SELECTION IN CLADOCERA ON THE BASIS OF A PHYSIOLOGICAL CHARACTER BY ARTHUR M. BANTA PUBLISHED BY THE CARNEGIE INSTITUTION OF WASHINGTON WASHINGTON, 1921 CARNEGIE INSTITUTION OF WASHINGTON. PUBLICATION No. 305. PAPER No. 33 OF THE DEPARTMENT OP GENETICS. ANDREW B. GRAHAM COMPANY WASHINGTON. D. C. CONTENTS. Page General statement 5 Source of material 6 Culture methods 8 Pedigree records 11 Method of selection 13 General procedure 13 Environmental influences and reaction- time 17 Test series 18 Same-day broods 19 Possible criticisms of methods of selec- tion 20 Nature of swimming movements of Cla- docera species used in selection . 24 Behavior of young of the different species when released in the experimen- tal tank 24 Relation between environmental condi- tions and reproductive age 26 Statistical treatment of data 26 Analysis of data 28 Line 695 28 Presentation of data 28 Analysis of data for effect of selection 38 Relation between reproductive vigor and reactiveness to light 40 Line 689 46 Line 691 49 Line 711 51 Line 713 55 Line 714 56 Line 719 60 Line 751 66 Line 762 ] 68 Line 766 70 Line 768 79 General introduction for Simocephalus exspinosus lines 73 Line 794 74 Line 795 77 Line 796 80 Line 740 84 Line 757 89 Presentation of data 89 Detailed analysis of data for Line 757 97 Reaction-time means compared by longer periods 97 Other features of the data 104 Relation between relative vigor and mean reaction-time. . . 108 Page Analysis of data — Continued. Line 757 — Continued. Special features of the reaction-time curves 115 Extent of the change in reactiveness of the Line 757 plus strain 116 Reactiveness of both strains of Line 757 modified through selection. 118 Conclusion regarding effect of selection in Line 757 126 Environmental conditions as affecting reaction-time and vigor of stock 128 1. Temperature influences 128 2. Influence of substances exhaled from observer's breath 130 3. Relatively temporary chemical (?) differences in water used in ex- perimental tank 131 4. Occurrence of negatively reacting individuals 132 5. General influences operative through longer periods of time 132 Coincident changes in reactiveness . . 133 Seasonal changes in reactiveness. . . 135 Contemporaneous shifts in reaction- time means 135 General increase in reactiveness of Simocephalus exspinosus 137 Independent shifts in reaction-time means 140 Whether "depression periods " occur. 142 6. Environmental conditions as affect- ing vigor of stock 143 Reliability of the reproductive index. 143 Coincident fluctuations in vigor. ... 144 Independent fluctuations in vigor. . . 145 Differences between reproductive indices during different parts of experiment 146 Is the increased reactiveness for lines of Simocephalus exspinosus asso- ciated with increased vigor? .... 147 Possible divergence in vigor between the two strains of a selected line 148 Statement of results of selection 150 Discussion of results 152 Summary 163 References 169 LIST OF FIGURES. Page 1. Line 695. Reproductive vigor 42 A. Average number in first brood 42 B. Average age of mother at production of first brood 42 C. Reproductive indices, actual values 42 2. Line 695 43 A. Relative rates of descent of the two strains 43 B. Reproductive indices, superiority 43 C. Reaction-time curves 43 3. Line 689 48 A. Reproductive indices, actual values 48 B. Reproductive indices, superiority 48 C. Reaction-time curves 48 4. Line 691. Reaction-time curves 51 5. Line 711. Reaction-time curves 53 6. Line 713. Reaction-time curves 55 7. Line 714 58 A. Reproductive indices, actual values 58 B. Reproductive indices, superiority 58 C. Reaction-time curves 58 8. Line 719 64 A. Reproductive indices, actual values 64 B. Reproductive indices, superiority 64 C. Reaction-time curves 64 9. Line 751. Reaction-time curves 67 10. Reaction-time curves for lines of Daphnia longispina and composite curves for all D. pulex lines 69 A. Line 762. Reaction-time curves 69 B. Line 766. Reaction-time curves 69 C. Line 768. Reaction-time curves 69 D. Composite curves for all D. pulex lines with curves for Line 751 super- imposed 69 11. Line 794 77 A. Reproductive indices, actual values 77 B. Reproductive indices, superiority 77 C. Reaction-time curves 77 12. Line 795 79 A. Reproductive indices, actual values 79 B. Reproductive indices, superiority 79 C. Reaction-time curves 79 13. Line 796 82 A. Reproductive indices, actual values 82 B. Reproductive indices, superiority 82 C. Reaction-time curves 82 14. Line 740 87 A. Reproductive indices, actual values 87 B. Reproductive