Fig. 90.Fig.90. Stylonychia showing division into two. (After Stein.)
Fig.90. Stylonychia showing division into two. (After Stein.)
More recently one of Jennings's students, Middleton, has made a careful series of selection experiments with Stylonychia (fig. 90) in which he selected for lines showing more rapidor slower rates of division. His observations seem to show that his selection separated two such lines that came from the same original stock. The rapidity of the effects of selection seems to preclude the explanation that pseudo-parthenogenesis has complicated the results. Nevertheless, the results are of such a kind as to suggest that they were due to selection of vegetative (somatic) differences and that no genetic change of factors was involved, for his conclusion that the rapidity with which the effects gained by long selection might be suddenly reversed when selection was reversed is hardly consistent with an interpretation of the results based on changes in the "potencies" of the factors present.
Equally striking are the interesting experiments that Jennings has recently carried out with Difflugia (fig. 91). This protozoon secretes a shell about itself which has a characteristic shape, and often carries spines. The opening at one end of the shell through which the protoplasm protrudes to make the pseudopodia is surrounded by a rim having a characteristic pattern. The protoplasm containsseveral nuclei and in addition there is scattered material or particles called chromidia that are supposed to be chromatic in nature and related to the material of the nuclei, possibly by direct interchange.
Fig. 91.Fig.91. Difflugia Corona. (After Cash.)
Fig.91. Difflugia Corona. (After Cash.)
When Difflugia divides, part of the protoplasm protrudes from the opening and a new shell is secreted about this mass which becomes a daughter individual. The behavior of the nucleus and of the chromidia at this time is obscure, but there is some evidence that their materials may be irregularly distributedbetween parent and offspring. If this is correct, and if in the protozoa the chromatin has the same influence that it seems to have in higher animals, the mode of reproduction in Difflugia would be expected to give little more than random sampling of the germ plasm.
Fig. 92.Fig.92. Races of Difflugia. (After Leidy.)
Fig.92. Races of Difflugia. (After Leidy.)
Jennings was able by means of selection to get from the descendants of one original individual a number of different types that themselves bred true, except in so far as selection could affect another change in them. In this connection it is interesting to note that Leidyhas published figures of Difflugia (fig. 92) that show that a great many "types" exist. If through sexual union (a process that occurs in Difflugia) the germ plasm (chromatin) of these wild types has in times past been recombined, then selection would be expected to separate certain types again, if, at division, irregular sampling of the germ plasm takes place. Until these points are settled the bearing of these important experiments of Jennings on the general problem of selection is uncertain.
How Does Natural Selection Influence the Course of Evolution?
The question still remains: Does selection play any rôle in evolution, and, if so, in what sense? Does the elimination of the unfit influence the course of evolution, except in the negative sense of leaving more room for the fit? There is something further to be said in this connection, although opinions may differ as to whether the following interpretation of the term "natural selection" is the only possible one.
Fig. 93.Fig.93. Evolution of elephant's skulls. (After Dendy.)
Fig.93. Evolution of elephant's skulls. (After Dendy.)
If through a mutation a character appears that is neither advantageous nor disadvantageous, but indifferent, the chance that it may become established in the race is extremely small, although by good luck such a thing may occur rarely. It makes no difference whether the character in question is a dominant or arecessive one, the chance of its becoming established is exactly the same. If through a mutation a character appears that has aninjuriouseffect, however slight this may be, it has practically no chance of becoming established.
Fig. 94.Fig.94. Evolution of elephant's trunk. (After Lull.)
Fig.94. Evolution of elephant's trunk. (After Lull.)
If through a mutation a character appears that has abeneficialinfluence on the individual, the chance that the individual will survive is increased, not only for itself, but for all of itsdescendants that come to inherit this character. It is this increase in the number of individuals possessing a particular character, that might have an influence on the course of evolution. This gives a better chance for improvement by several successive steps; but not because the species is more likely to mutate again in the same direction. An imaginary example will illustrate how this happens: When elephants had trunks less than a foot long, the chance of getting trunks more than one foot long was in proportion to the length of trunks already present and to the number of individuals; but increment in trunk length is no more likely to occur from an animal having a trunk more than one foot long than from an animal with a shorter trunk.
