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New YORK ACADEMY OF SCIENCES, Nov. 8.-Prof. Newberry made a communication on the antimony mines of Southern Utah. Prof. D. S. Martin exhibited specimens of wax from the Carnauba palm of Northern Brazil, and gave an account of its production, etc.
Nov. 22.--Miss Adelina Bierck described the volcanic eruption at Lake Ilopango, San Salvador, and Mr. A. A. Julien gave an account of a visit to the great alum cave of Sevier county, Tennessee.
Nov. 29.--Prof. H. A. Ward gave a description of the Island of Volcano.
AMERICAN GEOGRAPHICAL SOCIETY. Nov. 18. - Rev. Owen Street read a paper on the changes in the physical geography of the ancient home of man in Central and Western Asia.
SELECTED ARTICLES IN SCIENTIFIC SERIALS. ZEITSCHRIFT FÜR WISSENSCHAFTLICHE ZOOLOGIE-Nov. 6. On the relationship of the Cephalopoda, by H. von Ihering. The organ of smell of the land pulmonates, by Dr. D. Sochaczewer. (Refers to the structure of the feelers, and Moquis Tandon's opinion that the end or “terminal button is the seat of the organ of smell. He then discusses the nature of the organ of Semper and of the foot-glands, and thinks that the latter are more properly organs of smell, as at the base of the excretory duct of the pedal gland are distinct ciliated sense-cells which are like the ciliated sense cells in the skin of mollusks discovered by Flemming.) On the cases of the Trichopterous larvæ of Santa Catharina, Brazil, by Fritz Müller. (Figures and describes a great variety of singular forms of caddis-fly cases.) Researches on the Dysideidan and Phorio-sponges, by W. Marshall. On two early human embryos, by W. Krause (with excellent figures). The pedal nerve-system of Paludina vivipara, by H. Simroth.
THE GEOLOGICAL MAGAZINE — November. Precambrian volcanos and glaciers, by H. Hicks. The Mammoth in Siberia, by H. H. Howorth.
DATES OF PUBLICATION OF THE NATURALIST FOR 1879 AND 1880. -1879: January No., January 4th; February, February 4th; March, February 27th ; April, March 26th ; June, May 20th ; July, June 17th ; September, August 22d; November, October 25th; December, December 4th. 1880: January, January 2d; February, January 31st; March, February 25th; April, March 21st; May. April 27th ; June, May 21st; July, June 18th; August, July 22d; October, September 21st; November, October 23d; December, November 25th.
VOL. xv.- FEBRUARY, 1881. — No. 2.
INCOMPLETE ADAPTATION AS ILLUSTRATED BY
THE HISTORY OF SEX IN PLANTS.1
BY LESTER F. WARD, A.M. THE
doctrine of abrupt changes or cataclysms in nature has a remarkable survival in the still prevalent belief in perfect adaptation. As it was formerly held that organisms were purposely made for their conditions and exactly adjusted to them, so now, since the law of self-adjustment has become current, it is supposed that the organism and the environment have in all cases reached a condition of complete correspondence. It is in virtue of this assumption that the law of cross-fertilization of plants has been called in question, and an eminent botanist once remarked to me that the slight difference between the results of Darwin's experiments under cross and under self-fertilization, amounting on an average to one-fifth of the whole, was sufficient to invalidate that law.
Nothing seems so difficult for the human mind to grasp as change through minute variations indefinitely continued. Even those who admit that this is nature's method, fail to realize it in concrete examples.
We may suppose that a given character not possessed by a given species would, as a matter of fact, be an advantage to such species if it could acquire it. We may further suppose that for any reason the species commences to vary in the direction of acquiring that character. The benefit will be proportional to the degree of completeness with which the character is attained.
Read before the Biological Section of the American Association for the Advancement of Science, at Boston, August 27, 1880.
Under the law of natural selection, the perfection of the character will ultimately be reached, but a very long period, to say the least, must elapse during which it is still incomplete.
Again, the conditions surrounding a species are constantly changing, usually slowly, but sometimes rapidly or suddenly. In this way the usefulness of certain characters is frequently destroyed, but the species cannot lose the character; it persists and gradually becomes atrophied or transformed into a different one. Such changes in organisms are very slow, and vast periods are passed through before they are completed.
Now, considering the changes going on at all times in the conditions under which species exist, it may often happen that the period during which adaptation is incomplete from both these causes, is greater than that during which it is complete. Indeed, as a matter of fact, the adaptation is never absolutely complete, the organism being always, as it were, behind its environment, as the tides are behind the moon.
If this be true, we ought to expect constantly to find examples of incomplete adaptation. A character which required to be complete before it could be advantageous could never be acquired by natural selection. All such characters as are acquired must be advantageous in proportion as they are complete.
