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|13 - Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Or||13-05 - Descent always used in classification||10||
With species in a state of nature, every naturalist has in fact brought descent into his classification; for he includes in his lowest grade, or that of a species, the two sexes; and how enormously these sometimes differ in the most important characters, is known to every naturalist: scarcely a single fact can be predicated in common of the males and hermaphrodites of certain cirripedes, when adult, and yet no one dreams of separating them.
The naturalist includes as one species the several larval stages of the same individual, however much they may differ from each other and from the adult; as he likewise includes the so-called alternate generations of Steenstrup, which can only in a technical sense be considered as the same individual.
He includes monsters; he includes varieties, not solely because they closely resemble the parent-form, but because they are descended from it.
He who believes that the cowslip is descended from the primrose, or conversely, ranks them together as a single species, and gives a single definition.
As soon as three Orchidean forms (Monochanthus, Myanthus, and Catasetum), which had previously been ranked as three distinct genera, were known to be sometimes produced on the same spike, they were immediately included as a single species.
|13 - Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Or||13-05 - Descent always used in classification||50||
The resemblance, in the shape of the body and in the fin-like anterior limbs, between the dugong, which is a pachydermatous animal, and the whale, and between both these mammals and fishes, is analogical.
Amongst insects there are innumerable instances: thus Linnaeus, misled by external appearances, actually classed an homopterous insect as a moth.
We see something of the same kind even in our domestic varieties, as in the thickened stems of the common and swedish turnip.
The resemblance of the greyhound and racehorse is hardly more fanciful than the analogies which have been drawn by some authors between very distinct animals.
|13 - Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Or||13-05 - Descent always used in classification||60||
On my view of characters being of real importance for classification, only in so far as they reveal descent, we can clearly understand why analogical or adaptive character, although of the utmost importance to the welfare of the being, are almost valueless to the systematist.
For animals, belonging to two most distinct lines of descent, may readily become adapted to similar conditions, and thus assume a close external resemblance; but such resemblances will not reveal will rather tend to conceal their blood-relationship to their proper lines of descent.
We can also understand the apparent paradox, that the very same characters are analogical when one class or order is compared with another, but give true affinities when the members of the same class or order are compared one with another: thus the shape of the body and fin-like limbs are only analogical when whales are compared with fishes, being adaptations in both classes for swimming through the water; but the shape of the body and fin-like limbs serve as characters exhibiting true affinity between the several members of the whale family; for these cetaceans agree in so many characters, great and small, that we cannot doubt that they have inherited their general shape of body and structure of limbs from a common ancestor.
So it is with fishes.
|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||13-05 - Descent always used in classification||20||
But it may be asked, what ought we to do, if it could be proved that one species of kangaroo had been produced, by a long course of modification, from a bear?
Ought we to rank this one species with bears, and what should we do with the other species?
The supposition is of course preposterous; and I might answer by the argumentum ad hominem, and ask what should be done if a perfect kangaroo were seen to come out of the womb of a bear?
According to all analogy, it would be ranked with bears; but then assuredly all the other species of the kangaroo family would have to be classed under the bear genus.
The whole case is preposterous; for where there has been close descent in common, there will certainly be close resemblance or affinity.
|13 - Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Or||13-05 - Descent always used in classification||30||
As descent has universally been used in classing together the individuals of the same species, though the males and females and larvae are sometimes extremely different; and as it has been used in classing varieties which have undergone a certain, and sometimes a considerable amount of modification, may not this same element of descent have been unconsciously used in grouping species under genera, and genera under higher groups, though in these cases the modification has been greater in degree, and has taken a longer time to complete?
I believe it has thus been unconsciously used; and only thus can I understand the several rules and guides which have been followed by our best systematists.
We have no written pedigrees; we have to make out community of descent by resemblances of any kind.
Therefore we choose those characters which, as far as we can judge, are the least likely to have been modified in relation to the conditions of life to which each species has been recently exposed.
Rudimentary structures on this view are as good as, or even sometimes better than, other parts of the organisation.
We care not how trifling a character may be let it be the mere inflection of the angle of the jaw, the manner in which an insect's wing is folded, whether the skin be covered by hair or feathers if it prevail throughout many and different species, especially those having very different habits of life, it assumes high value; for we can account for its presence in so many forms with such different habits, only by its inheritance from a common parent.
We may err in this respect in regard to single points of structure, but when several characters, let them be ever so trifling, occur together throughout a large group of beings having different habits, we may feel almost sure, on the theory of descent, that these characters have been inherited from a common ancestor.
And we know that such correlated or aggregated characters have especial value in classification.
We can understand why a species or a group of species may depart, in several of its most important characteristics, from its allies, and yet be safely classed with them.
This may be safely done, and is often done, as long as a sufficient number of characters, let them be ever so unimportant, betrays the hidden bond of community of descent.
Let two forms have not a single character in common, yet if these extreme forms are connected together by a chain of intermediate groups, we may at once infer their community of descent, and we put them all into the same class.
As we find organs of high physiological importance those which serve to preserve life under the most diverse conditions of existence are generally the most constant, we attach especial value to them; but if these same organs, in another group or section of a group, are found to differ much, we at once value them less in our classification.
We shall hereafter, I think, clearly see why embryological characters are of such high classificatory importance.