Evolution and AdaptationMacmillan, 1903 - Всего страниц: 470 |
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Стр. 14
... period afterwards ; or , second , by having a mild form of the disease as in the case of smallpox , where immunity is brought about by vaccination , i.e. by giving the individual a mild form of smallpox ; or , third , by introducing ...
... period afterwards ; or , second , by having a mild form of the disease as in the case of smallpox , where immunity is brought about by vaccination , i.e. by giving the individual a mild form of smallpox ; or , third , by introducing ...
Стр. 20
... period of birth of the young , but extends at least throughout the time necessary to care for the young . It has even been suggested that this length- ening of the life period has been acquired on account of its use to the species ...
... period of birth of the young , but extends at least throughout the time necessary to care for the young . It has even been suggested that this length- ening of the life period has been acquired on account of its use to the species ...
Стр. 40
... period between the first deposits of stratified rocks and the present time , we might still have expected to find , despite the imperfections of the record , sufficient evidence to show how the present groups have arisen , and how they ...
... period between the first deposits of stratified rocks and the present time , we might still have expected to find , despite the imperfections of the record , sufficient evidence to show how the present groups have arisen , and how they ...
Стр. 41
... period of evolution of the verte- brates the fishes , amphibians , reptiles , and birds remained still in existence , although they , or some of them , may have at one time given origin to new forms . In fact , all these groups are ...
... period of evolution of the verte- brates the fishes , amphibians , reptiles , and birds remained still in existence , although they , or some of them , may have at one time given origin to new forms . In fact , all these groups are ...
Стр. 43
... period of human history we do not know of a single instance of the transformation of one species into another one , if we apply the most rigid and extreme tests used to distinguish wild species from each other.1 It may be claimed that ...
... period of human history we do not know of a single instance of the transformation of one species into another one , if we apply the most rigid and extreme tests used to distinguish wild species from each other.1 It may be claimed that ...
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acquired characters adaptation adult advantage amongst animals and plants aphids appear arisen artificial selection assume become believe birds breed butterflies caterpillars causes cells chromosomes color connection crawl crustaceans Darwin Darwinian degeneration degrees descendants determined direction elementary species embryo environment evidence evolution examine exist experiments explain fact fertilized eggs flowers fluctuating variations gemmules geotropic germ-cells give heliotropic hybrid hypothesis important individuals inheritance of acquired insects instance instinct kinds Lamarck Lamarckian large number larvæ later leaves less light living long-styled male and female mammals ment moths mutation theory Nägeli natural selection nest offspring organs origin outcome pangenesis parent form parthenogenetic polar body pollen probable produced question race reproduction resemblance response result rotifers seeds sexual reproduction sexual selection shown side sort spermatozoon stages stamens structure supposed surface takes place tion tropisms tube variability varieties Vries Weismann wings young
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Стр. 117 - If such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind ? On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation of favourable individual differences and variations, and the destruction of those which are injurious, I have called Natural...
Стр. 131 - To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree.
Стр. 116 - When we reflect on this struggle, we may console ourselves with the full belief, that the war of nature is not incessant, that no fear is felt, that death is generally prompt, and that the vigorous, the healthy, and the happy survive and multiply.
Стр. 116 - Can the principle of selection, which we have seen is so potent in the hands of man, apply under nature? I think we shall see that it can act most efficiently. Let the endless number of slight variations and individual differences occurring in our domestic productions, and, in a lesser degree, in those under nature, be borne in mind ; as well as the strength of the hereditary tendency.
Стр. 113 - The amount of food for each species of course gives the extreme limit to which each can increase ; but very frequently it is not the obtaining food, but the serving as prey to other animals, which determines the average numbers of a species.
Стр. 119 - ... be, compared with those accumulated by nature during whole geological periods. Can we wonder, then, that nature's productions should be far "truer" in character than man's productions; that they should be infinitely better adapted to the most complex conditions of life, and should plainly...
Стр. 129 - LONG before the reader has arrived at this part of my work, a crowd of difficulties will have occurred to him. Some of them are so serious that to this day I can hardly reflect on them without being in some degree staggered...
Стр. 117 - Variations neither useful nor injurious would not be affected by natural selection, and would be left either a fluctuating element, as perhaps we see in certain polymorphic species, or would ultimately become fixed, owing to the nature of the organism and the nature of the conditions.
Стр. 116 - Can it, then, be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other variations, useful in some way to each being in the great and complex battle of life, should occur in the course of many successive generations ? If such do occur, can we doubt (remembering that many more...
Стр. 108 - Again, it may be asked, how is it that varieties, which I have called incipient species, become ultimately converted into good and distinct species, which in most cases obviously differ from each other far more than do the varieties of the same species...