| Bronn, again, asks how natural selection can account for differences between species, which appear to be of no service to these species, such as the length of the ears or tail, or the folds of the enamel in the teeth, of the several species of hares and mice?
With respect to plants, this subject has been recently discussed by Nägeli in an admirable essay.
He admits that natural selection has effected much, but he urges that the families of plants differ chiefly from each other in morphological characters, which seem quite unimportant for the welfare of the species.
He consequently believes in an innate tendency towards perfection or progressive development.
He specifies the arrangement of the cells in the tissues, and of the leaves on the axis, as cases in which natural selection would fail to act.
To these may be added the numerical divisions in the parts of the flower, the position of the ovules, the shape of the seed, when not of any use for dissemination, &C. Professor Weismann, in discussing Nägelis essay, accounts for such differences by the nature of the varying organism under the action of certain conditions; and this is the same with what I have called the direct and definite action of the conditions of life, causing all or nearly all the individuals of the same species to vary in the same manner.
When we remember such cases as the formation of the more complex galls, and certain monstrosities, which cannot be accounted for by reversion, cohesion, &c., and sudden strongly-marked deviations of structure, such as the appearance of a moss-rose on a common rose, we must admit that the organisation of the individual is capable through its own laws of growth, under certain conditions, of undergoing great modifications, independently of the gradual accumulation of slight inherited modifications.
Various morphological differences probably come under this head, to which we shall recur; but many differences may at the present time be of high service, or may formerly have been so, although we are not able to perceive their use; and these will have been acted on by natural selection.
A still larger number of morphological differences may certainly be looked at as the necessary result—through pressure, the withdrawal or excess of nutriment, an early-formed part affecting a part subsequently developed, correlation, &c.—of other adaptive changes, through which all species must have passed during their long course of descent and modification.
No one will maintain that we as yet know the uses of all the parts of any one plant, or the functions of each cell in any one organ.
Five or six years ago, endless peculiarities of structure in the flowers of orchids, great ridges and crests, and the relative positions of the various parts would have been considered as useless morphological differences; but now we know that they are of great service, and must have been under the dominion of natural selection.
No one at present can explain why the leaves in a spire diverge from each other at certain angles; but we can see that their arrangement is related to their standing at equal distances from the leaves on all sides; and we may reasonably expect that the angles will hereafter be shown to follow from some such cause, as the addition of new leaves to the crowded spire in the bud, as inevitably as the angles of a bees cell follow from the manner in which the insects work together.
In certain whole groups of plants the ovules stand erect, and in others they are suspended; and in some few plants within the same ovarium one ovule holds the former and a second ovule the latter position.
These positions seem at first purely morphological and of no physiological signification; but Dr. Hooker informs me that of the ovules within the same ovarium, in some cases the upper ones alone and in other cases the lower ones alone are fertilised; and he suggests that this probably depends on the direction in which the pollen-tubes enter.
If so, the position of the ovules, even when one is erect and the other suspended, would follow from the selection of any slight deviation in position which might favour their fertilisation and the production of seed.
Several plants belonging to distinct orders habitually produce flowers of two kinds,—the one open and of the ordinary structure, the other closed and imperfect.
In the latter the petals are almost always reduced to the merest rudiments; the pollen-grains are reduced in diameter; five of the alternate stamens are rudimentary in Ononis columnæ and in some species of Viola three stamens are in this state, two retaining their proper function, but being of very small size.
In six out of thirty of the closed flowers in an Indian violet (name unknown, for the plants have not as yet produced perfect flowers), the sepals were reduced from the normal number of five to three.
In one section of the Malpighiaceæ the closed flowers, according to A. de Jussieu, are still further modified, for the five stamens which stand opposite to the sepals are all aborted, a sixth stamen standing opposite to a petal being alone developed; and this stamen is not present in the ordinary flowers of these species; the style is aborted; and the ovaria are reduced from three to two.
In all the foregoing plants the minute closed flowers are of high service, for they yield with perfect security, and with the expenditure of extremely little pollen, or other organised matter, a large supply of seed; whilst the perfect flowers permit occasional crosses with distinct individuals.
Therefore, these changes may have been, and no doubt have been, effected through natural selection; and I may add that nearly all the gradations between the perfect and imperfect flowers may sometimes be observed on the same plant.
With respect to modifications which necessarily follow from other changes—through the withdrawal or excess of nutriment—through pressure and other unknown influences—there is space here only for a few brief illustrations.
In the Spanish chesnut, and in certain fir-trees, the angles of divergence of the leaves differ, according to Schacht, in the nearly horizontal and in the upright branches.
In the common rue and some other plants, one flower, usually the central or terminal one, opens first, and has five sepals and petals, and five divisions to the ovarium; whilst all the other flowers on the plant are tetramerous.
In the British Adoxa the uppermost flower generally has two calyx-lobes with the other organs tetramerous, whilst the surrounding flowers generally have three calyx-lobes with the other organs pentamerous; and this difference appears to follow from the manner in which the flowers are closely packed together.
