly. The new character does not arise from a
pre-existing variety by any process of gradual selection, conscious or
otherwise. It turns up suddenly complete in itself, and thereafter it can
be associated by crossing with other existing characters to produce a gamut
of new varieties. If, for example, the character of hooding in the standard
(cf. Pl. II., 7) suddenly turned up in such a family as that shown on Plate
IV. we should be able to get a hooded form corresponding to each of the
forms with the erect {84} standard; in other words, the arrival of the new
form would give us the possibility of fourteen varieties instead of seven.
As we know, the hooded character already exists. It is recessive to the
erect standard, and we have reason to suppose that it arose as a sudden
sport by the omission of the factor in whose presence the standard assumes
the erect shape characteristic of the wild flower. It is largely by keeping
his eyes open and seizing upon such sports for crossing purposes that the
horticulturist "improves" the plants with which he deals. How these sports
or _mutations_ come about we can now surmise. They must owe their origin to
a disturbance in the processes of cell division through which the gametes
originate. At some stage or other the normal equal distribution of the
various factors is upset, and some of the gametes receive a factor less
than others. From the union of two such gametes, provided that they are
still capable of fertilisation, comes the zygote which in course of growth
develops the new character.
Why these mutations arise: what leads to the surmised unequal division of
the gametes: of this we know practically nothing. Nor until we can induce
the production of mutations at will are we likely to understand the
conditions which govern their formation. Nevertheless there are already
hints scattered about the recent literature of experimental biology which
lead us to hope that we may know more of these matters in the future. {85}
In respect of the evolution of its now multitudinous varieties, the story
of the sweet pea is clear and straightforward. These have all arisen from
the wild by a process of continuous loss. Everything was there in the
beginning, and as the wild plant parted with factor after factor there came
into being the long series of derived forms. Exquisite as are the results
of civilization, it is by the degradation of the wild that they have been
brought about. How far are we justified in regarding this as a picture of
the manner in which evolution works?
There are certainly other species in which we must suppose that this is the
way that the various domesticated forms have arisen. Such, for example, is
the case in the rabbit, where most of the colour varieties are recessive to
the wild agouti form. Such also is the case in the rat, where the black and
albino varieties and the various pattern forms are also recessive to the
wild agouti type. And with the exception of a certain yellow variety to
which we shall refer later, such is also the case with the many fancy
varieties of mice.
Nevertheless there are other cases in which we must suppose evolution to
have proceeded by the interpolation of characters. In discussing reversion
on crossing, we have already seen that this may not occur until the F_2
generation, as, for example, in the instance of the fowls' combs (cf. p.
65). The reversion to the single comb occurred as the result of the removal
of the two factors {86} for rose and pea. These two domesticated varieties
must be regarded as each possessing an additional factor in comparison with
the wild single-combed bird. During the evolution of the fowl, these two
factors must be conceived of as having been interpolated in some way. And
the same holds good for the inhibitory factor on which, as we have seen,
the dominant white character of certain poultry depends. In pigeons, too,
if we regard the blue rock as the ancestor of the domesticated breeds, we
must suppose that an additional melanic factor has arisen at some stage.
For we have already seen that black is dominant to blue, and the characters
of F_1, together with the greater number of blacks than blues in F_2,
negatives the possibility that we are here dealing with an inhibitory
factor. The hornless or polled condition of cattle, again, is dominant to
the horned condition, and if, as seems reasonable, we regard the original
ancestors of domestic cattle as having been horned, we have here again the
interpolation of an inhibitory factor somewhere in the course of evolution.
On the whole, therefore, we must be prepared to admit that the evolution of
domestic varieties may come about by a process of addition of factors in
some cases and of subtraction in others. It may be that what we term
additional factors fall into distinct categories from the rest. So far,
experiment seems to show that they are either of the nature of melanic
factors, or of inhibitory {87} factors, or of reduplication factors as in
the case of the fowls' combs. But while the data remain so scanty,
speculation in these matters is too hazardous to be profitable.
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