Vernon L. (Vernon Lyman) Kellogg.

Darwinism to-day; a discussion of present-day scientific criticism of the Darwinian selection theories, together with a brief account of the principal other proposed auxilary and alternative theories of species-forming online

. (page 11 of 38)
Online LibraryVernon L. (Vernon Lyman) KelloggDarwinism to-day; a discussion of present-day scientific criticism of the Darwinian selection theories, together with a brief account of the principal other proposed auxilary and alternative theories of species-forming → online text (page 11 of 38)
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sexual dimorphism or dichromatism in even more striking
degree, while in many others the differences are less con-
spicuous but nevertheless perfectly obvious if some attention
is given to looking for them. These differences in size,
colour, general appearances, and various specific structural
details in head, trunk, wings, feet, plumage, etc., are over
and beyond those primary radical differences existing in all
species in which the two sexes are differentiated. Some of
these differences may, however, have obvious relation to
the primary differences, in that they may be connected im-
mediately with the act of pairing or with the work of rear-
ing the young. The presence in male insects of complexly
developed holding organs, and in female mammals of milk
glands exemplifies differences of this category. A great many
sexual differences, however, have no such obvious direct
relation to the function of producing and rearing the young.
1 06


Such are the metallic purple and bronze colours of the male
grackles compared with the dull brown of the females ; the
long tails and brilliant coloration of the male pheasants,
the great, spreading, patterned tail of the peacock, the
larger size or the winglessness of many female insects, etc.
All these differences between male and female of the same
species of animal, beyond or in addition to the differences
between the actual primary reproductive organs, are known
as secondary sexual differences, or the characters themselves,
which may be characteristics of physiology and habit as
well as the more familiar ones of structure, are called sec-
ondary sexual characters. The layman may not readily
appreciate the abundance and the great variety of these char-
acters, but it is a fact that almost all species of animals,
excepting those in the lower invertebrate branches, show
them, and if one will try to recall the aspect of the two
sexes in one after another of the species of animals with
which one is familiar, mammals, birds, insects, etc., one will
begin to realise how widespread and significant are these
secondary sexual characters.

Various biologists have made up classifications, from
various points of view, of the different kinds of these char-
Classifications acters > one classification, like that of Kramer *
of secondary sex- for the secondary sexual characters of insects,
ual characters. mav ^ ^ ase( j on ^ faff erent parts of the body

showing the differences between male and female indi-
viduals, as head, antennae, thorax, wings, legs, abdomen,
etc., and on the character of the differences themselves, as
whether differences in structure or in colour and pattern.
Another type of classification, and one more useful for our
present discussion, is that based on the apparent significance
or actual use of the differing characters. An excellent
classification of this type is that of Plate. 2 The following
are the principal subdivisions of this classification with a
few examples illustrating each :



Group A. Characters which are useful to the possessor, or to its
young, or have an indirect relation to reproduction.

Sub-group I. Specialisations in organs which aid in the finding of
individuals of the other sex. Examples, the extra-develop-
ment of the antennae in many male moths and beetles, the
enlarged and divided eyes of certain flies and May-flies, the
enlarged tactile feelers of male Daphnias, the larger or better
wings of many male insects, the swimming membrane, in the
breeding season, on the hind-legs of Molge paradoxa.

Sub-group 2. Specialisations that aid in mating. Examples, the
clasping organs of many male crabs, the hectocotylus of
octopuses, the expanded tarsi of many male insects, and in
general, the accessory copulatory organs of innumerable vari-
ous animal species.

Sub-group 3. Special size and form of the female due to the extra-
development of the ovaries. Examples, in Psychid moths and
parasitic Crustaceans.

Sub-group 4. Differences connected with care of the young. Ex-
amples, mammae of female mammals, brood-pouch of mar-
supials, brood-sacs of male sea-horses, and brood-cavities in
the back of male Pipa (a frog).

Sub-group 5. Specialisations for defence or offence. Examples,
protective coloration of female birds and insects, mimicry
by female butterflies, antlers of the stag, strong canines of
many male mammals (wild boars, etc.), sting of the female
honey-bee, spurs of the cock, greater size and strength of
many male mammals and birds.

