In his important work, Mechanische-physiologische Theorie der Abstammungslehre, published in 1884, Nägeli, as a convinced opponent of the theory of selection, attempted an explanation. He was quite aware that his assumption of an inward 'perfecting principle' would not suffice to explain the mutual adaptations of flowers and insects, and he refers the transformation of the first inconspicuous blossoms into flowers to the mechanical stimulus which the visiting insects exerted upon the parts of the blossom. By the pressure of their footsteps, the pushing and probing with their proboscis, they have, he says, transformed gradually, for instance, the little covering leaves at the base of a pollen vessel into large flower petals, caused the conversion of short flower-tubes into long ones, and of the pollen, once dry and dusty, into the firmly adhesive mass formed in the anther lobes of our modern flowers. The colour of the flowers depends, according to him, upon the influence of light, which certainly no more explains the yellow ring on a blue ground in the forget-me-not than it does the many other nectar-guides which show the insect the way to the honey. Nägeli works with the Lamarckian principle in the most daring way, and with the same naïveté as Lamarck himself in his time, that is, without offering any sort of explanation as to how the minute impression made, say by the foot or by the proboscis of an insect, upon a flower, is to be handed on to the flowers of succeeding generations. He treats the unending chain of generations as if it were a single individual, and operates with his 'secular' stimulus, and with 'weak stimuli, lasting through countless generations,' as though they were a proved fact. But I have not even touched upon the question as to whether these 'stimuli' could produce the changes he ascribes to them, even if they were continually affecting the flower. How the scale-like covering leaves of the pollen vessels could become larger and petal-like through the treading of an insect's foot is as difficult to see as why a honey-tube should become longer because of the butterfly's honey-sucking: might it not just as well become wider, narrower, or even shorter? I see no convincing reason why it should become longer! And even if it did so, it would necessarily continue to lengthen as time went on, and this is not the case, for we find corolla-tubes of all possible lengths, but, it is to be noted, always in harmony with the length of the proboscis of the visiting insect. In a similar way Henslow has recently attempted to refer the origin of flowers to the mechanical stimulus exercised upon it by the visiting insects. 'An insect hanging to the lower petal of a flower elongates the same by its weight, and the lengthened petal is transmitted by heredity.'...'The irritation caused by its feet in walking along the flower causes the appearance of colouring matter, and the colour is likewise transmitted.'...'As it probes for honey it causes a flow of sweet sap to that part, and this also becomes hereditary!'
In this case, also, it is simply taken for granted that every little passing irritation not only produces a perceptible effect, but that this effect is transmissible. In a later lecture we shall have to discuss in detail the question of the inheritance of functional modifications. It is enough to say here that, if this kind of transmission really took place even in the case of such minute and transitory changes, there could be no dispute as to the correctness of the 'Lamarckian principle,' since every fairly strong and lasting irritation could be demonstrated with certainty to produce an effect. When a butterfly, floating freely in the air, sucks honey from a tube, the irritation must be almost analogous to that caused by a comb lightly drawn by some one through our hair, and this is supposed to effect the gradual lengthening of the corolla-tube of the flower!
The secretion of honey, too, depends upon the persistent irritation of the proboscis! Then 'deceptive flowers,' like the Cypripedium we have mentioned, could not exist at all, for they contain no honey, although the proboscis of the bee must cause the same irritation in them as in other orchids which do contain honey. This whole 'theory' of direct effect is, moreover, only a crude and apparent interpretation, which explains the conditions only in so far as they can be seen from a distance; it fails as soon as they are more exactly examined; all the great differences in the position of the honey, its concealment from intelligent insects, its protection from rain by means of hairs, and against unwelcome guests by a sticky secretion, the development of a corolla-tube which corresponds in length to the length of the visiting insect's proboscis, the development of spurs on the flower, in short, all the numerous contrivances which have reference to cross-fertilization by insects remain quite unintelligible in the light of this theory—it is a mere pis aller explanation for those who continue to struggle against accepting the theory of selection.
LECTURE XI
SEXUAL SELECTION
Decorative colouring of male butterflies and birds—Wallace's interpretation—Preponderance of males—Choice of the females—Sense by sight in butterflies—Attractive odours—Scent-scales—Fragrance of the females—The limits of natural and sexual selection not clearly defined—Odours of particular species—Odours of other animals at the breeding season—Song of the Cicadas, and of birds—Diversity of decoration successively acquired—Humming-birds—Substitution of other aids to wooing in place of personal decoration—Smelling organs of male insects and crabs—Contrivances for seizing and holding the female—Small size of certain males—Weapons of males used in struggle for the females—Turban eyes of Ephemerids—Hoods that can be inflated on the head of birds—Absence of secondary sexual characters in lower animals—Transference of male characters to the females—Lycæna—Parrots—Fashion operative in the phyletic modifications of colour—Pattern of markings on the upper surface of a butterfly's wing simpler than on the under side—Conclusion.
We found in the process of Natural Selection an explanation of numerous effective adaptations in plants and animals, as regards form, colouring, and metabolism, of the most diverse weapons and protective devices, of the existence of those forms of blossoms which we call flowers, of instincts, and so on. The origin of the most characteristic parts of whole orders of insects can only be understood as adaptations to the environment brought about by means of natural selection. Impressed by this, we have now to ask whether all the transformations of organisms may not be referred to adaptation to the continually changing conditions of life? We shall return to this question later, but in the meantime we are far from being able to answer it in the affirmative, for there are undoubtedly a great many characters, at least in animals, which cannot have owed their origin to natural selection in the form in which we have studied it so far.