Page:Popular Science Monthly Volume 33.djvu/869

Rh were constant in any one part of a plant, in the course of time the respose would become hereditary. Thus the adhesive pads of certain creepers (Ampelopsis) form before the tendril has come in contact with the wall. A similar development is true of the aerial roots of ivy, etc.

Nectaries have developed according to the same theory—namely, insects having been attracted to the juicy parts, withdraw the liquid and cause a flow of secretion, and finally a gland results. This is not unlike the statement that the excellence of milch-cows is largely due to the stimulation produced upon the milk-glands by the hands of the milker. Floral nectaries are correlated with pollination, they being invariably so placed as to subserve cross-pollination by insects.

It is shown by a long list of examples that sensitiveness and irritability are common phenomena in plants, whether in protoplasm, movements of organs, or formation of tissues, and the theory in hand is but an application of a general principle to the development of a particular part of the flower. The existing floral structures have been evolved under the mechanical and physiological impulses due to insect agencies. The colors of flowers, with all the display of streaks, etc., as path-finders to the nectaries, accord with the theory. "Instead, therefore, of a flower having first painted a petal with a golden streak to invite the insect and to show it the right way of entering, the first insect visitors themselves induced the flower to do it, and so benefited the future comers." The author's discussion of heterostylism—i. e., the different lengths of styles and stamens in the same species—is of great interest, especially that portion which deals with the origin of this condition in flowers. He assumes a homomorphic form as the primitive type, and dimorphism has been effected by varying degrees of stimulus, through insects, being applied to stamens and pistils, so that one set of organs may have been raised while the other was lowered. The lengths finally became so fixed that the best adaptation for cross-fertilization is thereby secured. This view not only ascribes to insects the original cause of variation, but that of the final stability in the dimorphic or trimorphic type. In summing up the treatment of metamorphosis of flowers, both progressive and retrogressive, Prof. Henslow concludes that the vascular cord is the fundamental floral unit, and, as these cords are identical before differentiation, it is not beyond expectation that petals may arise in place of stamens or stamens take the ordinary position of pistils. The primary cause may be less apparent, but doubtless it is in accordance with the governing action of environment modified by heredity. The conclusion drawn upon the somewhat obscure subject of fertilization is, "not that self-fertilization is per se in any way injurious, but that flowers which are normally sterile by having become so highly differentiated through insect stimulation do not now spontaneously set seed, and self-fertilization is not so efficient as crossing." Prof. Henslow has endeavored to make "a good theory of variation," and gives a direct cause for structural forms. He claims that the use of the expression "natural selection" leaves the subject where it was before." Instead, therefore, of using this term as the cause of anything and everything, I prefer to attribute effects to hypertrophy, atrophy, resistance to strains, responsive action to irritations, and so on." It is possible that "natural selection" may underlie all these, and be so understood by many. As to the origin of a flower, the author thinks that it is only necessary to assume a leaf-bud, some of the members of which have differentiated into floral organs of the simplest type, as seen in the gymnosperm. Insects frequently search for pollen only, and by piercing the juicy tissues of such primitive flowers would introduce a series of changes which in time result in conspicuous blossoms. Whether or not the theory is accepted in all its many bearings, it is certain that every botanist must feel under obligations to Prof. Henslow for the excellent volume he has prepared, for the great array of facts therein, and the clear, concise manner in which the theory has been presented from first to last.

author of this essay has evidently thought long and well on his topic. He has sought to treat the subject of industrial liberty in a manner somewhat different from that in which it has usually, been discussed; and, aiming to keep in view principles rather