Sub-group 6. Differences in coloration which aid in the recogni-
tion of the sexes ("recognition marks" of Wallace).

Sub-group 7. Differences connected with various special habits of
life. Examples, the pollen-baskets of the worker honey-bees,
the winglessness of male fig-caprifying insects (Blastophaga),
the large differences between males and females of certain
insects where one sex lives parasitically, the other independ-
ently, as the scale insects, the Strepsiptera, etc., the beak
differences in the New Zealand bird, Hcteralocha acutiros-
tris, whose male chisels out the hard wood with a short,
broad beak, while the female extracts insect larvae from decay-
ing wood by means of a long, curved beak.

Group B. Exciting organs. These are found almost exclusively in
the males only, and serve to indicate the sexual excitement
of their possessors, and at the same time to stimulate or excite


the females. The excitation of the male is manifest to the
female through her senses of sight, hearing, and smell (in each
case through one or more of these), and this perception gives
rise reflexively to an excitation on the part of the female.
Sub-group i. The male characters may appeal to the sense of sight
of the female: (a) by colours, as in the breeding plumage, or
coloration, of many birds, fishes, amphibians, and reptiles, or,
as in the constant brilliancy of colour and pattern in many
butterflies, other -insects, and spiders; or (b) by striking form,
as the beard and hairy tufts of many monkeys and the
extraordinary "horns" and processes of certain lamellicorn
beetles; or (c) by movable processes (often strongly coloured),
as the wattles and movable feathers (tail, etc.) of certain birds,
swelling cheek or neck sacs of turkeys, etc.; or (d) by strik-
ing movements, as the dancing on the ground or tumbling
and whirling in flight of certain birds, the mating-time bat-
tles of mammals, birds, and fishes, and the "love-dances" of

Sub-group 2. The male characters appeal to the sense of hearing
of the females, as the song of birds, the cries and calls of
many mammals, frogs, and insects.

Sub-group 3. The male characters appeal to the sense of smell of
the females, as the odours given off by goats, chamois, musk-
deer, beaver, etc., and from the scent-scales (androconia) of
many male butterflies.

Group C. Reciprocal organs; that is, organs which exist in func-
tional condition in one sex but are inherited by the other only
in rudimentary and often non-functional form.

Examples; the reduced mammae of male mammals, the brood
pouch of the male Thylacinus ; wingless female butterflies often
liave a rudimentary sucking proboscis, which in some cases is in-
herited by the males without any reduction of the wings ; in
cases of mimicry by female butterflies, the males often show
some traces of the changed colour-pattern ; traces of spurs in
female pheasants, reduced horns of female antelopes and goats,
small "horns" on female lamellicorn beetles, undeveloped stridu-
lating apparatus in female crickets, katydids, etc.
Group D. Indifferent characters, without any apparent utility.

Sub-group i. Rudimentary organs, which are wholly non-func-
tional in one sex, although still functional in the other.
Examples, the reduced wings of many female insects, the
rudimentary alimentary canal of male Rotatoria.
Sub-group 2. Negative characters, that is, those wholly failing in
one sex, although present in the other. This lack can be a


primary one, that is, indicate an older phyletic condition, as
the absence of antlers in the female deer; or be a secondary
one, that is, gradually acquired by progressive reduction, as
the loss of wings by many female insects.

Sub-group 3. Atavistic characters, as the more marked hairiness
on the breast of men as compared with women.

Sub-group 4. Correlated characters, which may be called into ex-
istence by other organs present; with the female mollusc
Anodonta, the shell is more strongly bowed in adaptive
correlation with the expanded brood chamber between the

Sub-group 5. A large number of secondary sexual characters
which are incapable of specific classification, as the minute
differences between the sexes in size and habitus ; slight dif-
ferences in wing form in humming-birds, dragon-flies, and
butterflies; small differences in character and number of
tarsal and antennal segments of many insects.

