WALLACE’S SECTION OF THE JOINT MEMOIR READ BEFORE THE LINNEAN SOCIETY
JULY 1, 1858.
The communication by Alfred Russel Wallace was entitled “On the
Tendency of Varieties to depart indefinitely from the Original Type.”
An abstract of it is given below.
[Sidenote: WALLACE’S ESSAY.]
_Varieties_ produced in domesticity are more or less unstable, and
often tend to return to the parent form. This is usually thought to be
true for all varieties, and to be a strong argument for the original
and permanent distinctness of species.
On the other hand, races forming “permanent or true varieties” are
well known, and there are generally no means of determining which is
the _variety_ and which the original _species_. The hypothesis of a
“permanent invariability of species” is satisfied by supposing that,
while such varieties cannot diverge from the species beyond a certain
fixed limit, they may return to it.
This argument is founded on the assumption that _varieties_ in nature
are in all respects identical with those of domestic animals. The
object of the paper is to show that this is false, and “that there is
a general principle in nature which will cause many _varieties_ to
survive the parent species and to give rise to successive variations
departing further and further from the original type.” The same
principle explains the tendency of domestic animals to return to the
parent form.
“The life of wild animals is a struggle for existence.” To procure
food and escape enemies are the primary conditions of existence, and
determine abundance and rarity, frequently seen in closely allied
species.
“Large animals cannot be so abundant as small ones; the carnivora must
be less numerous than the herbivora,” eagles and lions than pigeons
and antelopes. Fecundity has little or nothing to do with this. The
least prolific animals would increase rapidly if unchecked. But wild
animals do not increase beyond their average; hence there must be an
immense amount of destruction. The abundance of species in individuals
bears no relation whatever to their fertility. Thus the excessively
abundant passenger pigeon of the United States lays only one or two
eggs. Its abundance is explained by the widespread supply of food
rendered available by its powers of flight. The food-supply “is almost
the sole condition requisite for ensuring the rapid increase of a
given species.” This explains why the sparrow is more abundant than
the red-breast, why aquatic species of birds are specially numerous
in individuals, why the wild cat is rarer than the rabbit. “So long
as a country remains physically unchanged, the numbers of its animal
population cannot materially increase.” If one species does so, others
must diminish. In the immense amount of destruction the weakest must
die, “while those that prolong their existence can only be the most
perfect in health and vigour--those who are best able to obtain food
regularly and to avoid their numerous enemies. It is, as we commenced
by remarking, ‘a struggle for existence,’ in which the weakest and
least perfectly organised must always succumb.”
This tendency must apply to species as well as individuals, the best
adapted becoming abundant, the others scarce or even extinct. If we
knew the whole of the conditions and powers of a species “we might be
able even to calculate the proportionate abundance of individuals,
which is the necessary result.”
Hence, first, _the animal population of a country is generally
stationary (due to food and other checks)_; second, _comparative
abundance or scarcity of individuals is entirely due to organisation
and resulting habits, the varying measure of success in the struggle
being balanced by a varying population in a given area_.
Variations from type must nearly always affect habits or capacities.
Even changes of colour may promote concealment, while changes in the
limbs or any external organs would affect the mode of procuring food,
etc. “An antelope with shorter or weaker legs must necessarily suffer
more from the attacks of the feline carnivora”; the passenger pigeon
with less powerful wings could not always procure sufficient food.
Hence species thus modified would gradually diminish; but, on the other
hand, if modified in the direction of increased powers, would become
more numerous. Varieties will fall under these two classes--those
which will never rival, and those which will eventually outnumber,
the parent species. If, then, some alteration in conditions occurred
making existence more difficult to a certain species, first the less
favourable variety would suffer and become extinct, then the parent
species, while the superior variety would alone remain, “and on a
return to favourable circumstances would rapidly increase in numbers
and occupy the place of the extinct species and variety.”
The superior _variety_ would thus replace the _species_, to which it
_could not_ return, for the latter could never compete with the former.
Hence a tendency to revert would be checked. But the superior variety,
when established, would in time give rise to new varieties, some of
which would become predominant. Hence _progression and continued
divergence_ would follow, but not invariably, for the criteria of
success or failure would vary, and would sometimes render a race
which was under other conditions the most favoured now the least so.
Variations without any effect on the life-preserving powers might also
occur. But it is contended that certain varieties must, on the average,
tend to persist longer than the parent species, while the scale on
which nature works is so vast that an average tendency must in the end
attain its full result.
Comparing domestic with wild animals, the very existence of the latter
depends upon their senses and physical powers. Not so with the former,
which are defended and fed by man.
Any favourable variety of a domestic animal is utterly useless to
itself; while any increase of the powers and faculties of wild animals
is immediately available, creating, as it were, a new and superior
animal.
Again, with domestic animals all variations have an equal chance, and
those which would be extremely injurious in a wild state are, under the
artificial conditions, no disadvantage. Our domestic breeds could never
have come into existence in a wild state, and if turned wild “_must_
return to something near the type of the original wild stock, _or
become altogether extinct_.”[E]
Hence we cannot argue from domestic to wild animals, the conditions of
life in the two being completely opposed.
Lamarck’s hypothesis of change produced by the attempts of animals to
increase the development of their own organs has been often refuted,
but the view here proposed depends upon the action of principles
constantly working in nature. Retractile talons of falcons and cats
have not been developed by volition, but by _the survival of those
which had the greatest facilities for seizing prey_. The long neck of
the giraffe was not produced by constant stretching, but by the success
which any increase in the length of neck ensured to its possessors.
Even colours, especially of insects, are explained in the same way,
for among the varieties of many tints, those “having colours best
adapted to concealment ... would inevitably survive the longest.” We
can similarly explain deficiency of some organs with compensating
development of others, “great velocity making up for the absence of
defensive weapons,” etc. Varieties with an unbalanced deficiency could
not long survive. The action of the principle is like the governor
of a steam-engine, checking irregularities almost before they become
evident. Such a view accords well with “the many lines of divergence
from a central type”; the increasing efficiency of a particular organ
in a series of allied species; the persistence of unimportant parts
when important ones have changed; the “more specialised structure,”
said by Owen to be characteristic of recent as compared with extinct
forms.
Hence there is a tendency of certain classes of _varieties_ to progress
further and further from the original type, and there is no reason for
assigning any limit to this progression. Such gradual changes “may,
it is believed, be followed out so as to agree with all the phenomena
presented by organised beings, their extinction and succession in past
ages, and all the extraordinary modifications of form, instinct, and
habits which they exhibit.”
Wallace’s Essay has been reprinted without any alteration in his
“Essays on Natural Selection,” recently re-issued combined with
“Tropical Nature.”