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|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||120||
We will suppose the letters A to L to represent allied genera, which lived during the Silurian epoch, and these have descended from a species which existed at an unknown anterior period.
Species of three of these genera (A, F, and I) have transmitted modified descendants to the present day, represented by the fifteen genera (a14 to z14) on the uppermost horizontal line.
Now all these modified descendants from a single species, are represented as related in blood or descent to the same degree; they may metaphorically be called cousins to the same millionth degree; yet they differ widely and in different degrees from each other.
The forms descended from A, now broken up into two or three families, constitute a distinct order from those descended from I, also broken up into two families.
Nor can the existing species, descended from A, be ranked in the same genus with the parent A; or those from I, with the parent I.
But the existing genus F14 may be supposed to have been but slightly modified; and it will then rank with the parent-genus F; just as some few still living organic beings belong to Silurian genera.
So that the amount or value of the differences between organic beings all related to each other in the same degree in blood, has come to be widely different.
Nevertheless their genealogical arrangement remains strictly true, not only at the present time, but at each successive period of descent.
All the modified descendants from A will have inherited something in common from their common parent, as will all the descendants from I; so will it be with each subordinate branch of descendants, at each successive period.
If, however, we choose to suppose that any of the descendants of A or of I have been so much modified as to have more or less completely lost traces of their parentage, in this case, their places in a natural classification will have been more or less completely lost, as sometimes seems to have occurred with existing organisms.
All the descendants of the genus F, along its whole line of descent, are supposed to have been but little modified, and they yet form a single genus.
But this genus, though much isolated, will still occupy its proper intermediate position; for F originally was intermediate in character between A and I, and the several genera descended from these two genera will have inherited to a certain extent their characters.
|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||40||
Any one of these characters singly is frequently of more than generic importance, though here even when all taken together they appear insufficient to separate Cnestis from Connarus.' To give an example amongst insects, in one great division of the Hymenoptera, the antennae, as Westwood has remarked, are most constant in structure; in another division they differ much, and the differences are of quite subordinate value in classification; yet no one probably will say that the antennae in these two divisions of the same order are of unequal physiological importance.
Any number of instances could be given of the varying importance for classification of the same important organ within the same group of beings.
Again, no one will say that rudimentary or atrophied organs are of high physiological or vital importance; yet, undoubtedly, organs in this condition are often of high value in classification.
No one will dispute that the rudimentary teeth in the upper jaws of young ruminants, and certain rudimentary bones of the leg, are highly serviceable in exhibiting the close affinity between Ruminants and Pachyderms.
Robert Brown has strongly insisted on the fact that the rudimentary florets are of the highest importance in the classification of the Grasses.
|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.
|13 - Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Or||13-06 - Analogical or adaptive characters||10||
As members of distinct classes have often been adapted by successive slight modifications to live under nearly similar circumstances, to inhabit for instance the three elements of land, air, and water, we can perhaps understand how it is that a numerical parallelism has sometimes been observed between the sub-groups in distinct classes.
A naturalist, struck by a parallelism of this nature in any one class, by arbitrarily raising or sinking the value of the groups in other classes (and all our experience shows that this valuation has hitherto been arbitrary), could easily extend the parallelism over a wide range; and thus the septenary, quinary, quaternary, and ternary classifications have probably arisen.
As the modified descendants of dominant species, belonging to the larger genera, tend to inherit the advantages, which made the groups to which they belong large and their parents dominant, they are almost sure to spread widely, and to seize on more and more places in the economy of nature.
The larger and more dominant groups thus tend to go on increasing in size; and they consequently supplant many smaller and feebler groups.
Thus we can account for the fact that all organisms, recent and extinct, are included under a few great orders, under still fewer classes, and all in one great natural system.
As showing how few the higher groups are in number, and how widely spread they are throughout the world, the fact is striking, that the discovery of Australia has not added a single insect belonging to a new order; and that in the vegetable kingdom, as I learn from Dr. Hooker, it has added only two or three orders of small size.