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|OriginOfSpecies - 475 Rows|
|13 - Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Or||13-05 - Descent always used in classification||40||
Geographical distribution may sometimes be brought usefully into play in classing large and widely-distributed genera, because all the species of the same genus, inhabiting any distinct and isolated region, have in all probability descended from the same parents.
We can understand, on these views, the very important distinction between real affinities and analogical or adaptive resemblances.
Lamarck first called attention to this distinction, and he has been ably followed by Macleay and others.
|13 - Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Or||13-03 - Rules and difficulties in classification, explained on the theory of descent with modification||100||
Geographical distribution has often been used, though perhaps not quite logically, in classification, more especially in very large groups of closely allied forms.
Temminck insists on the utility or even necessity of this practice in certain groups of birds; and it has been followed by several entomologists and botanists.
Finally, with respect to the comparative value of the various groups of species, such as orders, sub-orders, families, sub-families, and genera, they seem to be, at least at present, almost arbitrary.
Several of the best botanists, such as Mr Bentham and others, have strongly insisted on their arbitrary value.
|06 - Difficutiles in Theory||06-09 - Cases of Difficulty||50||
From this simple stage we may pass through an inexhaustible number of contrivances, all for the same purpose and effected in essentially the same manner, but entailing changes in every part of the flower.
The nectar may be stored in variously shaped receptacles, with the stamens and pistils modified in many ways, sometimes forming trap-like contrivances, and sometimes capable of neatly adapted movements through irritability or elasticity.
From such structures we may advance till we come to such a case of extraordinary adaptation as that lately described by Dr. Cruger in the Coryanthes.
This orchid has part of its labellum or lower lip hollowed out into a great bucket, into which drops of almost pure water continually fall from two secreting horns which stand above it; and when the bucket is half full, the water overflows by a spout on one side.
The basal part of the labellum stands over the bucket, and is itself hollowed out into a sort of chamber with two lateral entrances; within this chamber there are curious fleshy ridges.
The most ingenious man, if he had not witnessed what takes place, could never have imagined what purpose all these parts serve.
But Dr. Cruger saw crowds of large humble-bees visiting the gigantic flowers of this orchid, not in order to suck nectar, but to gnaw off the ridges within the chamber above the bucket; in doing this they frequently pushed each other into the bucket, and their wings being thus wetted they could not fly away, but were compelled to crawl out through the passage formed by the spout or overflow.
Dr. Cruger saw a "continual procession" of bees thus crawling out of their involuntary bath.
The passage is narrow, and is roofed over by the column, so that a bee, in forcing its way out, first rubs its back against the viscid stigma and then against the viscid glands of the pollen-masses.
The pollen-masses are thus glued to the back of the bee which first happens to crawl out through the passage of a lately expanded flower, and are thus carried away.
Dr. Cruger sent me a flower in spirits of wine, with a bee which he had killed before it had quite crawled out with a pollen-mass still fastened to its back. When the bee, thus provided, flies to another flower, or to the same flower a second time, and is pushed by its comrades into the bucket and then crawls out by the passage, the pollen-mass necessarily comes first into contact with the viscid stigma, and adheres to it, and the flower is fertilised.
Now at last we see the full use of every part of the flower, of the water-secreting horns, of the bucket half full of water, which prevents the bees from flying away, and forces them to crawl out through the spout, and rub against the properly placed viscid pollen-masses and the viscid stigma.
The construction of the flower in another closely allied orchid, namely the Catasetum, is widely different, though serving the same end; and is equally curious.
Bees visit these flowers, like those of the Coryanthes, in order to gnaw the labellum; in doing this they inevitably touch a long, tapering, sensitive projection, or, as I have called it, the antenna.
This antenna, when touched, transmits a sensation or vibration to a certain membrane which is instantly ruptured; this sets free a spring by which the pollen-mass is shot forth, like an arrow, in the right direction, and adheres by its viscid extremity to the back of the bee.
The pollen-mass of the male plant (for the sexes are separate in this orchid) is thus carried to the flower of the female plant where it is brought into contact with the stigma, which is viscid enough to break certain elastic threads, and retaining the pollen, fertilisation is effected.
|01 - Variations Under Domestication||01-08 - Breeds of the Domestic Pigeons, their Differences and Origin||70||
From these several reasons, namely,- the improbability of man having formerly made seven or eight supposed species of pigeons to breed freely under domestication;- these supposed species being quite unknown in a wild state, and their not having become anywhere feral;- these species presenting certain very abnormal characters, as compared with all other Columbidae, though so like the rock-pigeon in most respects;- the occasional reappearance of the blue colour and various black marks in all the breeds, both when kept pure and when crossed;- and lastly, the mongrel offspring being perfectly fertile;- from these several reasons taken together, we may safely conclude that all our domestic breeds are descended from the rock-pigeon or Columba livia with its geographical sub-species.