The case is analogous to tossing pennies. At any stage in the game the chance of accumulating a hundred heads is in proportion to the number of heads already obtained, and to the number of throws still to be made. But the number of heads obtained has no influence on the number of heads that will appear in the next throw.
Fig. 95.Fig.95. Evolution of elephant's trunk: above Maeritherium, in the middle Tetrabelodon (After Lancaster); below African elephants (After Gambier Bolton).
Fig.95. Evolution of elephant's trunk: above Maeritherium, in the middle Tetrabelodon (After Lancaster); below African elephants (After Gambier Bolton).
Owing then to this property of the germ plasm to duplicate itself in a large number of samples not only is an opportunity furnished to an advantageous variation to become extensively multiplied, but the presence of a large number of individuals of a given sort prejudices the probable future result.
The question may be raised as to whether it is desirable to call selection acreativeprocess. There are so many supernatural and mystical implications that hang around the term creative that one can not be too careful in stating in what sense the term is to be used. If by creative is meant that something is made out of nothing, then of course there is no need for the scientist to try to answer such a question. But if by a creative process is meant that something is made out of something else, then there are two alternatives to be reckoned with.
First, if it were true that selection of an individual of a certain kind determines that new variations in the same direction occur as a consequence of the selection, then selection would certainly be creative. How this could occur might be quite unintelligible, but of course itmight be claimed that the point is not whether we can explain how creation takes place, but whether we can get verifiable evidence that such a kind of thing happens. This possibility is disposed of by the fact that there is no evidence that selection determines the direction in which variation occurs.
Second, if you mean by a creative process that by picking out a certain kind of individual and multiplying its numbers a better chance is furnished that a certain end result will be obtained, such a process may be said to be creative. This is, I think, the proper use of the term creative in a mechanistic sense.
Conclusions
In reviewing the evidence relating to selection I have tried to handle the problem as objectively as I could.
The evidence shows clearly that the characters of wild animals and plants, as well as those of domesticated races, are inherited both in the wild and in the domesticated forms according to Mendel's Law.
The causes of the mutations that give riseto new characters we do not know, although we have no reason for supposing that they are due to other than natural processes.
Evolution has taken place by the incorporation into the race of those mutations that are beneficial to the life and reproduction of the organism. Natural selection as here defined means both the increase in the number of individuals that results after a beneficial mutation has occurred (owing to the ability of living matter to propagate) and also that this preponderance of certain kinds of individuals in a population makes some further results more probable than others. More than this, natural selection can not mean, if factors are fixed and are not changed by selection.