As Plate justly remarks the foregoing classification can,
of course, make no pretension to completeness. But it

indicates sufficiently clearly certain important
Apparent sig- ;

nificanceoftbe differences among the secondary sexual char-
acters ; differences especially important in con-
nection with any attempt to get at an explanation of the
why and how of these characters. Such a classification
shows that many of these characters have uses which are
of a kind directly helpful in the struggle for existence, as the
strong antlers of the stags, useful in defence against attacking
enemies ; the brood sacs of the kangaroos and opossum, use-
ful in caring for their helpless young; the milk-glands and
teats of all female mammals, the pollen-baskets and wax-
glands of the honey-bee which make the workers more
effective food-gatherers and food-storers, and the protective
colours and patterns of many insects and birds. But others,,
many others, indeed, of these secondary sexual characters,
are either of a kind apparently useless in the struggle for
life, or even of a kind actually harmful. Of apparent useless-
ness are the reduced wings of some male insects, and the
host of slight differences in coloration, pattern, size, or


shape, of different body-parts or of the whole body, the
beards and hair-tufts of many male mammals and the combs
and wattles of male gallinaceous birds. Of apparent harm-
fulness are those ultra-developed pro-thoracic and head
processes, "horns," of stag and other lamellicorn beetles, the
conspicuous staring colours of many male birds, the long
dangling plumes, the .weighty crests, and heavy dragging
tails of others, all these parts also usually being dangerously
conspicuously coloured. The lively loud song of many
male birds, and the dancing and leaping of numerous male
spiders and some male birds must also involve some danger
to the performers by attracting the attention of their enemies.
In fact most of those secondary sexual characters that are
classified under the general head of "exciting organs" are
apparently of a sort that should be actually disadvantageous
in the struggle for existence. They are of a character tend-
ing to make their possessors conspicuous and thus readily
perceived by their carnivorous enemies. How is to be
explained the existence of so many and such highly de-
veloped structural and physiological characters of this kind,
a condition that seems to stand in direct opposition to the
theory of natural selection ? Darwin's answer to this ques-
tion is contained in his theory of sexual selection.

This theory, in few words, is that there is practically a

competition or struggle for mating, and that those males are

successful in this struggle which are the stron-

Darwin's the-
ory of sexual gest and best equipped for battle among them-
selves, or which are most acceptable, by reason
of ornament or other attractiveness, to the females. In the
former case mating with a certain female depends upon
overcoming in fight the other suitors, the female being the
passive reward of the victor ; in the second case the female
is presumed to exercise a choice, this choice depending upon
the attractiveness of the male (due to colour, pattern, plumes,
processes, odour, song, etc.). The actual fighting among


males and the winning of the females by the victor, are ob-
served facts in the life of numerous animal species. But a
special sexual selection theory is hardly necessary to explain
the development of the fighting equipment, antlers, spurs,
claws, etc. This fighting array of the male is simply a special
phase of the already recognised intra-specific struggle ; it is
not a fight for room or food, but for the chance to mate.
But this chance often depends on the issue of a life-and-
death struggle. Natural selection would thus account for
the development of the weapons for this struggle.

For the development, however, of such secondary sexual
characters as ornament, whether of special plumage, colour,
pattern, or processes, and song, and special odours, and
""love-dancing," the natural selection theory can in no way
account; the theory of sexual selection was the logical and
necessary auxiliary theory, and when first proposed by
Darwin * met with quick and wide acceptance. Wallace in
particular took up the theory and applied it to explain many
cases of remarkable plumage and pattern development
among birds. Later as he analysed more carefully his cases,
and those proposed by others, he became doubtful, and
finally wholly sceptical 4 of the theory.

The theory as proposed by Darwin was based on the fol-
lowing general assumptions, for the proof of each of which a
Postnlat d ^ ew * man y ^ ac ^ s were adduced. First, many
bases of the secondary sexual characters are not explicable
by natural selection ; they are not useful in the
struggle for life. Second, the males seek the females for
the sake of pairing. Third, the males are more abundant
than the females. Fourth, in many cases there is a struggle
among the males for the possession of the females. Fifth,
in many other cases the females choose, in general, those
males specially distinguished by more brilliant colours, more
conspicuous ornaments, or other attractive characters.
Sixth, many males sing, or dance, or otherwise draw to


themselves the attention of the females, Seventh, the sec-
ondary sexual characters are especially variable. Darwin
believed that he had observed certain other conditions to ex-
ist which helped make the sexual selection theory probable,
but the conditions noted are sufficient if they truly exist.

