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|04 - Natural Selection||04-09 - Circumstances favourable for the production of new forms through Natural Selection||10||
This is an extremely intricate subject.
A great amount of variability, under which term individual differences are always included, will evidently be favourable.
A large number of individuals, by giving a better chance within any given period for the appearance of profitable variations, will compensate for a lesser amount of variability in each individual, and is, I believe, a highly important element of success.
Though Nature grants long periods of time for the work of natural selection, she does not grant an indefinite period; for as all organic beings are striving to seize on each place in the economy of nature, if any one species does not become modified and improved in a corresponding degree with its competitors, it will be exterminated.
|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.
|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.
|08 - Hybridism||08-01 - Distinction between the sterility of first crosses and of hybrids||10||
THE view generally entertained by naturalists is that species, when intercrossed, have been specially endowed with the quality of sterility, in order to prevent the confusion of all organic forms.
This view certainly seems at first probable, for species within the same country could hardly have kept distinct had they been capable of crossing freely.
The importance of the fact that hybrids are very generally sterile, has, I think, been much underrated by some late writers.
On the theory of natural selection the case is especially important, inasmuch as the sterility of hybrids could not possibly be of any advantage to them, and therefore could not have been acquired by the continued preservation of successive profitable degrees of sterility.
I hope, however, to be able to show that sterility is not a specially acquired or endowed quality, but is incidental on other acquired differences.
In treating this subject, two classes of facts, to a large extent fundamentally different, have generally been confounded together; namely, the sterility of two species when first crossed, and the sterility of the hybrids produced from them.
Pure species have of course their organs of reproduction in a perfect condition, yet when intercrossed they produce either few or no offspring.
Hybrids, on the other hand, have their reproductive organs functionally impotent, as may be clearly seen in the state of the male element in both plants and animals; though the organs themselves are perfect in structure, as far as the microscope reveals.
In the first case the two sexual elements which go to form the embryo are perfect; in the second case they are either not at all developed, or are imperfectly developed.
This distinction is important, when the cause of the sterility, which is common to the two cases, has to be considered.
The distinction has probably been slurred over, owing to the sterility in both cases being looked on as a special endowment, beyond the province of our reasoning powers.
The fertility of varieties, that is of the forms known or believed to have descended from common parents, when intercrossed, and likewise the fertility of their mongrel offspring, is, on my theory, of equal importance with the sterility of species; for it seems to make a broad and clear distinction between varieties and species.
First, for the sterility of species when crossed and of their hybrid offspring.
It is impossible to study the several memoirs and works of those two conscientious and admirable observers, Koelreuter and Gaertner, who almost devoted their lives to this subject, without being deeply impressed with the high generality of some degree of sterility.
Koelreuter makes the rule universal; but then he cuts the knot, for in ten cases in which he found two forms, considered by most authors as distinct species, quite fertile together, he unhesitatingly ranks them as varieties.
Gaertner, also, makes the rule equally universal; and he disputes the entire fertility of Koelreuter's ten cases.
But in these and in many other cases, Gaertner is obliged carefully to count the seeds, in order to show that there is any degree of sterility.
He always compares the maximum number of seeds produced by two species when crossed and by their hybrid offspring, with the average number produced by both pure parent-species in a state of nature.
But a serious cause of error seems to me to be here introduced: a plant to be hybridised must be castrated, and, what is often more important, must be secluded in order to prevent pollen being brought to it by insects from other plants.
Nearly all the plants experimentised on by Gaertner were potted, and apparently were kept in a chamber in his house.
That these processes are often injurious to the fertility of a plant cannot be doubted; for Gaertner gives in his table about a score of cases of plants which he castrated, and artificially fertilised with their own pollen, and (excluding all cases such as the Leguminosae, in which there is an acknowledged difficulty in the manipulation) half of these twenty plants had their fertility in some degree impaired.
Moreover, as Gaertner during several years repeatedly crossed the primrose and cowslip, which we have such good reason to believe to be varieties, and only once or twice succeeded in getting fertile seed; as he found the common red and blue pimpernels (Anagallis arvensis and coerulea), which the best botanists rank as varieties, absolutely sterile together; and as he came to the same conclusion in several other analogous cases; it seems to me that we may well be permitted to doubt whether many other species are really so sterile, when intercrossed, as Gaertner believes.