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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.
|14 - Recapitulation and Conclusion||14-03 - Causes of the general belief in the immutability of species||40||
If we admit that the geological record is imperfect in an extreme degree, then such facts as the record gives, support the theory of descent with modification.
New species have come on the stage slowly and at successive intervals; and the amount of change, after equal intervals of time, is widely different in different groups.
The extinction of species and of whole groups of species, which has played so conspicuous a part in the history of the organic world, almost inevitably follows on the principle of natural selection; for old forms will be supplanted by new and improved forms.
Neither single species nor groups of species reappear when the chain of ordinary generation has once been broken.
The gradual diffusion of dominant forms, with the slow modification of their descendants, causes the forms of life, after long intervals of time, to appear as if they had changed simultaneously throughout the world.
The fact of the fossil remains of each formation being in some degree intermediate in character between the fossils in the formations above and below, is simply explained by their intermediate position in the chain of descent.
The grand fact that all extinct organic beings belong to the same system with recent beings, falling either into the same or into intermediate groups, follows from the living and the extinct being the offspring of common parents.
As the groups which have descended from an ancient progenitor have generally diverged in character, the progenitor with its early descendants will often be intermediate in character in comparison with its later descendants; and thus we can see why the more ancient a fossil is, the oftener it stands in some degree intermediate between existing and allied groups.
Recent forms are generally looked at as being, in some vague sense, higher than ancient and extinct forms; and they are in so far higher as the later and more improved forms have conquered the older and less improved organic beings in the struggle for life.
|01 - Variations Under Domestication||01-01 - Causes of Variability||30||
I may add, that as some organisms breed freely under the most unnatural conditions (for instance, rabbits and ferrets kept in hutches), showing that their reproductive organs are not easily affected;
so will some animals and plants withstand domestication or cultivation, and vary very slightly- perhaps hardly more than in a state of nature. Some naturalists have maintained that all variations are connected with the act of sexual reproduction;
but this is certainly an error; for I have given in another work a long list of "sporting plants," as they are called by gardeners;- that is, of plants which have suddenly produced a single bud with a new and sometimes widely different character from that of the other buds on the same plant.
These bud variations, as they may be named, can be propagated by grafts, offsets, &c., and sometimes by seed.
They occur rarely under nature, but are far from rare under culture.
As a single bud out of the many thousands, produced year after year on the same tree under uniform conditions, has been known suddenly to assume a new character; and as buds on distinct trees, growing under different conditions, have sometimes yielded nearly the same variety- for instance, buds on peach-trees producing nectarines, and buds on common roses producing moss-roses- we clearly see that the nature of the conditions is of subordinate importance in comparison with the nature of the organism in determining each particular form of variation;-
perhaps of not more importance than the nature of the spark, by which a mass of combustible matter is ignited, has in determining the nature of the flames.
|01 - Variations Under Domestication||01-04 - Inheritance||30||
The laws governing inheritance are for the most part unknown.
No one can say why the same peculiarity in different individuals of the same species, or in different species, is sometimes inherited and sometimes not so; why the child often reverts in certain characters to its grandfather or grandmother or more remote ancestor; why a peculiarity is often transmitted from one sex to both sexes, or to one sex alone, more commonly but not
exclusively to the like sex.
It is a fact of some importance to us, that peculiarities appearing in the males of our domestic breeds are often transmitted, either exclusively or in a much greater degree, to the males alone.
A much more important rule, which I think may be trusted, is that, at whatever period of life a peculiarity first appears, it tends to reappear in the offspring at a corresponding age, though sometimes earlier. In many cases this could not be otherwise;
thus the inherited peculiarities in the horns of cattle could appear only in the offspring when nearly mature; peculiarities in the silkworm are known to appear at the corresponding caterpillar or cocoon stage.
But hereditary diseases and some other facts make me believe that the rule has a wider extension, and that, when there is no apparent reason why a peculiarity should appear at any particular age, yet that it does tend to appear in the offspring at the same period at which it first appeared in the parent.
I believe this rule to be of the highest importance in explaining the laws of embryology.
These remarks are of course confined to the first appearance of the peculiarity, and not to the primary cause which may have acted on the ovules or on the male element; in nearly the same manner as the increased length of the horns in the offspring from a short-horned cow by a long-horned bull, though appearing late in life, is clearly due to the male element.