In many Compositæ and Umbelliferæ, and in some other plants, the circumferential flowers have their corollas much more developed than those of the centre; and this is probably the result of natural selection, for all the flowers are thus rendered much more conspicuous to those insects which are useful or even necessary for their fertilisation.
In connection with the greater development of the corolla, the reproductive organs are frequently more or less aborted.
It is a more curious fact that the achenes or seeds of the circumference and of the centre sometimes differ greatly in form, colour, and other characters.
In Carthamus and some other Compositæ the central achenes alone are furnished with a pappus; and in Hyoseris the same head yields achenes of three different forms.
In certain Umbelliferæ the exterior seeds, according to Tausch, are orthospermous, and the central one cœlospermous, and this difference has been considered by De Candolle as of the highest systematic importance in the family.
If in such cases as the foregoing all the leaves, flowers, fruits, &c., on the same plant had been subjected to precisely the same external and internal conditions, all no doubt would have presented the same morphological characters; and there clearly would have been no need to call in the aid of the principle of progressive development.
With the minute closed flowers, as well as with many degraded parasitic animals, if it be assumed that any such aid is requisite, we should have to call in an innate tendency to retrogressive development.
Many instances could be given of morphological characters varying greatly in plants of the same species growing close together, or even on the same individual plant; and some of these characters are considered as systematically important.
I will specify only a few cases which have first occurred to me.
It is not necessary to give instances of flowers on the same plant being indifferently tetramerous, pentamerous, &c.; but as when the parts are few, numerical variations are in all cases comparatively rare, I may mention that, according to De Candolle, the flowers of Papaver bracteatum offer two sepals with four petals (and this is the common type with poppies), or three sepals with six petals.
The manner in which the petals are folded in the bud is in most groups a constant morphological character; but Professor Asa Gray states that with some species of Mimulus, the æstivation is almost as frequently that of the Rhinanthideæ as of the Antirrhinideæ, to which tribe the genus belongs.
Aug. St. Hilaire gives the following cases: the genus Zanthoxylon belongs to a division of the Rutaceæ with a single ovary, but in some of the species flowers may be found on the same plant, and even in the same panicle, with either one or two ovaries.
In Helianthemum the capsule has been described as unilocular or 3-locular; and in H. mutabile, "Une lame, plus ou moins large, sétend entre le pericarpe et le placenta." In the flowers of Saponaria officinalis, Dr. Masters also observed instances of both marginal and free central placentation.
Lastly, St. Hilaire found towards the southern extreme of the range of Gomphia oleæformis two forms which he did not at first doubt were distinct species, but he subsequently saw them growing on the same bush; and he then adds, "Voilà donc dans un même individu des loges et un style qui se rattachent tantôt à un axe verticale et tantôt à un gynobase."
In the case of these plants, will it be said that they have been detected in the act of progressing towards a higher state of development?
On the contrary, I should infer from such characters varying so greatly, that they were of extremely small importance to the plants themselves, of whatever importance they may be to us in our classifications.
Although we are quite ignorant of the exciting cause of each modification, yet it seems probable from what we know of the relations of variability to changed conditions, that under certain conditions the one structure would have prevailed over the other, and thus might have been rendered almost or quite constant.
From the very fact of such differences being unimportant for the welfare of the species, any slight deviations which did occur would not be augmented or accumulated through natural selection; and they would be liable to obliteration through the occasional intercrossing of distinct individuals.
A structure which has been developed through long-continued selection, when it ceases to be of service to the species, will generally become variable, as we see with rudimentary organs; for it will no longer be regulated by this same power of selection; but on the other hand, when from the nature of the organism and from a change in the conditions definite modifications have been produced which are unimportant for the welfare of the species, they may be, and apparently often have been, transmitted in nearly the same state to numerous, otherwise modified descendants.
Hair has been transmitted to almost all mammals, feathers to all birds, and scales to all true reptiles.
A structure, whatever it may be, which is common to many allied forms, is ranked by us as of high systematic importance, and consequently is often assumed by us to be of high vital importance to the species.
Thus, as I am inclined to believe, morphological differences, which we consider as important—such as the arrangement of the leaves, the divisions of the ovarium, the position of the ovules, &c.—first appeared in many cases as fluctuating variations, which sooner or later became almost constant through the nature of the organism and of the surrounding conditions, as well as through intercrossing; for as these morphological characters do not affect the welfare of the species, any slight deviations in them would not be acted on or accumulated through natural selection.
It is a strange result which we thus arrive at, namely that characters of slight vital importance to the species, are the most important to the systematist; but, as we shall hereafter see when we treat of the genetic principle of classification, this is by no means so paradoxical as it at first appears.
Finally, whatever may be thought of this view, in none of the foregoing cases do the facts, as far as I can judge, afford any evidence of the existence of an innate tendency towards perfectibility or progressive development.
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