Gambar halaman
PDF
ePub

pleteness with which this object is accomplished is of all degrees, from Epilobium with its style merely turned to one side, to Iris with its short extrorse anthers hidden away under the broad styles stigmatic on the inaccessible side; from mere heterosty ly to complete dichogamy.

I need not review the conclusive reasoning by which all these morphological modifications are accounted for as the results of the long continued agency of insects. It is important only to point out that this influence has been powerful enough to reverse the entire course of sexual differentiation, which, as we have seen, has been in all lower forms constantly in the direction of a more and more complete separation of the sexes. It may be said that this proves too much, since progress in that advantageous direction once gained would not be likely to be lost. The sufficient reply to this is that, independently of the natural tendency to revert to the normal or monosexual state, when the separative influences are withdrawn, the reserve power of possible self-fertilization when for any cause cross-fertilization fails, as it clearly often may, is a positive advantage, and one whicn, under the proper circumstances, natural selection will insure.

The most significant fact which palæontology reveals is that of the simultaneous appearance of an insect fauna and a hermaphrodite flora. . When the insects came upon the scene they found only a diclinous flora with usually apetalous flowers destitute of both fragrance and color. The succeeding strata immediately commence to exhibit plants of the rose, mallow, magnolia, pulse, and crowfoot families with showy petals, often fragrant, and provided with special nectaries for the secretion of honey. Most of these had already made their appearance in the chalk formation, while during the Tertiary the still more perfectly organized Gamopetala were developed. The agency of insects in the fertilization of plants and even in the transformation of flowers to adapt them to their uses is no longer questioned by any at all familiar with the facts, but wide differences of opinion exist with regard to the degree of this influence, and also to the meaning of particular facts. Much of this confusion is due to the prevalence of the notion to which attention was called at the outset, that all adaptation must be regarded as completed at the present time. This assumption of a statical condition in nature now, while admitting the necessity of a dynamical condition in the past, is wholly gratuitous and belongs, as already remarked, to the same class of ideas as that by which all changes were once explained as the results of great and sudden catastrophes. It is due to the kind of reasoning which denies change to everything which can not be seen to move- -a kind of reasoning which leads the savage to deny that the great trees have ever been other than they are, while admitting growth in the herb and the sapling. In point of fact we find nearly all possible degrees of adaptation to the agency of insects. The mere existence of colored flowers must be regarded as an initial step in this direction, and the greater part of all flowering plants exhibit in a more or less marked manner this evidence of the influence which insects have exerted upon them. But it is evident that an ordinary hermaphrodite flower, however showy or fragrant, if devoid of special appliances for preventing self- and securing cross-fertilization, represents a very rudimentary and imperfect state of correlation to the insect world. This condition, which is now the predominant one, must therefore be regarded as constituting the first step of a long progressive series of morphological changes in the same direction, all tending to complete the degree of adaptation to insect life. The various specializations which a few species have already undergone mark so many additional steps taken by such species toward the same end and afford a faint idea of what the whole flora of the globe might become in the remote future, if wholly uninfuenced by man.

In the great majority of plants, self-fertilization is doubtless still the rule, and cross-fertilization the exception, but this occasional crossing, even though very rare, suffices to maintain the vigor of the stock. Such plants will appear to thrive as well when selffertilized as when cross-fertilized, and this would probably be the case if the experiment were repeated a great number of times, for it is not once or a score of times, or a hundred times even, that count in these processes of nature, but vast periods and innumerable repetitions, each with its minute differential to add to or subtract from the general sum. When these facts are properly understood, therefore, the partial or total failure of all human experiments on cross-fertilization becomes nothing more than naturalists ought to expect. The really surprising fact in such

See an address by Maj. J. W. Powell, delivered before the American Geographical Society, at Chickering Hall, New York, Dec, 29th, 1876.

experiments is that some of them actually do show a clear difference in favor of cross-fertilization. It may be compared to the attempt of astronomers to obtain the parallax of a fixed star. The result is in the highest degree satisfactory if it is certain that any positive angle is measured. And, as in the astronomical parallax, the greatest exactness is required to measure the vastness of space and its contents, so in the biological parallax equally great precision is needed to measure the vastness of time and its effects.

Independently of insect agency, however, the vegetable kingdom furnishes many facts which prove the unstable state in which the sexual relations are still found to exist.