Exposed to careful scrutiny and criticism an admira-
ble and convincing example of such scientific and impar-
tial criticism is Kramer's analysis of the

Scrutiny and

criticism of the secondary sexual characters of European m-
theory. sects ^ t h eor y o f sexua i selection has been

relieved of all necessity of explaining any but two categories
of secondary sexual characters, namely the special weapons
borne by males, and the special ornaments and excitatory
organs of the males and females. For examination has
disclosed the fact that males are not alone 5 in the possession
of special characters of attraction or excitation. Regarding
these two categories Plate, 8 in his able recent defence of
Darwinism, says, "the first part of this theory, the origin of
the special defensive and offensive weapons of males through
sexual selection is nearly universally accepted. The second
part of the theory, the origin of exciting organs has given
rise to much controversy. Undoubtedly the presumption
that the females compare the males and then choose only
those which have the most attractive colours, the finest song,
or the most agreeable odour, presents great difficulties, but
it is doubtful if it is possible to replace this explanation
by a better." Some of these difficulties may be briefly

The theory can be applied only to species in which the
males are markedly more numerous than the females, or in
which the males are polygamous. In other
more nnmerons cases there will be a female for each male
whether he be ornamented or not; and the
unornamented males can leave as many progeny
as the ornamented ones, which would prevent any cumula-


tion of ornamental variations by selection. As a matter of
fact in a majority of animal species, at least among the
vertebrates, males and females exist in approximately equal

Observation shows that in most species the female is
wholly passive in the matter of pairing, accepting the first
Female is turn- ma ^ e that offers. Note the cock and hens in
ally passive. the barnyard.

Ornamental colours are as often a characteristic of males
of kinds of animals in which there is no real pairing, as

among those which pair. How explain by
Ornaments oc- J

our on males that sexual selection the remarkable colours in the
do not pair. breeding season of many fishes, in which the
female never, perhaps, even sees the male which fertilises
her dropped eggs?

Choice on a basis of ornament and attractiveness implies
a high degree of aesthetic development on the part of the

females of animals for whose development in
astheTto develop- this line we have no (other) proof. Indeed this

ment in lower choice demands aesthetic recognition among

animals to which we distinctly deny such a

development, as the butterflies and other insects in which
secondary sexual characters of colour, etc., are abundant
and conspicuous. Similarly with practically all invertebrate
animals. Further, in those groups of higher animals where
aesthetic choice may be presumed possible we have repeated
evidence that preferences vary with individuals. Certainly
they do with man, the animal species in which such prefer-
ences certainly and most conspicuously exist. In some
human races hair on the face is thought beautiful ; in others,
ugly. Besides even if we may attribute fairly a certain
amount of aesthetic feeling to such animals as mammals and
birds, is this feeling to be so keen as to lead the female
to make choice among only slightly differing patterns of
songs ? Yet this assumption is necessary if the development


of ornament and other attracting and exciting organs is to
be explained by the selection and gradual cumulation through
generations of slight fortuitously appearing fluctuating
variations in the males.

There are actually very few recorded cases where the

observer believes that he has noted an actual choice by a

Few observed ^ ema ^ e - .Darwin records eight cases among

oases of choice birds. Since Darwin not more than half a

dozen other cases, all doubtful, have been

recorded. Also a few instances, all more illustrative of

sexual excitation of females resulting from the perception

of odour or actions, than any degree of choice by females,

have been listed.