indices, superiority 87 15. Line 740. Reaction-time curves with reaction-time curves for Line 757 super- imposed 87 16. Line 757. Relative rates of descent of the two strains 97 17. Line 757. Reproductive vigor 101 A. Average number in first brood 101 B. Average age of mother at production of first brood 101 C. Reproductive indices, actual values 101 18. Line 757 102 A. Reproductive indices, superiority 102 B. Reaction-time curves, with composite curves for all other Simocephalus exspinosus lines superimposed 102 19. Line 757. Reaction-time curves by six-month periods with similar curves for Line 740 superimposed 103 LIST OF TABLES. Page 1. History of lines of Cladocera used in selection experiments 9 2. Data from an early test series of Line 695 to illustrate differences in general reactiveness to light on successive days 18 3. Summary of selection data by broods for Line 695 plus 29 4. Summary of selection data by broods for Line 695 minus 32 5. Selection data summarized by two-month periods for Line 695 plus 35 6. Selection data summarized by two-month periods for Line 695 minus 36 7. Selection summary for Line 695 37 8. Same-day broods. Summary of data for Line 695 38 9. Selection data summarized by two-month periods for Line 689 plus 46 10. Selection data summarized by two-month periods for Line 689 minus 46 11. Selection summary for Line 689 47 12. Same-day broods. Summary of data for Line 689 47 13. Selection summary for Line 691 49 14. Same-day broods. Summary of data for Line 691 50 15. Selection data summarized by two-month periods for Line 711 plus 51 16. Selection data summarized by two-month periods for Line 711 minus 52 17. Selection summary for Line 711 52 18. Same-day broods. Summary of data for Line 711 52 19. Selection summary for Line 713 54 20. Same-day broods. Summary of data for Line 713 54 21. Selection summary for Line 714 57 22. Same-day broods. Summary of data for Line 714 57 23. Selection data summarized by two-month periods for Line 719 plus 61 24. Selection data summarized by two-month periods for Line 719 minus 61 25. Selection summary for Line 719 62 26. Same-day broods. Summary of data for Line 719 62 27. Selection summary for Line 751 66 28. Same-day broods. Summary of data for Line 751 66 29. Selection summary for Line 762 68 30. Selection summary for Line 766 70 31. Selection summary for Line 768 71 32. Same-day broods. Summary of data for Line 768 71 33. Selection summary for Line 794 74 34. Same-day broods. Summary of data for Line 794 74 35. Selection summary for Line 795 78 36. Same-day broods. Summary of data for Line 795 78 37. Selection summary for Line 796 81 38. Same-day broods. Summary of data for Line 796 81 39. Selection summary for Line 740 84 40. Same-clay broods. Summary of data for Line 740 84 41. Summary of selection data by broods for Line 757 plus 91 42. Summary of selection data by broods for Line 757 minus 94 43. Selection data summarized by two-month periods for Line 757 plus 98 44. Selection data summarized by two-month periods for Line 757 minus . . . . .t 99 45. Selection summary for Line 757 100 46. Same-day broods. Summary of data for Line 757 100 47. Summary of data for more reactive individuals of Line 757 107 48. Comparison of reaction-time means for the plus strain of Line 757 (within which selection was effective) with corresponding means for the plus strains for S. exspinosus lines in which selection was not effective (Line 740) and the shorter S. exspinosus lines. Lines 794, 795, and 796 119 49. Comparison of reaction-time means for the minus strain of Line 757 (within which selection was effective) with corresponding means for the minus strains for S. exspinosus lines in which selection was not effective (Line 740) and the shorter S. exspinosus lines. Lines 794, 795, and 796 121 SELECTION IN CLADOCERA ON THE BASIS OF A PHYSIOLOGICAL CHARACTER. GENERAL STATEMENT. For more than 8 years (January 1920) the writer has been rearing parthenogenetic "pure lines" of Cladocera. The original object of rearing this material was the conducting of experiments on selection within the pure line. These experiments were completed in May 1917. Data bearing on other problems have also been secured from the handling of this material. The selection experiments only will be treated in the present paper. The writer undertook the selection experiments in order to get additional data on the effects of selection within the pure line. The Cladocera material was chosen for several reasons: 1. It reproduces rapidly. 