site sex. While the chromosomal correlation is here forced to failure the metabolic correlalation here persists. The chromosomal constitution is not an efficient cause of sex; it is but a sign or index and possibly an assistance in the normal maintenance of that which is essential-namely, two different metabolic levels. But the requisite metabolic level of the germ may be established in the absence of the usual or appropriate chromosome complex, and the sex of the offspring made to correspond to the acquired grade or level of metabolism.

These facts which we consider firmly established in the pigeons carry the further essential analysis of sex practically into the field of physiology and bio-chemistry. Further analysis of the basis of sexual difference-in germ or in adult is to be sought in studies of the metabolic differences of the two kinds of sexgerms, of adults of the two sexes, and of individuals of intermediate sex. Now that the problem of sex has been shown to belong in the field of metabolism we shall be able to note, in connection with our diagram, that a number of the requisite data bearing on germinal and adult sexual differences are already at hand.

Turning now to the diagram we note that egg and adult stages are considered. In the egg of the pigeon we have identified maleness and femaleness by three differentials. Femaleness in the egg stage being accompanied by low metabolism, lower percentage of H,O, and higher total fat and phosphorus, or of phosphatides. Maleness is here accompanied by high metabolism, higher percentage of water, and lower total fat and phosphatides. Now there are valid reasons for treating these three differentials not as absolutely separate and disconnected facts, but rather as aspects or corrollaries of the same fact. For example, a high metabolism in a cell is consonant with less storage of fat and phosphatides, and with a more highly hydrated state of the cell-colloids. It follows that where data for either of

4 Since the chromosomes are structural characters they can not be expected readily to alter their numbers, etc., in response to new quantitative levels attained (permanently) by the fundamental cell-functions.

For what forms then are such data available? And, what is now known of the persistence of this definite type of differentiation of the two kinds of sex-germs into adult stages of the two sexes? Recently Lawrence and Riddles have shown that one of these differentials-or one aspect of the differential which my own work has demonstrated in the egg-is clearly continued in the blood of the adult male and female (see Table I.). Fowls were substituted for doves in this case in order to increase the size of the sample, and thus increase the accuracy of the analytical results. In birds, therefore, we have fairly clear evidence that the metabolic differences of male and female germs persist in the male and female adults. In mammals too these aspects of sexual differences of the adults have been fully demonstrated. Almost simultaneously with the above determinations, data were published by Goettler and Baker, which as we have pointed out, show that the blood of the human male contains less fat, that of the female more." Further, the basal metabolism of the human male and female has recently been accurately

5''Sexual Differences in the Fat and Phosphorous Content of the Blood of Fowls,' "Amer. Jour. of Phys., Vol. XLI., September, 1916. 6 Jour. Biol. Chem., XXV., June, 1916.

7 This result seems to have been anticipated by Gorup-Besanez in 1878.

determined by Benedict and Emmes; they find that the metabolism of man is 5 per cent. to 6 per cent. higher than that of woman.

Have we any measure of either of our differentials in any mammalian egg? I think that the experiments on sex-determination in cattle, together with an observation by van der Stricht, afford some evidence that the watercontent of the male-producing egg is high, and that of the female-producing egg is low. Thury reported in 1862 that from fertilizations made in the early period of heat in cattle an excess of females were produced; and that later (delayed) fertilizations give rise to an excess of males. Similar experiments have been four or five times repeated by others, and these have all shown an excess of one or the other sex in accordance with such early or late fertilization. No one definitely knows whether the ovum of the cow absorbs water in the Fallopian tubes in this interval between ovulation and fertilization, but we do know that every amphibian, reptilian and avian egg that has been investigated does absorb very appreciable amounts of water while being passed from the ovary to the exterior. And, van der Stricht has described phenomena of growth or swelling of the yoke granules in one mammal-the bat-which, I am sure from my own studies on yolk, indicate the taking up of water by the egg of this mammal. It is highly probable, therefore, that precisely that time relation which leads to an excess of males in cattle is preceded or accompanied by an increased hydration of the ovum. In mammals therefore there is some evidence that a shift of the metabolic level-as indicated by one partly known sex-differential-is associated with the observed changes in the sex-ratio of the germs which are thus modified. Further, in one adult mammal-man-two of the three sexdifferentials have been definitely demonstrated. These results for both the egg and adult stages of the mammal are at every point in 8 Jour. Biol. Chem., Vol. XX., 1915. These authors give references to earlier literature.

The use of the terms early and late fertilizations assumes that some ovulation occurs either immediately before, or shortly after, the beginning of heat.

complete agreement with our data for both the egg and adult stages of the bird.

Experiments on the frog and the toad have afforded evidence for the control of sex. This evidence by many is not thought conclusive. Though selective fertilization has been eliminated as a possibility by Kuschekewitch there remains the possibility of parthenogenetic development to account for the excessive maleproduction in his experiments with the frog. But this appeal makes it impossible to explain the great excess of females obtained by Dr. King on the eggs of the toad, and leaves such doubters to lean here upon the discredited staff of selective fertilization-a proposition wholly disproved for the related frog and for the pigeon.

