The Project Gutenberg ebook of Darwinism (1889), by Alfred Russel Wallace

"if crosses act by virtue of being a cross, and not by virtue of

removing an hereditary taint, then the greater the difference between

the two animals crossed the more beneficial will that act be." He then

shows that, the wider the difference the less is the benefit, and

concludes that a cross, as such, has no beneficial effect. A parallel

argument would be, that change of air, as from inland to the sea-coast,

or from a low to an elevated site, is not beneficial in itself, because,

if so, a change to the tropics or to the polar regions should be more

beneficial. In both these cases it may well be that no benefit would

accrue to a person in perfect health; but then there is no such thing

as "perfect health" in man, and probably no such thing as absolute

freedom from constitutional taint in animals. The experiments of Mr.

Darwin, showing the great and immediate good effects of a cross between

distinct strains in plants, cannot be explained away; neither can the

innumerable arrangements to secure cross-fertilisation by insects, the

real use and purport of which will be discussed in our eleventh chapter.

On the whole, then, the evidence at our command proves that, whatever

may be its ultimate cause, close interbreeding _does_ usually produce

bad results; and it is only by the most rigid selection, whether natural

or artificial, that the danger can be altogether obviated.

_Fertile Hybrids among Animals._
One or two more cases of fertile hybrids may be given before we pass on

to the corresponding experiments in plants. Professor Alfred Newton

received from a friend a pair of hybrid ducks, bred from a common duck

(Anas boschas), and a pintail (Dafila acuta). From these he obtained

four ducklings, but these latter, when grown up, proved infertile, and

did not breed again. In this case we have the results of close

interbreeding, with too great a difference between the original species,

combining to produce infertility, yet the fact of a hybrid from such a

pair producing healthy offspring is itself noteworthy.
Still more extraordinary is the following statement of Mr. Low: "It has

been long known to shepherds, though questioned by naturalists, that the

progeny of the cross between the sheep and goat is fertile. Breeds of

this mixed race are numerous in the north of Europe."[55] Nothing

appears to be known of such hybrids either in Scandinavia or in Italy;

but Professor Giglioli of Florence has kindly given me some useful

references to works in which they are described. The following extract

from his letter is very interesting: "I need not tell you that there

being such hybrids is now generally accepted as a fact. Buffon

(_Supplements_, tom. iii. p. 7, 1756) obtained one such hybrid in 1751

and eight in 1752. Sanson (_La Culture_, vol. vi. p. 372, 1865) mentions

a case observed in the Vosges, France. Geoff. St. Hilaire (_Hist. Nat.

Gén. des reg. org._, vol. iii. p. 163) was the first to mention, I

believe, that in different parts of South America the ram is more

usually crossed with the she-goat than the sheep with the he-goat. The

well-known 'pellones' of Chile are produced by the second and third

generation of such hybrids (Gay, 'Hist, de Chile,' vol. i. p. 466,

_Agriculture_, 1862). Hybrids bred from goat and sheep are called

'chabin' in French, and 'cabruno' in Spanish. In Chile such hybrids are

called 'carneros lanudos'; their breeding _inter se_ appears to be not

always successful, and often the original cross has to be recommenced to

obtain the proportion of three-eighths of he-goat and five-eighths of

sheep, or of three-eighths of ram and five-eighths of she-goat; such

being the reputed best hybrids."

doubt that races of hybrids between these very distinct species have

been produced, and that such hybrids are fairly fertile _inter se_; and

the analogous facts already given lead us to believe that whatever

amount of infertility may at first exist could be eliminated by careful

selection, if the crossed races were bred in large numbers and over a

considerable area of country. This case is especially valuable, as

showing how careful we should be in assuming the infertility of hybrids

when experiments have been made with the progeny of a single pair, and

have been continued only for one or two generations.

of two moths (Bombyx cynthia and B. arrindia) were proved in Paris,

according to M. Quatrefages, to be fertile _inter se_ for eight

generations.

_Fertility of Hybrids among Plants._
Among plants the cases of fertile hybrids are more numerous, owing, in

part, to the large scale on which they are grown by gardeners and

nurserymen, and to the greater facility with which experiments can be

made. Darwin tells us that Kölreuter found ten cases in which two plants

considered by botanists to be distinct species were quite fertile

together, and he therefore ranked them all as varieties of each other.

