« EelmineJätka »
11. A new species of Dalmania from Port Jervis, New York.Dr. S. T. Barrett, of Port Jervis, has recently described a new species of Dalmania from the Lower Helderberg of that vicinity, and named it D. dentata. A description by him, accompanied with a plate, will appear in the next number of this Journal.
J. D. D. III. BOTANY AND ZOOLOGY. 1. Naudin on the Nature of Heredity and Variability in Plants.- Why is it the nature and essence of species to breed true, and why do species sometimes vary? In other words, why is offspring like parent, and when unlike in certain particulars, what is the cause and origin of the difference? We commonly and properly enough take these two associated yet opposed facts as first principles. But it is equally proper and legitimate to enquire after the cause of them.
M. Naudin, a good many years ago, took up the study of hybrid plants, and followed up, for a series of generations, the course of life of certain self-fertile ones, notably of Datura. We gave at the time an abstract of his observations of the manner in which the characters of two closely related common species, D. Stramonium and D. Tatula, were mixed, and in which the characters of the two began to separate in the close-bred progeny of the next generation, ending in a complete division of the amalgamated fornis into those of the two constituent species after a few generations.
The Comptes Rendus of Sept. 27th and Oct. 4th, 1875, contain an abstract of a paper communicated by M. Naudin to the French Académie des Sciences, of which the text was suggested by a hybrid between the wild Lactuca virosa and a variety of L. sitiva, the common Lettuce. The hybrid was an accidental one: its seeds were fully fertile; a great number of young plants were raised from them, of which twenty were preserved for full development and study. Like other hybrids the original showed no character which was not evidently derived from the two parents; and, fertilized by its own pollen, the offspring all agreed in this respect, although they varied exceedingly among themselves in the division of the parental heritage, no two being quite alike. This exceeding vacillation between the two parental forms but not overpassing the limits on either hand,—which Naudin finds to be the common characteristic of fertile hybrids, close-bred — he names disordered variation (variation désordonnée). His explanation is, that the hybrid is a piece of living mosaic, that two specific natures are at strife in it; in the progeny each endeavors to reclaim its own, like seeks like; whence in the course of a very few generations (as he first showed in Datura), a segregation takes place, part of the progeny reverting completely to one ancestral type, part to the other. What Naudin now insists upon is that out of all this disturbance comes nothing new ; that there is here no variation beyond the line of inheritance; and therefore from crossing no possible development of species.
To this proposition we accede, so far as respects the direct consequence of crossing. To fill up the interval more or less between two forms or species with intermediate patterns may tend to the fusion or confusion of the two, but not to the origination of new forms or species. Although Naudin's own experiments lead him to deny all tendency to variation overpassing these limits, we do not forget that his countryman, the late M. Vilmorin,working in a different way and with another object,-arrived at a different conclusion. He succeeded, as we understand, in originating floricultural novelties from species which refused to vary per se, by making a cross,-not to infuse the character of the male parent, for he fertilized the progeny with the pollen of the female parent, and thus early bred out the other blood, but to induce variation, which, once initiated in the internal disorder consequent upon the crossing, was apt to proceed, or might be led on by selection, to great lengths, according to Vilmorin. The variations in question, being mainly such as are sought in floriculture, may not have passed the line laid down by Naudin, or actually have introduced new features. But such plants would surely have no exemption from the ordinary liability to variation. If other plants vary, in the sense of producing something new, so may these.
This brings us to another inference which Naudin draws. Having observed that his hybridis in their manifold variation exhibited nothing which was not derivable from their immediate ancestry, he directly (and in our opinion too confidently) concludes that all variation is atavism,—that when real variations are set up in ordinary species, this is not an origination but a reversion, a breaking out of some old ancestral character, a particular and long deferred instance of this variation désordonnée, which would thus appear to be the only kind of variation. This view has been presented before, but not, perhaps, so broadly. Adducing some theoretical considerations in its favor—to which we may revert-and some sound reasons against the view that variation is caused by external influences, he declares it “infinitely more probable that variation of species properly so called is due to ancestral influences rather than to accidental actions." We might think so if these two categories were exhaustive, and external conditions must be supposed to act immediately, as the cause rather than the occasion of variation. But the supposition that “accidental actions," whatever they may be, and external influences of every sort do not produce but educe and conduct variation—which is our idea of what natural selection means-avoids the force of Naudin's arguments.