INDEXAbnormal abdomen109Abraxas78-81Allantois17Allelomorphs83-84Altenburg112Amnion16-17Andalusian fowl45,46Annelids22Antlered wing111Apterous wing11Arc wing111Aristae104Bar eye67,108,169Bateson18,34,36Beaded wing11,115Beans147-149,157Belgian hare171Bent wing116Bergson30,31Bildungstrieb34Biogenetic law15,18,19,21Biometricians156Bird21,23Bithorax65,112,113Black body color111,133Blakeslee152Bridges114,143,163British Association36Brünn40Buff eye color109Bufon27Castle176-180Cat33Cell90,91Chance variations37Chick16,17,20Chromatin184Chromosome group of Drosophila102Chromosomes91,95,96,98,130,131,132Cleavage21,22,94Clover butterfly62Club wing69,70,108Colias philodice62Color blindness77,125Comb of Drosophila103Combs of fowls33,54Comparative anatomy7,8,9,14Corn150,153,172Correns41Cosmogonies27Cream eye color163,164Crepidula22Criss-cross inheritance78Crossing over131-133Cuénot178Curled wing115Curved wing111Curve of probability149Cut wing11,104Dachs legs112Dahlgren62Darwin15,24,28,32,35-37,64,145,146,152,154-156Dendy188De Vries18,147,156Dexter170Dichaete114Difflugia184-187Discontinuous variation13Disuse31Drosophila ampelophila10,12,13,48-50,60,75,84,85,93,100,103,119,155,162,169Drosophila repleta76Duplication of legs109Dwarf114East170,172Ebony50,55,56,115Egg91,94Elephant191Elephants' skulls188Elephants' trunks190Embryology13-23Emerson172Environment27Eosin eye color61,107,163Erdmann183Evolution Creatrice30Evolution—three kinds of1,2,4Eye color13Eyeless66,115Factorial theory89Factors of Drosophila143Fantails172,175Fertilization91Fish16,20,21Flatworms22Fluctuations12Forked bristles106Fowl77Fused veins107,108Galton154Geneticist26Germ-plasm142Geoffroy St. Hilaire27Giant114Gill-slits20,21,23Groups I, II, III, IV100-118Haeckel15Haemophilia77Heliotropism106,107Himalyan rabbits83History1,6Hoge66Horse, evolution of6Indian corn172,173Interference137,138Janssens132Jaunty wing111Jennings161,181-184,186Johannsen156,157,159-161,166,182Lamarck31-34Langshan77Leaves147Leidy186Lethal105Linkage groups103Lizard23Localization of factors118MacDowell155,170,171Macritherium191Mammal16,21,23Man20,77,125,126Map of Chromosomes136Maroon eye color114Mendel40,41,52,89Mendelian heredity39Mendel's law41-59,64,124Mendel's second law52Mesenchyme cells22Mesoderm cells22Metaphysician30Mice33,178Middleton183Miniature wing108Mirabilis42Modifiers163,164,170,171Molluscs22Mouse83Muller112,167Mutations35,39,84Nägeli34,35Natural Selection36,145,146,187-194Nisus formativus34Non-disjunction139-142Notch wing104-106Nucleus91Origin of Species35,145Orthogenesis34Paleontology24-27Papilio polytes63Papilio turnus63Paramecium181,182Paratettix81Peach eye color114Pea comb54Pearl161Peas47Pigeons172,174,175Pink eye color114,115Planarian22Plymouth Rock77Podarke22Polar bodies126Pole arms5Protozoa181Pseudo-parthenogenesis183Purple eye color109Purpose4Rabbits83,170Rats176-180Reduction division182Reproductive cells96Ruby eye color106Rudimentary organ116Rudimentary wing70,71,107Sable body color107Science definition of6Segregation41Selenka94Sepia eye color13,114Sex chromosomes118Sex linked inheritance75,118-130Sexual dimorphism62Sheep33Single comb54Sooty body color50,114,115Speck68,69,111Spencer145Spermatozoön91,98Stars, evolution of6St. Hilaire27-30Strap wing110,111Stumpy wing11Sturtevant76,143Stylonychia183Survival of the fittest146Systematist85Tails33Tan flies106,107Tetrabelodon191Trefoil111Truncate wing111,112,167,168Unfolding principle34Unio22Unit character74,75Use31Variation discontinuous13Vermilion eye color108,163Vestigial wing11,55,56,109,133Vital force34Wallace36Walnut comb54Weismann17,31-33Wilson, E. B.125Wingless67Winiwarter126White eye color13,75,119-130Whiting eye color163,164Woodruff183Yellow body color108,133Yolk sac16,17Zeleny169
INDEX
INDEX
Abnormal abdomen109Abraxas78-81Allantois17Allelomorphs83-84Altenburg112Amnion16-17Andalusian fowl45,46Annelids22Antlered wing111Apterous wing11Arc wing111Aristae104
Abnormal abdomen109
Abraxas78-81
Allantois17
Allelomorphs83-84
Altenburg112
Amnion16-17