In some cases these were grown for six to ten successive generations,

but after a time the fertility decreased, as we saw to be the case in

animals, and presumably from the same cause, too close interbreeding.
Dean Herbert, who carried on experiments with great care and skill for

many years, found numerous cases of hybrids which were perfectly fertile

_inter se_. Crinum capense, fertilised by three other species--C.

pedunculatum, C. canaliculatum, or C. defixum--all very distinct from

it, produced perfectly fertile hybrids; while other species less

different in appearance were quite sterile with the same C. capense.

are invariably fertile. Lobelia syphylitica and L. fulgens, two very

distinct species, have produced a hybrid which has been named Lobelia

speciosa, and which reproduces itself abundantly. Many of the beautiful

pelargoniums of our greenhouses are hybrids, such as P. ignescens from a

cross between P. citrinodorum and P. fulgidum, which is quite fertile,

and has become the parent of innumerable varieties of beautiful plants.

All the varied species of Calceolaria, however different in appearance,

intermix with the greatest readiness, and the hybrids are all more or

less fertile. But the most remarkable case is that of two species of

Petunia, of which Dean Herbert says: "It is very remarkable that,

although there is a great difference in the form of the flower,

especially of the tube, of P. nyctanigenaeflora and P. phoenicea the

mules between them are not only fertile, but I have found them seed much

more freely with me than either parent.... From a pod of the

above-mentioned mule, to which no pollen but its own had access, I had a

large batch of seedlings in which there was no variability or difference

from itself; and it is evident that the mule planted by itself, in a

congenial climate, would reproduce itself as a species; at least as much

deserving to be so considered as the various Calceolarias of different

districts of South America."[56]
Darwin was informed by Mr. C. Noble that he raises stocks for grafting

from a hybrid between Rhododendron ponticum and R. catawbiense, and that

this hybrid seeds as freely as it is possible to imagine. He adds that

horticulturists raise large beds of the same hybrid, and such alone are

fairly treated; for, by insect agency, the several individuals are

freely crossed with each other, and the injurious influence of close

interbreeding is thus prevented. Had hybrids, when fairly treated,

always gone on decreasing in fertility in each successive generation, as

Gartner believed to be the case, the fact would have been notorious to

nurserymen.[57]

_Cases of Sterility of Mongrels._
The reverse phenomenon to the fertility of hybrids, the sterility of

mongrels or of the crosses between _varieties_ of the same species, is a

comparatively rare one, yet some undoubted cases have occurred. Gartner,

who believed in the absolute distinctness of species and varieties, had

two varieties of maize--one dwarf with yellow seeds, the other taller

with red seeds; yet they never naturally crossed, and, when fertilised

artificially, only a single head produced any seeds, and this one only

five grains. Yet these few seeds were fertile; so that in this case the

first cross was almost sterile, though the hybrid when at length

produced was fertile. In like manner, dissimilarly coloured varieties of

Verbascum or mullein have been found by two distinct observers to be

comparatively infertile. The two pimpernels (Anagallis arvensis and A.

coerulea), classed by most botanists as varieties of one species, have

been found, after repeated trials, to be perfectly sterile when crossed.

wondered at, when we consider how very few experiments have been made

with natural varieties; while there is good reason for believing that

domestic varieties are exceptionally fertile, partly because one of the

conditions of domestication was fertility under changed conditions, and

also because long continued domestication is believed to have the effect

of increasing fertility and eliminating whatever sterility may exist.

This is shown by the fact that, in many cases, domestic animals are

descended from two or more distinct species. This is almost certainly

the case with the dog, and probably with the hog, the ox, and the sheep;

yet the various breeds are now all perfectly fertile, although we have

every reason to suppose that there would be some degree of infertility

if the several aboriginal species were crossed together for the first

time.

the crossing of different stocks and the evil effects of close

interbreeding, up to the partial or complete sterility induced by

crosses between species belonging to different genera, we have, as Mr.

Darwin points out, a curious parallelism with the effects produced by

change of physical conditions. It is well known that slight changes in

the conditions of life are beneficial to all living things. Plants, if

constantly grown in one soil and locality from their own seeds, are

greatly benefited by the importation of seed from some other locality.