Moreover Naudin's view, regarded as an hypothesis for explaining variation, leaves the problem just where it finds it. To explain the occurrence of present and actual variations, hypothetical ones like those of a former time are assumed; the present diversity implies not only equal but the very same anterior diversity, and so on backwards. Or rather it demands a much greater diversity at the outset than now; for these aberrant forms are the rare exception, and if due to atavism they imply the loss of the many and the incidental reappearance of the few. Else they would be the rule instead of the exception, and atavism would be simply heredity. This comes to the view which Mr. Agassiz strongly maintained, that really there are no varieties,-meaning, we understand, that all the forms are aboriginal, except the transient ones evidently due to circumstances.
That some variation is atavism is clear enough. This is the natural explanation of the appearance of characters wanting in the immediate parents but known in their ancestors or presumed ancestors. But the assumption of hypothetical ancestors to account for variation generally is quite another thing. Besides its inutility as an explanation, to wbich we have adverted, its improbability as an hypothesis is set in a strong light by Naudin's own forcible conception of the nature of heredity. What is heredity ? be asks. In other words, what keeps species so true, offspring like parent, through the long line of generations? He illustrates hereditary force by comparing its action with that of physical force, in which the movement from one state of equilibrium to another is always that in which there is least resistance. From which it follows that when it has once begun to proceed in a certain course, its tendency to continue in that direction increases, because it facilitates its way as it overcomes obstacles. In other words this line becomes fixed by habit; vires acquirit eundo; the stream deepens its bed by flowing; and the more remote the commencement of a certain course, the more fixed its direction, and the greater its power of overcoming opposition. The species is kept true in its course by the sum of the heredities which press each individual forward in its actual direction. So that, as Naudin remarks, if we could calculate the energy with which millions of ancestors tend to impel the living representatives of the line onward in the same direction, we should better apprehend the persistence of species, and feel the great improbability that the stream will ever escape from its ancient and well-worn bed, and strike into new courses.
Now, in the first place, the more lively the conception we thus form of the invariability of species, through a bappy metaphorical illustration of it, the more unlikely does it appear that early characters, long lost in the flow, should re-appear through atavism as varieties. To continue the simile, the more impetuous the stream, the less the possibility of its turning back upon itself, and resuming old characteristics. The eddies of atavism (the resumption of dropped characters) are not likely to extend back very far; and it seems gratuitous to have recourse to them in explanation of new forms. Moreover, although the stream has made its bed and lies in it, not escaping from its own valley, it is flexible enough to obstacles, is ever changing its particular course as it flows, and may by its own action send off here and there a bayou (variety) or branch into a delta of channels (derivative species).
Like Agassiz, Naudin conceives of species as originating with a large number of individuals of the same structure, and of which
numerous reciprocal crosses have determined the direction of the line in which their posterity have evolved. But he mantains that these individuals, and all existing species, had a common origin in a “proto-organism ;" and that the various lines of descent acquired fixity into species only as they acquired sexuality. If we rightly apprehend it, Naudin's idea of the purport of sexual reproduction (as contrasted with that by buds) is, to give fixity to species. Our idea is a different one, both as to the essential meaning of sexuality, and as to its operation in respect to fixity. His conception may be tested by enquiring which are the more variable or sportive, seedlings or plants propagated from buds. This we suppose can be answered in only one way.
M. Naudin is a veteran and excellent investigator; and nothing which be writes is to be slighted. We have frankly set down our impressions upon a first perusal of his important communication ; but are ready to revise them, if need be, upon more deliberate consideration.