Andalusian fowl45,46
Annelids22
Antlered wing111
Apterous wing11
Arc wing111
Aristae104
Bar eye67,108,169Bateson18,34,36Beaded wing11,115Beans147-149,157Belgian hare171Bent wing116Bergson30,31Bildungstrieb34Biogenetic law15,18,19,21Biometricians156Bird21,23Bithorax65,112,113Black body color111,133Blakeslee152Bridges114,143,163British Association36Brünn40Buff eye color109Bufon27
Bar eye67,108,169
Bateson18,34,36
Beaded wing11,115
Beans147-149,157
Belgian hare171
Bent wing116
Bergson30,31
Bildungstrieb34
Biogenetic law15,18,19,21
Biometricians156
Bird21,23
Bithorax65,112,113
Black body color111,133
Blakeslee152
Bridges114,143,163
British Association36
Brünn40
Buff eye color109
Bufon27
Castle176-180Cat33Cell90,91Chance variations37Chick16,17,20Chromatin184Chromosome group of Drosophila102Chromosomes91,95,96,98,130,131,132Cleavage21,22,94Clover butterfly62Club wing69,70,108Colias philodice62Color blindness77,125Comb of Drosophila103Combs of fowls33,54Comparative anatomy7,8,9,14Corn150,153,172Correns41Cosmogonies27Cream eye color163,164Crepidula22Criss-cross inheritance78Crossing over131-133Cuénot178Curled wing115Curved wing111Curve of probability149Cut wing11,104
Castle176-180
Cat33
Cell90,91
Chance variations37
Chick16,17,20
Chromatin184
Chromosome group of Drosophila102
Chromosomes91,95,96,98,130,131,132
Cleavage21,22,94
Clover butterfly62
Club wing69,70,108
Colias philodice62
Color blindness77,125
Comb of Drosophila103
Combs of fowls33,54
Comparative anatomy7,8,9,14
Corn150,153,172
Correns41
Cosmogonies27
Cream eye color163,164
Crepidula22
Criss-cross inheritance78
Crossing over131-133
Cuénot178
Curled wing115
Curved wing111
Curve of probability149
Cut wing11,104
Dachs legs112Dahlgren62Darwin15,24,28,32,35-37,64,145,146,152,154-156Dendy188De Vries18,147,156Dexter170Dichaete114Difflugia184-187Discontinuous variation13Disuse31Drosophila ampelophila10,12,13,48-50,60,75,84,85,93,100,103,119,155,162,169Drosophila repleta76Duplication of legs109Dwarf114
Dachs legs112
Dahlgren62
Darwin15,24,28,32,35-37,64,145,146,152,154-156
Dendy188
De Vries18,147,156
Dexter170
Dichaete114
Difflugia184-187
Discontinuous variation13
Disuse31
Drosophila ampelophila10,12,13,48-50,60,75,84,85,93,100,103,119,155,162,169
Drosophila repleta76
Duplication of legs109
Dwarf114
East170,172Ebony50,55,56,115Egg91,94Elephant191Elephants' skulls188Elephants' trunks190Embryology13-23Emerson172Environment27Eosin eye color61,107,163Erdmann183Evolution Creatrice30Evolution—three kinds of1,2,4Eye color13Eyeless66,115
East170,172
Ebony50,55,56,115
Egg91,94
Elephant191
Elephants' skulls188
Elephants' trunks190
Embryology13-23
Emerson172
Environment27
Eosin eye color61,107,163
Erdmann183
Evolution Creatrice30
Evolution—three kinds of1,2,4
Eye color13
Eyeless66,115
Factorial theory89Factors of Drosophila143Fantails172,175Fertilization91Fish16,20,21Flatworms22Fluctuations12Forked bristles106Fowl77Fused veins107,108
Factorial theory89
Factors of Drosophila143
Fantails172,175
Fertilization91
Fish16,20,21
Flatworms22
Fluctuations12
Forked bristles106
Fowl77
Fused veins107,108
Galton154Geneticist26Germ-plasm142Geoffroy St. Hilaire27Giant114Gill-slits20,21,23Groups I, II, III, IV100-118
Galton154
Geneticist26
Germ-plasm142
Geoffroy St. Hilaire27
Giant114
Gill-slits20,21,23
Groups I, II, III, IV100-118
Haeckel15Haemophilia77Heliotropism106,107Himalyan rabbits83History1,6Hoge66Horse, evolution of6
Haeckel15
Haemophilia77
Heliotropism106,107
Himalyan rabbits83
History1,6
Hoge66
Horse, evolution of6
Indian corn172,173Interference137,138
Indian corn172,173
Interference137,138
Janssens132Jaunty wing111Jennings161,181-184,186Johannsen156,157,159-161,166,182
Janssens132
Jaunty wing111
Jennings161,181-184,186
Johannsen156,157,159-161,166,182