2. Under favorable conditions it is readily handled in the laboratory. 3. It reproduces parthenogenetically with (under carefully con- trolled conditions) no possible question as to the occurrence of sexual reproduction. 4. In the maturation of the parthenogenetic eggs of Cladocera there is a single division without reduction (Weismann, 1886; Kiihn, 1908). Hence with this material there are presumably no compli- cations of segregation during maturation or of fertilization as with sexually reproducing forms. 5. No selection experiments had been made with a purely physiological character as the basis for selection. Aside from the fact that a physiological character had probably not been used previously as a basis for studies on the effect of selection, a purely physiological character seemed desirable to use for two additional reasons: (a) Physiological differences are frequently readily measur- able and measurable in a precise way. Numerical series of measure- ments provide very usable series of data free from the errors of estimation and personal equation applying to series not directly numerical. The light reactions of Cladocera afford a character per- haps as definitely measurable as any readily handled physiological characteristic. (6) In both the ontogenetic and phylogenetic history of organisms physiological modifications may occur without observed morphological changes, and in the modification of organisms physio- logical modifications may readily precede distinguishable or measur- able morphological changes. This latter consideration (6) had the greatest weight in determining the use of a physiological character as the basis for these experiments. 6 SELECTION IN CLADOCERA ON THE BASIS OF The rapidly breeding, easily propagated, parthenogenetic organ- ism possessing a readily measurable physiological character afforded just the material desired. The experimental object was to attempt to isolate by selection, through a number of generations, a strain more responsive to light than a second strain from the same pure line1 selected for its reduced responsiveness to light.2 SOURCE OF MATERIAL. The Cladocera material to be considered in this paper was obtained from ponds near Cold Spring Harbor. Pond I is a surface- water pond in an open lot at an altitude of about 160 feet above tide- water. This is a temporary pond, some 60 by 40 feet in dimensions when moderately filled, and contains water only from early winter to May or somewhat later in the summer. Pond II was3 a permanent spring-fed pond in a shaded situation in the woods on a northeast slope. It was about one-fourth mile distant from Pond I, at an altitude of about 60 feet, was approximately 18 by 10 feet, and had a depth of only 10 inches. Pond III is another surface-water pond on the upland, three-eighths of a mile from Pond I. It overflows occasionally after excessive rains and frequently becomes dried up in summer.4 At its maximum it is approximately 65 by 40 feet, but it ordinarily contains less than a fourth of this area and does not exceed 2 feet in depth at its deepest portion. This pond is near and receives the surface drainage from a barnyard and a pasture lot. Hence it is rich in organic solution constituents. Pond IV is also a shallow surface-water pond. It is on the upland, an eighth of a mile from Pond I, ordinarily has an area of perhaps half an acre, and has become dry only once in the past 7 years. This pond also receives the surface drainage from a barnyard, but because of its larger drainage basin it is not so rich in organic solution constituents as Pond III. The Daphnia pulex material used in the selection experiments was obtained from Pond I (November 1911 and October 1912) and from Pond II (November 1911). Only two lines (689, 751) from Pond I were retained very long. The remainder of the lines of D. pulex used in these experiments came from Pond II, the small spring-fed pond in the woods. The D. longispina used in selection were derived from stock from Ponds I and IV. Collections were 1 These were not "pure lines" in the narrowed sense of Johannsen's definition, but were clones in the sense adopted by Johannsen and his followers. The term "pure line" has not in the general mind been restricted to Johannsen's usage, however. Nevertheless, to avoid loose- ness of expression and possible misunderstanding, it might be wise to abandon this term to Johannsen's limited meaning and for a general term, embracing the Johannsen pure line and the clone, to adopt the term pure lineage or pure descent. *Two short notes relating to this series of experiments have been previously published (Banta, 1913, 1919). The references there made to Simocephalus vetulus should read Simo- cephalus exspinosus = vetulus. 