FIG. 75.-Diagram showing variations in elytral pattern of the California
flower beetle, Diabrotica soror. (After Kellogg and Bell.)

to show the spines. In eighty-nine individuals of this species of locust collected at Ithaca, N. Y., the number of spines in the

outer row of the right tibiæ varies from nine to fifteen, in the inner row from eleven to sixteen. One not given to the systematic study of insects may think spines on the hind legs very trivial structures indeed; but the entomologist, using exactly such characteristics as the number of these structures as a means in helping him to distinguish and define his species, knows how considerable this variation really is.

The dog-days cicada (Fig. 79) also has spines on its hind tibie, but only a few, usually, indeed, two. But in any series

of individuals of this insect some individuals will be found with but a single spine, some with three, and a few with four even, although the very great majority will have two. For example,

in a series of 98 male individuals collected at Indianapolis, Indiana, at one time, 12 individuals had one spine in the outer row of the right tibiæ, 83 had two spines, 2 had three spines, and one had four spines. In the outer row of the left tibia of the same individuals, there were three spines in 6 individuals, two in 75, and one in 17. In the inner rows of tibial spines in these same b

SL

4R

FIG. 77.-Diagram showing variation in pattern of the prothorax of a flower bug, (After Kellogg and Bell.)

individuals there were in the right tibiæ, five spines in 5, four spines in 40, three spines in 43, two spines in 9, and one spine in 1 individual: in the left tibiæ, five spines in 2 individuals, four spines in 48, three spines in 39, and two spines in 8.

In the paper from which we have taken these illustrations of the actuality of variation, studied and statistically tabulated, are given the data showing the actual extent and frequency of variations in various characters, such as color patterns of head, thorax, and abdomen, character of antennal segments, number of tibial spines, character of elytral striation, character of vena

tion, number of wing hooks, etc., in two dozen different insect species. Long ago Dr. J. A. Allen, of the American Museum of Natural History, gave similar data of the actual variation in

various familiar American bird species, his data referring chiefly to variations in dimensions; as length of whole body, length of tail, of wing, of bill, of tarsus and claw, etc.

CARDINALIS VIRGINIANUS 58 specimens, Florida.

Tail.

Length of Bird.

Wing.

FIG. 80.-Diagram showing variation in length of tail, body, and wing in fifty-eight specimens of the cardinal, Cardinalis (formerly called virginianus), from Florida. (After Allen.)

And anyone with means of collecting considerable series of individuals of single species can, if he but give the time and study to it, reveal similar variations in almost any part or characteristic of any species or kind of plant or animal.

parts vary?" some one asks. All parts vary, but some more than others.

Darwin, in Chapter V of his "Origin of Species," postulated certain so-called laws of variability, which attempt to answer this question, "What parts vary?" These so-called "laws" which to-day would hardly

be dignified with the name

VARIATION OF

Tail.

of law, are summed up by ICTERUS BALTIMORE.20.3 Darwin at the end of this chapter as follows:

Wing.

Tarsus.

Middle Toe.

Hind Toe.

Bill, Length.

"Our ignorance of the laws of variation is profound. Not in one case out of a hundred can we pretend to assign any reason why this or that part has varied. But whenever we have the means of instituting a comparison, the same laws appear to have acted in producing the lesser differences between varieties of the same species, and the greater differences between species of the same genus. Changed conditions generally induce mere fluctuating variability, but sometimes they cause direct and definite effects; and these may become strongly marked in the course of time, though we have not sufficient evidence on this head. Habit in producing constitutional peculiarities, and use in strengthening, and disuse in weakening and diminishing organs, appear in many cases to have been potent in their effects. Homologous parts tend to vary in the same manner, and homologous parts tend to cohere. Modifications in hard parts and in external parts sometimes affect softer and internal parts. When one part is largely developed, perhaps it tends to draw nourishment from the adjoining parts; and every part of the structure which can be saved without detriment will be saved.

Bill, Width.

FIG. 81.-Diagram showing variation in dimensions in twenty male specimens of the Baltimore oriole, Icterus galbula (formerly called baltimore). (After Allen.)

Changes of structure at an early age may affect parts subsequently developed; and many cases of correlated variation, the nature of which we are unable to understand, undoubtedly occur. Multiple parts are variable in number and in structure, perhaps arising from such parts not having been closely specialized for any particular function, so that their modifications have not been closely checked by natural selection. It follows, probably from this same cause, that organic beings low in the scale are more variable than those standing higher in the scale, and which have their whole organization more specialized. Rudimentary organs, from being useless, are not regulated by natural selection, and hence are variable. Specific characters—that is, the characters which have come to differ since the several species of the same genus branched off from a common parent―are more variable than generic characters, or those which have long been inherited, and have not differed within this same period. In these remarks we have referred to special parts or organs being still variable, because they have recently varied and thus come to differ; but we have also seen . . . that the same principle applies to the whole individual; for in a district where many species of a genus are found-that is, where there has been much former variation and differentiation, or where the manufactory of new specific forms has been actively at work-in that district and among these species we now find, on an average, most varieties. Secondary sexual characters are highly variable, and such characters differ much in the species of the same group. Variability in the same parts of the organization has generally been taken advantage of in giving secondary sexual differences to the two sexes of the same species, and specific differences to the several species of the same genus. Any part or organ developed to an extraordinary size or in an extraordinary manner, in comparison with the same part or organ in the allied species, must have gone through an extraordinary amount of modification since the genus arose; and thus we can understand why it should often still be variable in a much higher degree than other parts; for variation is a long-continued and slow process, and natural selection will in such cases not as yet have had time to overcome the tendency to further variability and to reversion to a less modified state. But when a species with an extraordinarily developed organ has become the parent of many modified descendants-which in our view must be a very slow process, requiring a long lapse of time-in this case, natural selection has succeeded in giving a fixed character to the organ, in however extraordinary a manner it may have been developed. Species inheriting nearly the same constitution from a common parent, and exposed to similar