In numerous cases the so-called attractive characters of

the males, described usually from preserved (museum)

specimens, have been found, in actual life, to

attractive'char- ^ e of such a character that they cannot be noted

acters not visible by th e female. For example, the brilliant

in nature. . ,

colours and the curious horns of the males
of the dung beetles are, in life, always so obscured by dirt
and filth that there can be no question of display to the
female eyes about them. The dancing swarms of many
kinds of insects are found to be composed of males alone
with no females near enough to see ; it is no case of an
excitatory flitting and whirling of many males before the
eyes of the impressionable females. Of many male katy-
dids singing in the shrubbery will not for any female that
particular song be the loudest and the most convincing that
proceeds from the nearest male, not the most expert or the
strongest stridulator? Similarly with the flitting male fire-
flies ; will not the strongest gleam be, for any female, that
from the male which happens to fly nearest her, and not
from the distant male with ever so much better, stronger

Stolzmann finds it difficult to understand, when nearly


related species differ widely in their ornamental plumage,

that this should be attributed to a difference in

Problem of preference among the females of the related

the Andean

humming-birds, species. The humming-bird, Schistes perso-

natus, lives in Ecuador on the west side of the
Andes, in a restricted range of four degrees of latitude. It
is distinguished from the nearly related Schistes geoffroyi
especially in the possession by its male of a brilliant spot over
each eye. Schistes geoffroyi lives on the east side of the
Andes from Colombia to Central Peru, with a range cover-
ing over twenty degrees of latitude, which range is divided
into two completely separated regions by the Maranon
valley. Now if isolation alone is sufficient to produce a
change in the taste of the females, one would expect to find
two sorts of males (as far as ornamental pattern goes) in-
side this one species. But there is but one kind of male
through the whole range. Why is the taste of the female
constant through twenty degrees of latitude, while it is
changed on the other side of the Andes in a limited range
of four degrees of latitude? Another case presented by
Stolzmann is even more striking. The Chilian hummer,
Eustephanus galeritus, which is green in both sexes, has
migrated from the continent to the Juan Fernandez Islands.
On Masatierra Island it has changed to Enst. fernandensis,
in Masafuera Island to Eust. leyboldi. These two species
agree in the females with the original continental form
(that is, are green) while the males have become red, but in
different pattern in .the two species. Eust. galerittis (the
Continental form) also occurs on Masatierra Island, in the
same form as on the continent, that is, with green male.
Now one must presume from this state of affairs that this
species (galeritus} has been able to reach Masatierra twice,
once long ago the descendants of the invasion having
changed to Eust. fernandensis and once more recently
the descendants of these later migrants showing as yet no


sign of a transformation of the male colour. Shall one in this
case and others like it, asks Stolzmann, assume a change of
beauty-ideal on the part of the females ? Much simpler and
much more reasonable, according to Stolzmann, is it to see in
the change of colour of the males of the earlier migrants the
results of the direct influence of the new environment ; the
islands are distinctly milder and warmer than the continent.
Even if the females do choose among the males on a basis
of attractiveness, how are the characters of the more at-
tractive males to become especially fostered
attractive' SIS and accumulated by selection? Do such males
acters to be produce more offspring or more vigorous ones
than the other males, which, though rejected
by the first females, find their mates among the females not
already mated ? Are we to attribute to the more ornamental
males a particular vigour? If so, may not that very vigour
be the cause of the extra-production of colour or plumage
or wattles, etc. ?

Darwin admits, in order to explain the beginnings of
colour and ornament development, a certain degree of differ-
ence between the male and female in regard to

Darwin's Big- _ ,.

nificant admis- their reaction to environmental influences. If
8ion - so, may not these admitted differences be really

sufficient to account for even a pretty high degree of differ-
ence in development of secondary sexual characters ?

The special display of colours, tufts, plumes, spreading

tails, and other secondary sexual characters by the males at

mating time is an observed fact; the "dances"

piwro ctSce? 7 of cranes and storks > tne serenades of the song-
birds, the evolutions of the male spiders are all
familiar phenomena in the mating season of these animals.
And they probably do exercise an exciting effect on the
females, and are probably actually displayed for this pur-
pose. But does this in any way prove, or even give basis
for a reasonable presumption for belief in a discriminating


and definitive choice among the males on the part of the
female? And it is this actual choosing which is the neces-
sary basis for the theory of sexual selection.

How explain the well-known cases of a similar extra-

Online LibraryVernon L. (Vernon Lyman) KelloggDarwinism to-day; a discussion of present-day scientific criticism of the Darwinian selection theories, together with a brief account of the principal other proposed auxilary and alternative theories of species-forming → online text (page 11 of 38)