3 It has since been drained. 4 It has become empty in the autumn three times in the last 7 years. A PHYSIOLOGICAL CHARACTER. 7 made in October 1912, in Pond I, and in Pond IV in November 1913. The Simocephalus exspinosus material used in selection experiments was obtained from Pond IV, August 1912 and December 1914, and from Pond I, October 1912. The stock obtained from Pond I, the surface-water pond, had in all probability only recently undergone sexual reproduction. The pond ordinarily remains dry from early summer until October or November. If as much as 3 to 5 weeks had elapsed between the filling of the pond and November 16, when the latest collection from this pond was made, the daphnids could have descended at most only one or two generations, allowing several days for the (fertilized) ephippial eggs (produced before the pond became dried up in the spring) to develop and 2 weeks for each generation at out-door temperature at that season. It is indeed quite probable that the individuals collected were themselves ex-ephippial individuals. There is no safe criterion for determining how long the material obtained from Pond II (the spring-fed pond) may have reproduced parthenogenetically since the stock had last undergone sexual repro- duction previous to its being taken into the laboratory. However in this pond and in other small ponds in which this species has been observed it has not ordinarily been found to occur for more than 3 to 5 weeks at a time, so that probably this material had comparatively recently descended from ephippial eggs. There is likewise no way of determining how recently the stock collected from Pond IV had undergone sexual reproduction. D. longispina occurs in this pond occasionally, and, so far as observations go, seems not to continue there long at a time. S. exspinosus, however, is found there the year round. No males or sexual eggs of either species have been found in this pond.1 The five lines of S. exspinosus originated from five different mothers collected from two ponds and at three different times. There is some evidence (see page 123) that Line 757 (from Pond I, October 1912) at the beginning of selection differed in its reactive- ness from Line 740 (from Pond IV, August 1914). The progenitors of these two lines were obtained from different ponds and with a time-interval of about two months. The other lines (794, 795, and 796, from Pond IV, December 1914) of this species seemed not to differ in their reactiveness from each other or from Line 740 obtained two years earlier from the same pond. Possibly these four lines belonged to the same clone, while Line 757 belonged to a different clone. It is possible that sexual reproduction may not occur in the pond from which these four lines (740, 794, 795, and 796) were obtained, and they may all have come from a common progenitor. While the material was not examined with this point in mind, 1 For the purpose of these experiments it is presumably quite immaterial at what time the lust previous sexual reproduction had occurred. 8 SELECTION IN CLADOCERA ON THE BASIS OF between 60 and 100 collections of S. exspinosus material from this pond were made at intervals of several days and no males or ephippial females were found. Males might readily be overlooked in col- lections not carefully examined microscopically, but ephippial females are unlikely to be overlooked by one who is accustomed to handling Cladocera material. However, sexual reproduction in Cladocera is spasmodic and frequently of short duration, so that the results of the examination of the collections do not entirely preclude the possibility (though making it seem improbable) that sexual reproduction may actually have occurred during the period of fre- quent examinations of material from this pond. The same general reactiveness appeared to exist in all D. pulex lines (see figs. 2c, 3c, 4, 5, 6, 7c, 8c, 9, and lOo), although they came from two ponds (Pond I and Pond II), were collected at different times, and presumably may have belonged to more than one clone. Sexual reproduction in this species was several times noted in a nearby pond (Pond III) from which the culture-water was obtained. Table 1 shows in condensed form certain data concerning the lines of Cladocera reared in the laboratory and used for the selection experiments. CULTURE