The American Naturalist, Volumen43Essex Institute, 1909 |
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Página 13
... course all restricted to the primitive disc . Soon after this stage the secondary hapteres begin to arise around the base of the stipe and become very abundant , quickly obliterating the primitive holdfast . The age at which branching ...
... course all restricted to the primitive disc . Soon after this stage the secondary hapteres begin to arise around the base of the stipe and become very abundant , quickly obliterating the primitive holdfast . The age at which branching ...
Página 22
... course the great elonga- tion . While this is especially noticeable in the stipe , the lamina likewise grows until it reaches a length of about 50 cm . , but its width increases scarcely at all , seldom ex- ceeding 4 cm . The ...
... course the great elonga- tion . While this is especially noticeable in the stipe , the lamina likewise grows until it reaches a length of about 50 cm . , but its width increases scarcely at all , seldom ex- ceeding 4 cm . The ...
Página 39
... course behind the œsophagus , are the two connected pedal - ganglia ( Fig . 19 ) , and at the center of the base of the velum , in front of where the œsophagus joins the stomach , is the supra - œsophageal ganglionic mass ( Figs . 13 ...
... course behind the œsophagus , are the two connected pedal - ganglia ( Fig . 19 ) , and at the center of the base of the velum , in front of where the œsophagus joins the stomach , is the supra - œsophageal ganglionic mass ( Figs . 13 ...
Página 58
... course of two weeks , while two months have elapsed before the tadpoles of the second lot had all metamorphosed . There is one point in connection with this experiment on which unfortunately Chambers gives no information , and yet it ...
... course of two weeks , while two months have elapsed before the tadpoles of the second lot had all metamorphosed . There is one point in connection with this experiment on which unfortunately Chambers gives no information , and yet it ...
Página 60
... course be also true in case of the sperms , which might thus be a factor determining the size of the young . At any rate , Chambers made an interesting observation that tadpoles developed from eggs of the same size begin to vary only ...
... course be also true in case of the sperms , which might thus be a factor determining the size of the young . At any rate , Chambers made an interesting observation that tadpoles developed from eggs of the same size begin to vary only ...
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Términos y frases comunes
adaptation AMERICAN NATURALIST animals appear bees breeding brown cells cent chromosomes corn correlation crinoids crossed curve Darwin dk.br ecologist ecology eggs elementary species environment evidence evolution experiment experimental fact factors female fertilization flowers forms genera genus germ growth gynandromorphs hair color heredity hybrids important individuals inheritance insects kelps L. H. BAILEY lamina large number larva larvæ Lessoniopsis lt.br male matter Mendelian Mendelian inheritance ment methods mice morphology muscular articulations mutation natural selection Notes and Literature observed offspring organs Origin of Species oyster pairs pangen paper Paramecium parent pigment pinnule plants present problem produced Professor progeny pure race RAYMOND PEARL rays Recapitulation Theory recent relation Science scientific seeds seems single soil specimens sporophytic stipe structure T. H. MORGAN theory tion toad variability variations varieties vascular vegetation Vries weight yellow
Pasajes populares
Página 170 - This has been effected chiefly through the natural selection of numerous successive, slight, favourable variations ; aided in an important manner by the inherited effects of the use and disuse of parts ; and in an unimportant manner, that is in relation to adaptive structures, whether past or present, by the direct action of external conditions, and by variations which seem to us in our ignorance to arise spontaneously.
Página 166 - If numerous species, belonging to the same genera or families, have really started into life at once, the fact would be fatal to the theory of evolution through natural selection.
Página 73 - No man would ever try to make a fantail till he saw a pigeon with a tail developed in some slight degree in an unusual manner, or a pouter...
Página 145 - We are thus brought to the question which has been largely discussed by naturalists, namely, whether species have been created at one or more points of the earth's surface. Undoubtedly there are very many cases of extreme difficulty, in understanding how the same species could possibly have migrated from some one point to the several distant and isolated points, where now found. Nevertheless the simplicity of the view that each species was first produced within a single region captivates the...
Página 85 - Our ignorance of the laws of variation is profound. Not in one case out of a hundred can we pretend to assign any reason why this or that part differs, more or less, from the same part in the parents.
Página 385 - ... 1 Papers from the Biological Laboratory of the Maine Agricultural Experiment Station. No.
Página 170 - ... and it has been stated that I attribute the modification of species exclusively to natural selection, I may be permitted to remark that in the first edition of this work, and subsequently, I placed in a most conspicuous position— namely, at the close of the Introduction — the following words: "I am convinced that natural selection has been the main but not the exclusive means of modification.
Página 83 - One of the most remarkable features in our domesticated races is that we see in them adaptation, not indeed to 'the animal's or plant's own good, but to man's use or fancy.
Página 151 - The direct action of changed conditions leads to definite or indefinite results. In the latter case the organisation seems to become plastic, and we have much fluctuating variability. In the former case the nature of the organism is such that it yields readily, when subjected to certain conditions, and all, or nearly all the individuals become modified in the same way.
Página 148 - ... to natural selection, by affording a better chance of the occurrence of profitable variations. Unless such occur, natural selection can do nothing. Under the term of " variations," it must never be forgotten that mere individual differences are included.