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The orchard oriole, Icterus spurius, has the head, back, and tail all black, the lower parts chestnut, and the body relatively small, as shown by the average measurements of different parts. In the hooded oriole, Icterus cucullatus, the head is all golden orange except the throat, which is black, the tail is black, and the wings are black and white. This species, with its subspecies, ranges through southern California and Arizona, and over much of Mexico. Our other orioles have the tail black and orange. In the common Baltimore oriole, Icterus galbula, of the east, the head is all black and the under parts orange. In the equally common Bullock oriole, Icterus bullocki, of the California region, the head is yellow on each side, the belly rather yellow than orange. The females of all the species are plain olivaceous, the color and proportions of parts varying with the different species, while in the males of each of the many species black, white, yellow, orange, and chestnut are variously and tastefully arranged. Each species again has a song of its own, and each its own way of weaving its hanging Inest.
That which interests us now is that not one of these varied traits is clearly related to any principle of utility. Adaptation is evident enough, but each species is as well fitted for its life as any other, and no transposition or change of the distinctive specific characters or any set of them would in any conceivable degree reduce this adaptation. No one can say that any one of the actual distinctive characters or any combination of them enables their possessors to survive in larger numbers than would otherwise be the case. One or two of these traits, as objects of sexual selection or as recognition marks, have a hypothetical value, but their utility in these regards is slight or uncertain. Naturalists now look with doubt on sexual selection as a factor in the evolution of ornamental structures, and the psychological reality of recognition marks is yet unproved, though not impossible. It may be noted in passing, that the prevalent dull yellowish and olivaceous hues of the female orioles of all species seem to be clearly of the nature of protective coloration.
It has been shown statistically that certain specific characters among insects have no relation to the process of selection. Among honey bees the variation in venation of the wings and in the number and character of the wing hooks is just as
great among the bees which first come from their cells as in a series of individuals long exposed to the struggle for existence.
Among ladybird beetles of a certain species (Hippodamia convergens), eighty-four different easily describable "aberrations” or variations in the number and arrangement of the black spots on the wing covers have been traced. These variations are again just as numerous in individuals exposed to the struggle for life as in those just escaped from the pupal state. In these characters, there is, therefore, no rigorous choice due to natural selection. Such specific characters, without individual utility, may be classed as indifferent, so far as natural selection is concerned, and the great mass of specific characters actually used in systematic classification are thus indifferent.
And what is true in the case of the orioles and the ladybirds is true as a broad proposition of the related species which constitute any one of the genera of animals or plants. All that survive are sufficiently fitted to live, each individual, and therefore, each species, matched to its surroundings as the dough is to the pan, or the river to its bed, but all adaptation lying apparently within a range of the greatest variety in nonessentials. Adaptation is presumably the work of natural selection; the division of forms into species is the result of existence under new and diverse conditions.
VARIATION AND MUTATION
It becomes imperative that we should carry out the most exact research possible by means of experiment and also wean ourselves of the convenient, but, as it seems to me, highly pernicious habit of theoretical explanations from general propositions. Otherwise there is great danger that the bright expectation which Darwin has opened out to us by his theory may be baffled—the prospect of gradually bringing even organic Being within reach of that method of inquiry which seeks to discern mechanical efficient causes.—SEMPER.
Thu's far in our discussion of evolution factors and theories we have taken for granted the actuality of the two fundamental factors, variation and heredity. No one disputes their reality: nor does anyone deny their fundamental and indispensable character in relation to the origin of species and the evolution of organisms. All the theories to explain evolution build on these two basic factors or vital conditions. The subjects of doubt or denial are such postulated factors as selection, mutation, orthogenetic progress, etc.; variation and heredity never.
But the character, the influence, the regularity or irregularity of variations, their behavior in heredity, whether transmissible or not, whether acquired or congenital, whether determinate or indeterminate, etc.—these are the problems that the factor variation or variability presents to biologists. Heredity, too, has its problems. These we shall take up in another chapter.
That variations exist is too obvious to everyone to need any discussion. Any litter of kittens or puppies, of mice or pigs, shows us the differences in pattern, shape, and physiology of individuals born at one time and of the same parents. In wild nature the variations among brothers and sisters are no less real than among these domesticated animals.
Collect a few thousand individuals, at one time in one place, of a single species of insect, as a spotted ladybird beetle; then go over these carefully, looking for variation in some single characteristic, as the color pattern. What do you find? Let us
FIG. 72.-Diagram showing variation in elytral pattern of the convergent ladybird,
Hippodamia convergens : 1, Mode; 2-9, variations in size of spots; 10-17, variations by coalescence of spots; 18-40, variations by reduction in number of spots. (After Kellogg and Bell.)
answer by calling attention to Figs. 72, 73, and 74 and what these variations signify. Note also Fig. 75, showing the
Fig. 73.-Diagram showing variations in elytral pattern of convergent ladybird,
Hippodamia convergens: 1-5, Variations by different reduction in number of spots in the two elytra; 6-9, variations by conditions of spots. (After Kellogg and Bell.)
variation in elytral blotching to be found in a series of individuals of the California flower beetle, Diabrotica soror; see also Fig. 76, showing the variations in the black and A
А yellow color pattern of the abdomen of the common yellow jacket (Vespa sp.); and Fig. 77 showing the variation in the pattern of the prothorax in a series of 178 individuals of a common Californian flower bug, all these individuals collected at one time by sweeping a net over a few rods of alfalfa and Baccharis on the campus Fig. 74.-Diagram showing variations in of Stanford University.
prothoracic pattern of the convergent These are all color and
ladybird, Hippodamia convergens. (After
Kellogg and Bell.) pattern variations; but insects show variations in structural parts as well. Fig. 78 shows a common red-legged locust and one of its hind tibiæ enlarged