The same thing happens with animals; and the benefit we ourselves

experience from "change of air" is an illustration of the same

phenomenon. But the amount of the change which is beneficial has its

limits, and then a greater amount is injurious. A change to a climate a

few degrees warmer or colder may be good, while a change to the tropics

or to the arctic regions might be injurious.
Thus we see that, both slight changes of conditions and a slight amount

of crossing, are beneficial; while extreme changes, and crosses between

individuals too far removed in structure or constitution, are injurious.

And there is not only a parallelism but an actual connection between the

two classes of facts, for, as we have already shown, many species of

animals and plants are rendered infertile, or altogether sterile, by the

change from their natural conditions which occurs in confinement or in

cultivation; while, on the other hand, the increased vigour or fertility

which is invariably produced by a judicious cross may be also effected

by a judicious change of climate and surroundings. We shall see in a

subsequent chapter, that this interchangeability of the beneficial

effects of crossing and of new conditions, serves to explain some very

puzzling phenomena in the forms and economy of flowers.

_Remarks on the Facts of Hybridity._

sufficiently conclusive to prove that the old belief, of the universal

sterility of hybrids and fertility of mongrels, is incorrect. The

doctrine that such a universal law existed was never more than a

plausible generalisation, founded on a few inconclusive facts derived

from domesticated animals and cultivated plants. The facts were, and

still are, inconclusive for several reasons. They are founded,

primarily, on what occurs among animals in domestication; and it has

been shown that domestication both tends to increase fertility, and was

itself rendered possible by the fertility of those particular species

being little affected by changed conditions. The exceptional fertility

of all the varieties of domesticated animals does not prove that a

similar fertility exists among natural varieties. In the next place, the

generalisation is founded on too remote crosses, as in the case of the

horse and the ass, the two most distinct and widely separated species of

the genus Equus, so distinct indeed that they have been held by some

naturalists to form distinct genera. Crosses between the two species of

zebra, or even between the zebra and the quagga, or the quagga and the

ass, might have led to a very different result. Again, in pre-Darwinian

times it was so universally the practice to argue in a circle, and

declare that the fertility of the offspring of a cross proved the

identity of species of the parents, that experiments in hybridity were

usually made between very remote species and even between species of

different genera, to avoid the possibility of the reply: "They are both

really the same species;" and the sterility of the hybrid offspring of

such remote crosses of course served to strengthen the popular belief.

species, not as a distinct entity due to special creation, but as an

assemblage of individuals which have become somewhat modified in

structure, form, and constitution so as to adapt them to slightly

different conditions of life; which can be differentiated from other

allied assemblages; which reproduce their like, and which usually breed

together--we require a fresh set of experiments calculated to determine

the matter of fact,--whether such species crossed with their near allies

do always produce offspring which are more or less sterile _inter se_.

Ample materials for such experiments exist, in the numerous

"representative species" inhabiting distinct areas on a continent or

different islands of a group; or even in those found in the same area

but frequenting somewhat different stations.
To carry out these experiments with any satisfactory result, it will be

necessary to avoid the evil effects of confinement and of too close

interbreeding. If birds are experimented with, they should be allowed as

much liberty as possible, a plot of ground with trees and bushes being

enclosed with wire netting overhead so as to form a large open aviary.

The species experimented with should be obtained in considerable

numbers, and by two separate persons, each making the opposite

reciprocal cross, as explained at p. 155. In the second generation these

two stocks might be themselves crossed to prevent the evil effects of

too close interbreeding. By such experiments, carefully carried out with

different groups of animals and plants, we should obtain a body of facts

of a character now sadly wanting, and without which it is hopeless to

expect to arrive at a complete solution of this difficult problem. There

are, however, some other aspects of the question that need to be

considered, and some theoretical views which require to be carefully

examined, having done which we shall be in a condition to state the

general conclusions to which the facts and reasonings at our command

seem to point.

_Sterility due to changed Conditions and usually correlated with other

Characters, especially with Colour._

reproductive system, and the curious irregularity with which infertility

or sterility appears in the crosses between some varieties or species

while quite absent in those between others, seem to indicate that

sterility is a characteristic which has a constant tendency to appear,

either by itself or in correlation with other characters. It is known to

be especially liable to occur under changed conditions of life; and, as

such change is usually the starting-point and cause of the development

of new species, we have already found a reason why it should so often

appear when species become fully differentiated.

species, we have some external differences with which it is correlated;

and though these differences are sometimes slight, and the amount of the

infertility is not always, or even usually, proportionate to the

external difference between the two forms crossed, we must believe that

there is some connection between the two classes of facts. This is

especially the case as regards colour; and Mr. Darwin has collected a

body of facts which go far to prove that colour, instead of being an

altogether trifling and unimportant character, as was supposed by the

older naturalists, is really one of great significance, since it is

undoubtedly often correlated with important constitutional differences.

