M Database Inspector (cheetah)
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|OriginOfSpecies - 475 Rows|
|12 - Geographical Distribution -- continued||12-30 - Absence of Batrachians and of terrestrial Mammals||30||
But as these animals and their spawn are known to be immediately killed by sea-water, on my view we can see that there would be great difficulty in their transportal across the sea, and therefore why they do not exist on any oceanic island.
But why, on the theory of creation, they should not have been created there, it would be very difficult to explain.
Mammals offer another and similar case.
I have carefully searched the oldest voyages, but have not finished my search; as yet I have not found a single instance, free from doubt, of a terrestrial mammal (excluding domesticated animals kept by the natives) inhabiting an island situated above 300 miles from a continent or great continental island; and many islands situated at a much less distance are equally barren.
The Falkland Islands, which are inhabited by a wolf-like fox, come nearest to an exception; but this group cannot be considered as oceanic, as it lies on a bank connected with the mainland; moreover, icebergs formerly brought boulders to its western shores, and they may have formerly transported foxes, as so frequently now happens in the arctic regions.
|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||30||
Perhaps from this cause it has partly arisen, that almost all naturalists lay the greatest stress on resemblances in organs of high vital or physiological importance.
No doubt this view of the classificatory importance of organs which are important is generally, but by no means always, true.
But their importance for classification, I believe, depends on their greater constancy throughout large groups of species; and this constancy depends on such organs having generally been subjected to less change in the adaptation of the species to their conditions of life.
That the mere physiological importance of an organ does not determine the classificatory value, is almost shown by the one fact, that in allied groups, in which the same organ, as we have every reason to suppose, has nearly the same physiological value, its classificatory value is widely different.
No naturalist can have worked at any group without being struck with this fact; and it has been most fully acknowledged in the writings of almost every author.
It will suffice to quote the highest authority, Robert Brown, who in speaking of certain organs in the Proteaceae, says their generic importance, `like that of all their parts, not only in this but, as I apprehend, in every natural family, is very unequal, and in some cases seems to be entirely lost.' Again in another work he says, the genera of the Connaraceae `differ in having one or more ovaria, in the existence or absence of albumen, in the imbricate or valvular aestivation.
|12 - Geographical Distribution -- continued||12-50 - On colonisation from the nearest source with subsequent modification||30||
New Zealand in its endemic plants is much more closely related to Australia, the nearest mainland, than to any other region: and this is what might have been expected; but it is also plainly related to South America, which, although the next nearest continent, is so enormously remote, that the fact becomes an anomaly.
But this difficulty almost disappears on the view that both New Zealand, South America, and other southern lands were long ago partially stocked from a nearly intermediate though distant point, namely from the antarctic islands, when they were clothed with vegetation, before the commencement of the Glacial period.
The affinity, which, though feeble, I am assured by Dr. Hooker is real, between the flora of the south-western corner of Australia and of the Cape of Good Hope, is a far more remarkable case, and is at present inexplicable: but this affinity is confined to the plants, and will, I do not doubt, be some day explained.
|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.