M Database Inspector (cheetah)
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|OriginOfSpecies - 475 Rows|
|06 - Difficutiles in Theory||06-08 - Means of Transition||60||
There is another possible mode of transition, namely, through the acceleration or retardation of the period of reproduction.
This has lately been insisted on by Prof. Cope and others in the United States.
It is now known that some animals are capable of reproduction at a very early age, before they have acquired their perfect characters; and if this power became thoroughly well developed in a species, it seems probable that the adult stage of development would sooner or later be lost; and in this case, especially if the larva differed much from the mature form, the character of the species would be greatly changed and degraded.
Again, not a few animals, after arriving at maturity, go on changing in character during nearly their whole lives.
With mammals, for instance, the form of the skull is often much altered with age, of which Dr. Murie has given some striking instances with seals; every one knows how the horns of stags become more and more branched, and the plumes of some birds become more finely developed, as they grow older. Prof. Cope states that the teeth of certain lizards change much in shape with advancing years; with crustaceans not only many trivial, but some important parts assume a new character, as recorded by Fritz Muller, after maturity. In all such cases,- and many could be given,- if the age for reproduction were retarded, the character of the species, at least in its adult state, would be modified; nor is it improbable that the previous and earlier stages of development would in some cases be hurried through and finally lost.
Whether species have often or ever been modified through this comparatively sudden mode of transition, I can form no opinion; but if this has occurred, it is probable that the differences between the young and the mature, and between the mature and the old, were primordially acquired by graduated steps.
|10 - On The Geological Succession of Organic Beings||10-08 - On the state of development of ancient forms||10||
There has been much discussion whether recent forms are more highly developed than ancient.
I will not here enter on this subject, for naturalists have not as yet defined to each other's satisfaction what is meant by high and low forms.
But in one particular sense the more recent forms must, on my theory, be higher than the more ancient; for each new species is formed by having had some advantage in the struggle for life over other and preceding forms.
If under a nearly similar climate, the eocene inhabitants of one quarter of the world were put into competition with the existing inhabitants of the same or some other quarter, the eocene fauna or flora would certainly be beaten and exterminated; as would a secondary fauna by an eocene, and a palaeozoic fauna by a secondary fauna.
I do not doubt that this process of improvement has affected in a marked and sensible manner the organisation of the more recent and victorious forms of life, in comparison with the ancient and beaten forms; but I can see no way of testing this sort of progress.
Crustaceans, for instance, not the highest in their own class, may have beaten the highest molluscs.
From the extraordinary manner in which European productions have recently spread over New Zealand, and have seized on places which must have been previously occupied, we may believe, if all the animals and plants of Great Britain were set free in New Zealand, that in the course of time a multitude of British forms would become thoroughly naturalized there, and would exterminate many of the natives.
On the other hand, from what we see now occurring in New Zealand, and from hardly a single inhabitant of the southern hemisphere having become wild in any part of Europe, we may doubt, if all the productions of New Zealand were set free in Great Britain, whether any considerable number would be enabled to seize on places now occupied by our native plants and animals.
Under this point of view, the productions of Great Britain, may be said to be higher than those of New Zealand.
Yet the most skilful naturalist from an examination of the species of the two countries could not have foreseen this result.
Agassiz insists that ancient animals resemble to a certain extent the embryos of recent animals of the same classes; or that the geological succession of extinct forms is in some degree parallel to the embryological development of recent forms.
I must follow Pictet and Huxley in thinking that the truth of this doctrine is very far from proved.
Yet I fully expect to see it hereafter confirmed, at least in regard to subordinate groups, which have branched off from each other within comparatively recent times.
For this doctrine of Agassiz accords well with the theory of natural selection.
In a future chapter I shall attempt to show that the adult differs from its embryo, owing to variations supervening at a not early age, and being inherited at a corresponding age.
This process, whilst it leaves the embryo almost unaltered, continually adds, in the course of successive generations, more and more difference to the adult.
Thus the embryo comes to be left as a sort of picture, preserved by nature, of the ancient and less modified condition of each animal.
This view may be true, and yet it may never be capable of full proof.
Seeing, for instance, that the oldest known mammals, reptiles, and fish strictly belong to their own proper classes, though some of these old forms are in a slight degree less distinct from each other than are the typical members of the same groups at the present day, it would be vain to look for animals having the common embryological character of the Vertebrata, until beds far beneath the lowest Silurian strata are discovered a discovery of which the chance is very small.
|13 - Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Or||13-08 - Extinction separates and defines groups||20||
There has been less entire extinction of the forms of life which once connected fishes with batrachians.
There has been still less in some other classes, as in that of the Crustacea, for here the most wonderfully diverse forms are still tied together by a long, but broken, chain of affinities.
|02 - Variations Under Nature||02-06 - Many of the Species included within the Larger Genera resemble Varieties in being very closely, but unequally, related to each other, and in having restricted ranges||10||
There are other relations between the species of large genera and their recorded varieties which deserve notice.
We have seen that there is no infallible criterion by which to distinguish species and well-marked varieties; and when intermediate links have not been found between doubtful forms, naturalists are compelled to come to a determination by the amount of difference between them, judging by analogy whether or not the amount suffices to raise one or both to the rank of species.
Hence the amount of difference is one very important criterion in settling whether two forms should be ranked as species or varieties.
Now Fries has remarked in regard to plants, and Westwood in regard to insects, that in large genera the amount of difference between the species is often exceedingly small.
I have endeavoured to test this numerically by averages, and, as far as my imperfect results go, they confirm the view.
I have also consulted some sagacious and experienced observers, and, after deliberation, they concur in this view.
In this respect, therefore, the species of the larger genera resemble varieties, more than do the species of the smaller genera.
Or the case may be put in another way, and it maybe said, that in the larger genera, in which a number of varieties or incipient species greater than the average are now manufacturing, many of the species already manufactured still to a certain extent resemble varieties, for they differ from each other by less than the usual amount of difference.
Moreover, the species of the larger genera are related to each other, in the same manner as the varieties of any one species are related to each other. No naturalist pretends that all the species of a genus are equally distinct from each other; they may generally be divided into sub-genera, or sections, or lesser groups.
As Fries has well remarked, little groups of species are generally clustered like satellites around other species.