In many cases it is difficult to determine whether the movement is at the present time towards a greater or a less degree of separation. In a former paper read before this Association? I endeavored to bring forward the evidence to prove that certain species of Lauraceæ, and notably the genera Sassafras and Lindera, had already passed through three different stages, of which traces are still left in the form of “ rudiments” or obsolete organs. In this case the movement has obviously been towards more complete sexual separation. In the majority of other common cases, such as Smilax, Ilex, Rumex, Rhus, Chamælirium, &c., where the rudiments of both stamens and pistils remain, though one or the other set is functionless and the plants are really diecious, the direction of development seems also to be towards sexual distinctness, and it may well be doubted whether the flowers of the oak, the alder, or the willow were ever hermaphrodite. Still, progress toward hermaphrodism may also be going on in some species where insect fertilization is found a sufficient substitute for the distinction of sex.

Upon the whole, however, it must be concluded that the special effect of the appearance of insects in the Mesozoic or Secondary age of geology was to render the evolution of new hermaphrodite forms possible, which vastly enriched the world's flora, since prior to that time only diclinous species could survive, and that this great army of plants, having been thus brought into existence in this imperfect condition, have since been gradually throwing off their encumbrance, and at different rates moving forward toward sexual independence.

Published in the Scientific American Supplement of Sept. 20, 1879, p. 3089.

1

A PARTIAL BIOGRAPHY OF THE GREEN LIZARD.

BY SARAH P. MONKS.

THE green lizard (Anolis principalis) of the Southern United

States is sometimes called the American chameleon, but it is not related to the chameleon of the Old World.

Its changeable coat, however, gives it a popular right to the name. Two specimens of Anolis that I have kept for months in a wire-cloth cage, have shown some interesting habits.

The female came from South Carolina in November, 1879, in good condition, but with the greater part of the tail wanting. She was placed in a small cage and supplied with flies, but refused to eat. During the winter the cage stood among house plants, in a room heated by a furnace, and although she was lively and ran around a good deal, she ignored the flies. Thus she remained without food and water (except an occasional drop that fell by accident when the plants were watered) for four or five months.

But when the warm spring days came, she greedily devoured the flies, and when water was sprinkled in the cage, she eagerly lapped it up with her tongue. It is said that the Old World chameleons drink in the same manner. She would not notice water that was in a small jar in the cage, although very thirsty.

Sometimes when I approach the cage she lifts her head and opens her mouth. I do not know whether she is conscious of asking for water, but I soon recognized this as an indication of thirst. In April a new tail began to show itself, looking like a small black wart, and since then it has grown nearly an inch. At first it was distinct and looked like a graft on the other portion, but now, after several moultings, it is continuous, although it can be easily distinguished from the rest; the scales are smaller, it always remains darker than the rest of the body.

About the middle of May another and larger specimen, a male, came from South Carolina, and I put them in a large box in which were twigs and a stick of wood. After the larger one had dined, their antics on seeing one another were exceedingly amusing.

First, one would raise itself to the full extent of its front legs, and bow its head and the fore part of its body in a regular and dignified manner. It worked as though there was a hinge joint at the shoulders. Then the other would repeat the gesture. The male, when bowing, erected a small nuchal crest, and after several bows, held its head still and stiff and distended a dew-lap.

This expansion, of which ordinarily there is no trace, is not inflated, but is a flattened disc about an inch in diameter. It is orange-red in reflected, and crimson in transmitted light. At this time the lizard is a beautiful sight, the body being green above and white below, and the vivid dew-lap edged with white.

I have seen them bowing several times, but they scamper off on finding themselves watched; and even in the midst of their ceremonious courtship, if a fly comes near they dart after it like a flash of green light.

There is a difference in the change of color in the two specimens, and the same cause does not affect them alike. The female, in the day time, is generally dark-brown, or drab, speckled with white, and has a lighter dorsal line. Sometimes, however, she is grayish. When very dark, even the under side is brown, but when lighter colored the under side is gray, or white. But at night she becomes some shade of green, rarely a pale-green. Once or twice during July I have seen her green in the day time. On the other hand the male is generally pale-green. Their colors are different shades of green, yellow and brown. When changing, the coming color does not suffuse the entire body at once, but first appears on the legs and sides of the head and the body, the dorsal line and tail often remaining darker long after the other parts are light-colored.

When they are green, yellow, or drab above, they are white below; when dark-brown, a lighter shade of the same color below; and sometimes I have seen them a uniform dark brown. Occasionally, the light-green color remains on the eye-lids and a few scattered scales of the body, after the other portions have become brown. They do not always grade regularly from brown, through yellow, to green, but sometimes change from dark brown to palegreen and white, without showing yellowish. The bronze (yellow) is the rarest color, and is very seldom assumed by the female. They change from one color to another in from two to eight minutes, and one changed from green to light-brown, then back to green again, in five minutes.

I see no reason, as yet, for this changing of color, for it comes regardless of the object on which they are placed, or amount of light and darkness. They become green or light-brown when placed in sun-light, but also assume the same colors in the darkest room. When disturbed, they sometimes get darker, and at other times do

« SebelumnyaLanjutkan »