How does this situation look in the light of the sex-differentials already noted for birds and mammals? Richard Hertwig,10 and later Kuschekewitch,11 allowed frog's eggs to overripen-a process during which the eggs take up water-and obtained (in the case of the latter author) in some cases a total of 100 per cent. of males. Dr. King12 did the converse of this experiment with toad's eggs-withdrawing water from them before fertilization-and obtained nearly or quite 90 per cent. of females in cases where the mortality was less than 7 per cent. According to our knowledge of the sex-differentials in the pigeon's eggs both of these experiments might have been predicted to result as these three investigators have reported.

In the spider-crabs Geoffrey Smith1 has shown that both the blood and the liver of the adult male crabs contain less fat than do the blood and liver of the females. Here once more the facts concerning one of the sex-differentials is in complete accord with all the preceding cases. In the parasitically castrated spidercrabs Smith and Robson were able to show, moreover, that the parasitized male crabs, which under these conditions gradually assume several female morphological character10 Verhand. deutsch. zool. Gesellsch., 1906. 11 Festschrift. f. R. Hertwig, 1910.

12 Jour. of Exp. Zool., Vol. 12, April, 1912. 13 The Quart. Jour. of Micr. Sci., Vol. 57, November, 1911.

istics, are also found to have assumed the type of fat metabolism which characterizes the normal female crab. How much these facts contribute to, and how completely they adjust themselves to, our own general theory, will be realized only after a moment's reflection.

A glance at the diagram indicates three other groups of animals which experimental work has thrown into the general question of the control of sex. The information at hand for these forms does not so expressly concern the egg as does that from the preceding cases, but all of these latter groups are concerned with early stages-some of them with the generation preceding the egg whose sex seems influenced by conditions. The results of studies of the first of these groups-Hydatina-are of such a kind as to show that they are in general accord with the metabolic differentials of all of the previously mentioned cases of sexcontrol. One can scarcely doubt that change of food, and increased oxygen supply are consonant with increased metabolism, just as the studies of Whitney14 particularly, and later of Shull, 15 have shown that these changes lead to the appearance of male-producing daughters.

The second of these groups-the Daphnidshave been studied by three independent investigators who agree upon two points that are of importance in the question of the control of sex, and to the general theory of sex as stated here, though the results throw little light on precisely what is causally involved. Issakowitch,16 Woltereck1 and Banta,18 all find numerous sex-intermediates in a material for which all agree that the type of reproductionsexual or asexual-is influenced by environmental conditions. All further agree that unfavorable conditions" (or is it a change from favorable conditions?) tends toward sex

14 SCIENCE, N. S., Vol. 39, June 5, pp. 832-33, 1914. Also Jour. Exp. Zool., Vol. 17, November, 1914, and later papers.

15 Abstracts of Amer. Soc. Zool., December meeting, SCIENCE, N. S., Vol. 43, 1916.

16 Biol. Centralbl., Vol. 25, 1905.

17 Intern. Rev. d. gesammt. Hydrobiol. u. Hydrogr., Vol. 4, 1911-12.

18 Carnegie Year Book, 1915, and Proc. Nat. Acad. Sci., Vol. 2, October, 1916.

ual reproduction, while "favorable conditions" favor asexual reproduction.

66

In the third of these groups-the mothsthe studies of Goldschmidt, and Goldschmidt and Poppelbaum, 19 and the work of Machida, have demonstrated again sex-intermediates of various grades. Moreover, it has been shown that from among the various geographical races of moths certain matings can be arranged which produce rather definite types of male or female-intermediates or sex-intergrades, as Goldschmidt elects to call them. And further, from pairs involving still other species still other levels or grades of sexintermediates may be freely obtained. A more or less factorial basis of the phenomena has hitherto been used in the discussion of these results; but recently Goldschmidt20 has stated that very important new facts will be published later which will probably enable us to replace the symbolistic Mendelian language, used here, by more definite physicochemical conceptions." Such newer descriptions-we would say-is wholly in line with the requirements of present data on sex. In Whitman's and our own material it has been clear from the first that the results far overstep the possibility of treating them in Mendelian terms, for it has been apparent from the beginning that we have had to do not with three or four points merely, but with a flowing graduated line. In the work with the moths, however, sex is clearly described in quantitative terms, and we can readily believe that when the functional basis of sex can there be identified, sex will be found to accord with metabolic grades there, as it does elsewhere.

It is clear then that all of the animal-forms for which there is reasonable evidence of sexcontrol show important correspondences with the situation fully elucidated in the pigeons. And that where the sex-differentials known to

19 Goldschmidt u. Poppelbaum, Ztschr. induct. Abstammungsl., Vols. VII. (1912), and XII. (1914), and other papers 1913-16 by both authors. See R. Goldschmidt, below.