Naturalists must therefore learn to regard a large proportion of the characters which they find to exist, as partial or uncompleted characters, useful to the species in proportion as they are developed, but capable of greater adaptation.
There are, moreover, two general classes of characters with respect to their usefulness and advantageousness to the species. Those of one of these classes are only useful to a certain limited degree, beyond which they may be injurious, and which only apply to particular species in their relations to definite existing conditions. Such characters may be called special.
The other class, which may be distinguished as general, apply to all organisms, and are less limited in their degrees of possible development.
Passing over the class of special characters, I propose to illustrate the principles above stated, by an example in the class of general characters taken from the vegetable kingdom.
The distinction of sex is a condition advantageous to all plants, and one in the process of attaining which a large number of gradations are to be found. The purely asexual state exists only in the lowest Protophytes, as in Saccharomyces, the Phycochromaceæ, and other unicellular forms. The simple phenomenon of conjugation or copulation seen in the Zygomycetæ and diatoms, forms the earliest step towards sexual differentiation, which is followed by the various intermediate steps represented by the pairing of active cells in Volvox, the formation of oöspores in the Confervæ and Fucaceæ, and of carpospores in the Fungi.
In the Characeæ we first find the well marked distinction of antheridia and carpogonia, the former furnishing in Nitella the active spermatozooids which differ immensely from the cells with which they combine. This latter feature continues to characterize all the higher Cryptogams, though in nearly all cases the organs of both sexes are borne on the same plant. The transition from the Cryptogams to the Phanerogams is effected by a primary differentiation of the spores, which in most Cryptogams are the independent asexual bodies that produce the sexually differentiated prothallium. This prothallium loses its independence and becomes the albumen of the seed; the male es are converted into pollen grains and the antheridia into the fertilizing pollentubes; the female spores are transformed into embryo-sacs containing corpuscles within which are the ultimate germ-cells.
In a certain sense this transition, instead of marking an advance in the process of sexual separation, constitutes a step backward, since the prothallia of Cryptogams, considered as distinct individuals, are respectively male and female, 'while the stamens and pistils of the Cycadaceæ and Coniferæ, the earliest Phænogams developed, though quite distinct in themselves, are both borne on the same plant. But the prothallium marks the highest development reached or possible to the Cryptogam. The Phänogam must begin from a point lower down, and in turn evolve sexually differentiated forms. The distinction of macrospores and microspores found only in the Rhizocarpeæ and Ligulatæ, and which, as already stated, initiated the transition from the Cryptogams to the Phænogams, took place in the same individual, both kinds of spores often occurring in the same sporangium, as in Salvinia. This, when the two kinds of spores at length came to represent the two sexual organs of the Cycad or the Conifer, necessarily reunited the sexes once more in the same plant, and the process of separation, so well completed in the higher Cryptogams, was required to be begun anew on the higher Phænogamic plane of development.
From this point, however, the history of this process is of the highest interest. In the Cycadaceæ complete diæcisin was reached before any of the few now existing forms were developed, and all present living species are male and female. In the Coniferæ, different families have attained to different degrees of diclinism. The Taxineæ, which many facts show to have been among the earliest forms developed, are diæcious, while the great pine and fir tribes, as well as most cedars, are still moncecious. Both these great orders have come down to us from the Carboniferous epoch, and indicate, along with the remnant which we possess of the then luxuriant cryptogamic flora, the kind of vegetation which prevailed in those remote ages. The flowers even of the highest forms were uniformly inconspicuous and odorless. The only possible substitute for sexual separation was the distribution of pollen by the winds. Forms so high in development, it would seem, could not continue to exist through self-fertilization alone, and hence, under the operation of natural selection, more or less complete sexual separation early took place.
The transition from the Gymnosperm to the Angiosperm is veiled in great obscurity. Certain considerations point to the gradual transformation of the Cycadaceæ into the Monocotylæ through the Palmaceæ or some allied family, on the one hand, and to that of the Coniferæ into the Dicotylæ through the Gnetaceæ and Casuarineæ, on the other. However this may be, the earliest known fossil species of Angiosperms, dating back to the early Trias, consist of poplars, beech, oak, chestnut, sycamore, and other unisexual and diæcious trees, all of which want the showy flowers characteristic of the present flora of the globe.
In view of the fact that this early flora was to so great an extent diclinous, it becomes an important question why so large a proportion of the present flora is hermaphrodite. We find that many of the plants of the most recent geological development possess the means of self-fertilization within the same flower nd no obvious means of crossing individuals. Upon closer observation, however, we perceive that many of these apparently perfect flowers possess arrangements of a more or less anomalous kind, which, inexplicable on any other theory, are all explainable as contrivances for the prevention of self-fertilization. The com