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wasps, that the young collector is slow to venture to put his hand on them. Again: the Danaidæ and Heliconidæ, already mentioned, are mimicked by other South-American butterflies, of genera which are eatable by birds. In all such cases it is observed that the kinds which mimic are far less abundant than the species which they resemble; and this must be so, if the mimickers are to pass unnoticed among the multitude of the really uneatable kinds. The imitation is so wonderfully close, that even experienced collectors can hardly distinguish the imitation heliconias and danaids from the true ones. If the naturalist can be so deceived, does it not seem certain that this similarity must also deceive the enemies of these butterflies, and that individuals of the imitating species must often escape by this means, when they would otherwise be captured and eaten? The first observations on these interesting cases of mimicry among butterflies are due to Mr. H. W. Bates, while Mr. Wallace has added greatly to our knowledge of the subject. Among plants, observed cases of mimicry are much more rare than with animals; and yet it has for years been one of the unexplained facts of botany, that certain plants of the orchis group produce flowers which closely resemble butterflies and other insects. It is evident, that, since the principal causes which operate in the destruction of plants are other than the attacks of animals, protective resemblances and warning-colors would be of comparatively little ise to a plant; and accordingly the botanist finds few or no cases which may certainly be classed under either one of these heads.
1 A full account of these facts will be found in his Naturalist on the River Amazon.
There are, however, some phenomena manifested by plants, which have an important bearing on the development theory, and which may best be stated in this connection. Darwin 1 and Wallace 2 have called the attention of botanists to the fact, that those fruits which are eaten by birds or otlier animals large enough to swallow the seeds whole are, in general, of some other color than green, and that the seeds or stones in them are usually so hard or tough, that, when swallowed whole, they cannot be digested, but pass nearly unchanged through the stomach or intestines of the animal. In some instances the softening process undergone in the digestive cavities of the animal seems even to hasten the sprouting of the seed. Now, from the two
1 Origin of Species, p. 161. 2 Tropical Nature, pp. 224, 225.
facts just stated, the coloration of fruits and the indigestibility of seeds, the most natural conclusion is, that the fruit is colored as a result of the action of natural selection; that is, colored fruit would be more readily found among the green leaves of the plant by birds or other animals, and so be eaten when uncolored fruits would be left. That this is not mere guess-work has been shown by Mr. Darwin's observations on the consumption by birds of pale and of bright-red berries of the English holly. He found that a much larger proportion of the bright-colored berries than of the pale ones were eaten. In the same way the indigestibility of seeds may be attributed to the action of natural selection, tending to perpetuate those variations in any
seed which would make it more able to resist digestion, and so to grow after having been swallowed. If it be asked, "What advantage can it be to a plant to have its seeds swallowed?” I answer, that much would be gained for the plant by transference of its seeds to another spot from that on which they were produced ; for certainly one of the greatest difficulties with which plants have to contend is overcrowding, and the worst kind of crowding is that which comes from individuals of the same species. It would be easy enough for many hazel-bushes to grow in the shade of an oak, for greenbriers and blackberry-bushes to crowd themselves in among the hazels, and for a large variety of herbaceous plants to grow closer to the ground, among the bushes. But two oaks could not have grown to full size in the place now occupied by the one, nor could the bushes, with the oak removed, have grown enough larger to make up for its absence, or the oak have done much more without the bushes than with them.
1 The words "seed” and “fruit” are used in this chapter in the popular, not the botanical sense.
2 Animals and Plants under Domestication, ii. p. 216.
So the economical growth of plants demands that provision be made for distributing seeds as widely as possible; and accordingly we find a variety of other methods at work, besides this rather complicated process of having the seeds swallowed. Burs are carried by un willing animals often to great distances; winged fruits and seeds are carried by the wind ; the thick husk of the cocoanut floats it over tropical seas, so that the feathered crown of the cocoa-palm is the first and often the only tree-like form encountered on coral-islands many hundred of miles from the mainland.
In the Mississippi basin there are several grasses, some of them very troublesome weeds, whose soft, plumy heads break off from the parent stem as soon as the seeds are ripe, and travel overland, rolling along before the wind, for vast distances. Not less curious than the arrangements for general distribution are some of the modes by which seeds are locally distributed ; that is, thrown or carried to a short distance from the plant that bore them. The balsam, or touch-me-not, throws its seeds several feet by the bursting of the capsule in which they are contained. In the wild crane's-bill the little pouches in which the seeds ripen are attached to elastic slips, which are at first united about a central column; but, as the seeds ripen, these slips break loose, and fly out into a little coil, while the seed is thrown to a considerable distance.
Stranger still, the spores? of such microscopic organisms as the red snow-plant (Protococcus) and many others are provided with little hairlike oars, called cilia, with which they swim off through the water. The instances just quoted, though only a few out of the multitudes that might be brought forward, will perhaps serve to give some idea of the generality and the importance of the provisions for scattering the seeds of plants.
1 Little particles answering nearly the purpose of seeds: in flowerless plants they take the place of seeds.