Now colour is one of the characters that most usually distinguishes

closely allied species; and when we hear that the most closely allied

species of plants are infertile together, while those more remote are

fertile, the meaning usually is that the former differ chiefly in the

_colour_ of their flowers, while the latter differ in the form of the

flowers or foliage, in habit, or in other structural characters.
It is therefore a most curious and suggestive fact, that in all the

recorded cases, in which a decided infertility occurs between varieties

of the same species, those varieties are distinguished by a difference

of colour. The infertile varieties of Verbascum were white and yellow

flowered respectively; the infertile varieties of maize were red and

yellow seeded; while the infertile pimpernels were the red and the blue

flowered varieties. So, the differently coloured varieties of

hollyhocks, though grown close together, each reproduce their own colour

from seed, showing that they are not capable of freely intercrossing.

Yet Mr. Darwin assures us that the agency of bees is necessary to carry

the pollen from one plant to another, because in each flower the pollen

is shed before the stigma is ready to receive it. We have here,

therefore, either almost complete sterility between varieties of

different colours, or a prepotent effect of pollen from a flower of the

same colour, bringing about the same result.
Similar phenomena have not been recorded among animals; but this is not

to be wondered at when we consider that most of our pure and valued

domestic breeds are characterised by definite colours which constitute

one of their distinctive marks, and they are, therefore, seldom crossed

with these of another colour; and even when they are so crossed, no

notice would be taken of any slight diminution of fertility, since this

is liable to occur from many causes. We have also reason to believe that

fertility has been increased by long domestication, in addition to the

fact of the original stocks being exceptionally fertile; and no

experiments have been made on the differently coloured varieties of wild

animals. There are, however, a number of very curious facts showing that

colour in animals, as in plants, is often correlated with constitutional

differences of a remarkable kind, and as these have a close relation to

the subject we are discussing, a brief summary of them will be here

given.

_Correlation of Colour with Constitutional Peculiarities._

deafness, and of the tortoise-shell marking with the female sex of the

same animal, are two well-known but most extraordinary cases. Equally

remarkable is the fact, communicated to Darwin by Mr. Tegetmeier, that

white, yellow, pale blue, or dun pigeons, of all breeds, have the young

birds born naked, while in all other colours they are well covered with

down. Here we have a case in which colour seems of more physiological

importance than all the varied structural differences between the

varieties and breeds of pigeons. In Virginia there is a plant called the

paint-root (Lachnanthes tinctoria), which, when eaten by pigs, colours

their bones pink, and causes the hoofs of all but the black varieties to

drop off; so that black pigs only can be kept in the district.[58]

Buckwheat in flower is also said to be injurious to white pigs but not

to black. In the Tarentino, black sheep are not injured by eating the

Hypericum crispum--a species of St. John's-wort--which kills white

sheep. White terriers suffer most from distemper; white chickens from

the gapes. White-haired horses or cattle are subject to cutaneous

diseases from which the dark coloured are free; while, both in Thuringia

and the West Indies, it has been noticed that white or pale coloured

cattle are much more troubled by flies than are those which are brown or

black. The same law even extends to insects, for it is found that

silkworms which produce white cocoons resist the fungus disease much

better than do those which produce yellow cocoons.[59] Among plants, we

have in North America green and yellow-fruited plums not affected by a

disease that attacked the purple-fruited varieties. Yellow-fleshed

peaches suffer more from disease than white-fleshed kinds. In Mauritius,

white sugar-canes were attacked by a disease from which the red canes

were free. White onions and verbenas are most liable to mildew; and

red-flowered hyacinths were more injured by the cold during a severe

winter in Holland than any other kinds.[60]

constitutional peculiarities, both in animals and plants, render it

probable that the correlation of colour with infertility, which has been

detected in several cases in plants, may also extend to animals in a

state of nature; and if so, the fact is of the highest importance as