processes involved in species-forming, that is, evolution. The methods and phenomena of evolution are intimately linked with-indeed throughout are based upon-the methods and phenomena of variation. What causes variation is a contrib 360 320 280 240 200 160 1200 80 400 utory cause in evolution, and one of the fundamental and all-important causes. Concerning the causes of variation, at least of those of congenital variation, we are almost wholly in the dark. Only such influences as can affect the actual germ cells are presumably potent to effect congenital variation. Such influences are not proved to the satisfaction of many biologists to be numerous. In the fusion of the germ cells of two individuals, the phenomenon called by him amphimixis, Weismann finds the most effective cause of variation. Now the wider apart the two parents are in structural and functional characteristics, the greater is the variation in their offspring likely to be. Hence hybridization, or the mating of unlike parents, even to the degree of race and species unlikeness, is a great resource of the breeder who would have in his hands large variation. But if the parents are too unlike, their mating, even if possible, proves sterile. Usually parents must be of the same species, although experiment has shown that considerable extraspecific hybridization is possible. Among cultivated plants and animals the artificially selected races differ very much, but these races are mostly easily hybridiz Classes: BEE MISCEL' Variates 313 40 55 388 109 total 905 FIG. 92. Frequency polygon of variation of elytral pattern in 905 specimens of the California flower beetle, Diabrotica soror, collected at Stanford University, October, 1902. (After Kellogg and Bell.) able. The nature and results of fertilization and amphimixis are treated in Chapter XIII. But parthenogenetically produced individuals (that is, young born from unfertilized eggs-as the honey-bee drones, certain whole generations of various gall flies, saw flies, aphids, etc., etc., regularly are) also vary. FIG. 93.-Fore and hind wings of honeybee (drone), showing normal venation. (After Kellogg and Bell.) In the case of male bees, male ants, female aphids, etc., etc., the individuals differ quite as much as do individuals of the same species of bisexual parentage. Comparing the variation in drone bees (parthenogenetically produced) as compared with that of the workers (from fertilized eggs), we find that this is true. The organs examined for variation in these series of bees were the wings, organs used by both drones and workers, and having no immediate relation either structurally or physiologically to the differentiation of those two castes or kinds of individuals of the honey-bee species. The workers are "incomplete" only in that most of them are infertile: in no other structural or physiological feature of their makeup are they less "complete" than the drones. They are indeed distinctly the more specialized of FIG. 94.-Part of costal margin of hind wing of honeybee, much magnified to show hooks. (After Kellogg and Bell.) the two, and according to one of the early Darwinian canons of variation might be expected to differ more than the drones. But the drones are males and, according to another commonly accepted belief, this is the explanation for a larger variation on their part, if such larger variation occurs. As a matter of fact, it does. The drones, in all the many series studied, show markedly more variation in the venation of the wings than do the workers, while they show quite as much variation as the workers in the number of the hooks which hold the two wings together in flight. (See Figs. 93 to 96.) Both these characters, i. e., wing venation and wing hooks, are not so-called "male char acters": they are not to be compared with those secondary sexual characters such as ornamental or aggressive spines, horns, patterns, etc., which are the characteristics that give males their special reputation for ultravariation. FIG. 95.-Fore wings of honeybee (drone), showing variations in venation. Finally, with regard to the causal influence in variation-producing of the "primary factors of evolution," such as temperature, light, humidity, pressure, and extrinsic physicochemical conditions generally, summed up commonly in the phrase climate and environment, we have one all-important considera G FIG. 96.-Hind wings of honeybee (drone), showing variations in venation. Note the interpolation of the cells. (After Kellogg and Bell.) tion to keep constantly in mind. However potent and obvious the effects of these influences are on the individual, we have no proof as yet of a nature to compel the general acceptance of biologists, that such effects can be carried directly over to the race or species. Only ten years after Darwin published the "Origin of Species," von Kölliker, the great German zoologist, in criticising the assumptions on which species-forming by natural selection was based in the Darwinian theory, proposed an alternative theory of heterogenesis or species-forming by leaps (saltations or mutations). These saltations need not of necessity to be large, but must be changes definite and fixed. Later, Korschinsky, a Russian botanist, outlined in some de tail and with greater emphasis such a theory of species-forming by mutations; and finally in 1901 Hugo de Vries, the famous botanist of Amsterdam, published in extenso the details of many years of observation and experiment on the subject of mutations, and reformulated definitively a theory of speciesforming by mutational or saltational variation, the now familiar mutation theory. The following paragraphs from Morgan ("Evolution and Adaptation," pp. 294–297, 1903) give a concise statement of the actual details of the mutations in the evening primrose observed by de Vries: "We may now proceed to examine the evidence from which de Vries has been led to the general conclusions given in the preceding pages. De Vries, found at Hilversam, near Amsterdam, a locality where a number of plants of the evening primrose, Enothera lamarckiana, grow in large numbers. This plant is an American form that has been imported into Europe. It often escapes from cultivation, as is the case at Hilversam, where for ten years it had been growing wild. Its rapid increase in numbers in the course of a few years may be one of the causes that have led to the appearance of a mutation period. The escaped plants showed fluctuating variations in nearly all of their organs. They also had produced a number of abnormal forms. Some of the plants came to maturity in one year, others in two, or in rare cases in three, years. "A year after the first finding of these plants de Vries observed two well-characterized forms, which he at once recognized as new elementary species. One of these was O. brevistylis, which occurred only as female plants. The other new species was a smooth-leafed form with a more beautiful foliage than O. lamarckiana. This is 0. lavifolia. It was found that both of these new forms bred true from self-fertilized seeds. At first only a few specimens were found, each form in a particular part of the field, which looks as though each might have come from the seeds of a single plant. "These two new forms, as well as the common O. lamarckiana, were collected, and from these plants there have arisen the three groups or families of elementary species that de Vries has studied. In his garden other new forms also arose from those that had been brought under cultivation. The largest group, and the most important one, is that from the original (). lamarckiana form. The accompanying table shows the mutations that arose between 1887 and 1899 from these plants. The seeds were selected in each case from selffertilized plants of the lamarckiana form, so that the new plants appearing in each horizontal line are the descendants in each generation of lamarckiana parents. It will be observed that the species, O. oblongata, appeared again and again in considerable numbers, and the same is true for several of the other forms also. Only the two species, 0. gigas and O. scintillans, appeared very rarely. "Thus de Vries had, in his seven generations, about fifty thousand plants, and about eight hundred of these were mutations. When the flowers of the new forms were artificially fertilized with pollen from |