Lamarck31-34Langshan77Leaves147Leidy186Lethal105Linkage groups103Lizard23Localization of factors118
Lamarck31-34
Langshan77
Leaves147
Leidy186
Lethal105
Linkage groups103
Lizard23
Localization of factors118
MacDowell155,170,171Macritherium191Mammal16,21,23Man20,77,125,126Map of Chromosomes136Maroon eye color114Mendel40,41,52,89Mendelian heredity39Mendel's law41-59,64,124Mendel's second law52Mesenchyme cells22Mesoderm cells22Metaphysician30Mice33,178Middleton183Miniature wing108Mirabilis42Modifiers163,164,170,171Molluscs22Mouse83Muller112,167Mutations35,39,84
MacDowell155,170,171
Macritherium191
Mammal16,21,23
Man20,77,125,126
Map of Chromosomes136
Maroon eye color114
Mendel40,41,52,89
Mendelian heredity39
Mendel's law41-59,64,124
Mendel's second law52
Mesenchyme cells22
Mesoderm cells22
Metaphysician30
Mice33,178
Middleton183
Miniature wing108
Mirabilis42
Modifiers163,164,170,171
Molluscs22
Mouse83
Muller112,167
Mutations35,39,84
Nägeli34,35Natural Selection36,145,146,187-194Nisus formativus34Non-disjunction139-142Notch wing104-106Nucleus91
Nägeli34,35
Natural Selection36,145,146,187-194
Nisus formativus34
Non-disjunction139-142
Notch wing104-106
Nucleus91
Origin of Species35,145Orthogenesis34
Origin of Species35,145
Orthogenesis34
Paleontology24-27Papilio polytes63Papilio turnus63Paramecium181,182Paratettix81Peach eye color114Pea comb54Pearl161Peas47Pigeons172,174,175Pink eye color114,115Planarian22Plymouth Rock77Podarke22Polar bodies126Pole arms5Protozoa181Pseudo-parthenogenesis183Purple eye color109Purpose4
Paleontology24-27
Papilio polytes63
Papilio turnus63
Paramecium181,182
Paratettix81
Peach eye color114
Pea comb54
Pearl161
Peas47
Pigeons172,174,175
Pink eye color114,115
Planarian22
Plymouth Rock77
Podarke22
Polar bodies126
Pole arms5
Protozoa181
Pseudo-parthenogenesis183
Purple eye color109
Purpose4
Rabbits83,170Rats176-180Reduction division182Reproductive cells96Ruby eye color106Rudimentary organ116Rudimentary wing70,71,107
Rabbits83,170
Rats176-180
Reduction division182
Reproductive cells96
Ruby eye color106
Rudimentary organ116
Rudimentary wing70,71,107
Sable body color107Science definition of6Segregation41Selenka94Sepia eye color13,114Sex chromosomes118Sex linked inheritance75,118-130Sexual dimorphism62Sheep33Single comb54Sooty body color50,114,115Speck68,69,111Spencer145Spermatozoön91,98Stars, evolution of6St. Hilaire27-30Strap wing110,111Stumpy wing11Sturtevant76,143Stylonychia183Survival of the fittest146Systematist85
Sable body color107
Science definition of6
Segregation41
Selenka94
Sepia eye color13,114
Sex chromosomes118
Sex linked inheritance75,118-130
Sexual dimorphism62
Sheep33
Single comb54
Sooty body color50,114,115
Speck68,69,111
Spencer145
Spermatozoön91,98
Stars, evolution of6
St. Hilaire27-30
Strap wing110,111
Stumpy wing11
Sturtevant76,143
Stylonychia183
Survival of the fittest146
Systematist85
Tails33Tan flies106,107Tetrabelodon191Trefoil111Truncate wing111,112,167,168
Tails33
Tan flies106,107
Tetrabelodon191
Trefoil111
Truncate wing111,112,167,168
Unfolding principle34Unio22Unit character74,75Use31
Unfolding principle34
Unio22
Unit character74,75
Use31
Variation discontinuous13Vermilion eye color108,163Vestigial wing11,55,56,109,133Vital force34
Variation discontinuous13
Vermilion eye color108,163
Vestigial wing11,55,56,109,133
Vital force34
Wallace36Walnut comb54Weismann17,31-33Wilson, E. B.125Wingless67Winiwarter126White eye color13,75,119-130Whiting eye color163,164Woodruff183
Wallace36
Walnut comb54
Weismann17,31-33
Wilson, E. B.125
Wingless67
Winiwarter126
White eye color13,75,119-130
Whiting eye color163,164
Woodruff183
Yellow body color108,133Yolk sac16,17
Yellow body color108,133
Yolk sac16,17
Zeleny169
Zeleny169