A. G. 2. First Forms in Vegetation ; by the Rev. Hugh MACMILLAN, LL.D., F.R.S.E. With numerous illustrations. Second edition, corrected and enlarged. London: Macmillan & Co. 1874. pp. 438, 18mo.—The first edition under a somewhat different title, was published in 1861. In the present volume it is brought up to the time, and we suppose much amplified. As it stands it forms an excellent popular introduction, of the readable sort, to lower Cryptogamic botany, from Mosses (and even Club-mosses) to Fungi. The materials of its chapters were first used for popular lectures, and this primary form and use gives its character to this re-written and now extended volume. The author wished to recast it in a systematic mold, but was deterred not only by the labor required, but by the doubt whether it were worth the while. We fancy it is better as it stands, and more likely to fulfill its purpose, which is “to kindle the sympathy and awaken the interest of the reader in a department of nature with which few, owing to the technical phraseology of botanical works, are familiar.” The book is full of information, possibly too full for the object in view, except that it cannot be amiss to gratify as well as awaken interest in the lower forms of vegetation by referring to as large a number as is practicable. The spirit in which the subject is handled is indicated by the motto: “ Deus magnus in magnis, maximus in minimis ;" and the sermonizings, being apposite, we have no right to intimate that they are too many and too long. There are good indexes of scientific and of popular names. It is not often that an amateur-botanist writes a book of this kind which is more free from serious errors or misunderstandings. But the statement that the antherozoids of mosses, although “furnished with cilia, like animalcules," yet "their motion is simply a hygrometical action, like that of the teeth which fringe the mouth of the capsule," must be one which unaccountably escaped revision, even in the edition of 1861. So also the suggestion that sexual reproduction may be gradually dispensed with in the lower plants and animals, which is in fact corrected in the latter part of the volume. In referring to Prof. Tuckerman, we hope it may be long before he must be designated as “the late distinguished American lichenologist.” The “ conjecture” which “ may be hazarded” that the Red Sea “ acquired its denomination from the prevalence of this red alga ( Trichodesmium erythræum] in its waters," was hazarded by Ehrenberg, if we mistake not, at the time of its discovery. The late Dr. M. A. Curtis, our American mycologist, informed Mr. Berkeley, as we remember, that he and his neighbors whom he instructed, procured no small supply of excellent food from the edible Fungi, which grew around his home, in the center of North Carolina, so abundantly that he opined he might have supported a regiment on them. This, we suppose, is the whole foundation for the extraordinary statement that, “During the latter part of the American war, when meat was scarce and dear, fungi, which grow in immense profusion and variety in America, formed the principal food of the Southern army."
A. G. 3. Seeds that float in water have a certain interest in connection with questions about dissemination. Many ranked as such are fruits, botanically speaking, with spongy or cellular pericarp, or with some air-space between the pericarp and the seed itself. But there are a considerable number of true seeds with specific gravity less than that of water, some as low as 0.75. Van Tieghem (Ann. Sci. Nat., ser. 6, i, 383) finds that this is due to different causes. More commonly the seed owes its lightness to its coats, either by a separation in drying between the two, or between the inner and the kernel, leaving an air-space, or by a loose cellular structure of the coat. Sometimes, as in castor-oil seeds, the integument is heavier than water, but the kernel is so much lighter as to float the seed. This comes from a separation of the two cotyledons during the natural desiccation, leaving a considerable cavity filled with air. This is strikingly the case in the large, flat, and very dense-coated seeds of Entada scandens, which are well known to have been wafted across the Atlantic from the West Indies to Northern Europe and left in a condition fit for germination; while in those of Guilandina Bonduc, which have been known to accompany the former, the air is interposed between the embryo and the bony coat. The embryo itself, in all such cases, is heavier than water. But to this rule Van Tieghem now brings to light a few exceptions, and these in leguminous seeds. In those of Erythrina crista-galli, the specific gravity of the whole seed is 0.91, that of the embryo itself 0.87. Those of our Apios tuberosa are a little lighter, of our Wistaria frutescens a little heavier than this, but still lighter than water; and various common leguminous seeds, although heavier than water, are much lighter than would be supposed. This proves to be owing to a very loose and open structure of the parenchyma of the inner or upper side of the cotyledons (that which answers to the upper face of the leaf), leaving abundant intercellular spaces and passages, filled with air, which renders this spongy stratum light enough in certain cases to float the otherwise heavy seeds.