20 R. Goldschmidt, Amer. Nat., Vol. L., December, 1916.

exist in the pigeon's ova have been traced in adults of the two sexes, the parallel rigorously holds there also. A general classification of male and female adult animals on the basis of a higher metabolism for the one, and a lower for the other, was indeed made by Geddes and Thomson21 many years ago. There can now be little question that this conclusion of these authors is a correct and important one.

It remains to point out that another very old, and much-worked line of investigation supplies further confirmatory evidence for our present point of view. Studies on the effects of castration, gonad-transplantation, and gonad-extract injection, constitute a large body of observations which deal with sexual phenomena associated with the internal secretions of the sex-glands. These internal secretions, let it be remembered, are themselves metabolites, which have the capacity to influence the metabolism of some, many, or of all the tissues with which they came in contact, or which they may reach indirectly. A partial list of the animal forms that have been most studied in this respect is written vertically on the top of our diagram—in a position intermediate to egg and adult. The number of these animal forms might be much increased, and the names of the investigators of this aspect of the modification of sex are quite too numerous22 to be mentioned here.

21 The Evolution of Sex," 1890, Humboldt Publ'g Co., New York.

22 The following partial references are suggested by the particular animals listed in the diagram: Stag-Darwin (1868); Caton (1881); Fowler (1894); Rörig (1900). Human-Hegar (1893); Selheim (1898); Hikmet and Renault (1906); C. Wallace (1907); Tandler and Gross (1909). Sheep-Shattock and Seligman (1904); Seligman (1906); Marshall and Hammond (1914). Guineapig-Bouin and Ancel (1903-09); Steinach (1910-13). Pheasant-Gurney (1888). Fowl and Duck-Darwin (1868); Gurney (1888); Foges (1903); Shattock and Seligman (1906-07); Goodale (1910-16). Pigeon-Riddle (1914). FrogNussbaum (1907); Pflüger (1907); Steinach (1910); G. Smith (1912). Inachus and Carcinus -Potts (1909); G. Smith (1910-12). Free-martin-Lillie (1916). Bonellia-Baltzer (1914).

But the present point of interest is that these results, as a whole, demonstrate that the extent of sexual modification in the experimental animal is, in general, in proportion to the immaturity of the treated animal. That is to say, the earlier the internal secretion of the gonad is supplied or withdrawn, the more profound is the sexual modification of the individual. The stag is a form that has long been known to show thus a considerable and beautiful series. The free-martin-another Ungulate is now known to exemplify a much earlier point at which the foreign internal secretion begins to act; and here, true to the rule that has been established elsewhere in all this general line of work, the resulting modification is correspondingly strong and striking. When, by whatever means, we effect a change in the metabolism (which is the essential thing) at a still earlier stage-in the eggstage, in our own and in some other experimental reversals of sex,-then we obtain individuals whose sexual nature is quite thoroughly reversed; in many cases completely so, and in still other cases with varying degrees of completeness.

Professor Whitman's main decisions concerning the nature of sex may here be briefly stated. These decisions were that the male proceeds from a "stronger" germ, has greater "developmental energy," and "carries the processes of development farther" than does the female. I am confident that his results fully justify his conclusions; and that these are in the completest harmony with the later and fuller developments of the sex-studies in the pigeons, and thus with the theory of sex which has been outlined in these pages.

In conclusion, our present definite knowledge of the metabolic basis of sexual difference, and the methods of attack which this new knowledge brings with it, offer the surest guarantees that the problem of sex can now be studied and, indeed, the basal facts of the problem must be studied-in the field of the elemental protoplasmic functions.

COLD SPRING HARBOR, N. Y.

OSCAR RIDDLE

PHYSICAL CHEMISTRY IN THE SERVICE OF PHYTOGEOGRAPHY 1

BIOLOGISTS, grown in the present generation from a mere squad of determined scouts to a splendid army of disciplined investigators, increasing daily in rank and equipment, have as their greatest task the placing of biology alongside physics and chemistry in the ranks of the exact sciences.

In the title of this paper, Phytogeography, which even its most ardent disciples must confess is one of the least quantitative of the biological sciences, is coupled with Physical Chemistry, which is conceded by all to be one of the most precise of the physical sciences. This contrast has been made, not to magnify the chasm which conventionally has been assumed to separate the exact from the descriptive sciences, but to emphasize to biologists and to chemists and to physicists alike, the fact that the methods of the most advanced physical sciences can now be successfully employed in such a confessedly descriptive phase of biology as ecology and phytogeography.

In turning to the task of the moment, which is to consider how certain of the simplest physico-chemical methods may be of service in ecology and phytogeography, it is important to place the group of problems to be investigated in its proper biological setting, and to state these problems in such a form that their relationship to a physico-chemical method of investigation is quite obvious.

1 A paper presented at the Symposium on Relations of Chemistry to Botany, before the joint session of Section G, American Association for the Advancement of Science, and the Botanical Society of America, December 27, 1916.