METHODS. The culture-water in which these animals were reared was at first obtained from the spring-fed pond (Pond II). Beginning a year and a half later, it was gotten from Pond III, the barnyard pond. This pond receives all its water from the surface drainage of a small pasture lot and from a barn and barnyard in which live-stock is kept. The amount of organic matter carried into the pond by the surface drainage is relatively large and the water is rich in organic solution constituents. The water is usually heavily colored a reddish or yellowish brown. In being collected, the culture-water is dipped up in a manner to obtain as much as possible of the loose, fluffy sediment from the bottom of the pond. After being brought into the laboratory it is strained through fine "India linen." The lighter portion of the sediment is gently rubbed through the straining-cloth and the coarser residue is discarded. The strained water is then allowed to stand for two days, when it is again (sediment included) strained through India linen. After a second interval of from one to several days the water is thoroughly stirred and a third straining is made through a standard silk bolting-cloth having 130 meshes to the inch. The water is then ready for use as culture-water. The first straining removes any Cladocera or Copepoda which may be in the pond- water and removes most (at least) of their eggs. The second strain- ing removes any young which as eggs may have passed through the A PHYSIOLOGICAL CHARACTER. 9 a-o° .2 gel *a 5 *-* fS'c! . ... -t~O • • -OiOO r~ •* OS IO 00 00 "* •* O N fe*>i-l " " " "US'* • • -TfCOiO "d O ? 00 5 :« :£ :.° : « : « : « : a :o : « : o : « .°? .S o.S 53 ^'Ib « g°B cSO P^ PH -fU ip^ -Pn -PH -Pn • PH -Pn • PH • CU • P-, - P, -P^ • PH g — — - — • "— ••-" "^ • i-i -N .N -M ~.tt -IN 'IN •*)< "^i -^i oj:.9 D o ^ > •> '>• ' > ' > ' > ' > ' ^; ' ±i 'ii ' > 'ti'jj 'e> ' d "c5 o ;o ;o ;o ;o ;o ;o ;"S ;« ;o ;o ; g ; o ;® ;S ;£ XJ ' '"" * ' ' ' '^ '^ '^ "^ ' a .5 'Ol 'O3 'HI "OJ *03 "03 "03 '00 '00 '00 " tB '00 "00 "00 "00 "00 5j •3'3'3'3'3'3'3'3'3'3'3'3'3'3-3-3 3.9 3.2 3.2 3.2 3-S 3.9 3.9 3.9 3.9 3.2 3.9 3.9 3 2 3.9 3 9 3 2 a xo[nd -Q ensoaidsxa -g 10 SELECTION IN CLADOCERA ON THE BASIS OF cloth on the first straining, they, meantime, having had time to develop before the later straining. The third straining is probably quite unnecessary, but is used as an added precaution. The culture-water contains a small amount of the original (now thrice-strained) sediment, has much material in suspension and in solution, and is dull brownish-gray in color. It settles and clears slowly, so that young daphnids placed in the culture-bottles are visible only with difficulty for a day or two. The animals when collected were isolated in 200-c. c. wide- mouthed bottles filled to a depth of near 5 cm. (about 100 c. c. in quantity) with culture-water. The young of the first brood from the wild mother were transferred during the first day of life to indi- vidual bottles. A single individual each was placed in the No. 1 and the No. 2 bottles, while three or more were placed in a third bottle designated as a "prime" bottle to serve as reserve stock. Transfers were made in like manner in subsequent generations, except that Sunday's broods were transferred Monday, when one day old. The bottles of the parent generation were retained as additional reserve stock until a second descendent generation was obtained. The grandmother generation was then discarded, a few individuals being preserved for morphological study if later desired.1 For many months after the cultures were begun, and at frequent intervals afterward, large samples of the culture-water, after its three strainings, were set aside in glass jars. Observations were made to determine if daphnids of any sort developed in this water from eggs passing through the straining-cloth or in any way introduced by accident during its handling. None appeared at any time. In addition to the three strainings of the culture-water and the general precautions observed in handling the stock, the system of handling the material in itself served to check out any possible con- tamination. The No. 1 and No. 2 bottles each contained a single individual and the prime bottles three or more individuals. Any contamination must have been detected, except in the case of an individual of the same species and very nearly the same age in a No. 1 or No. 2 bottle in which the individual belonging in the bottle died within a day after having been placed therein. The likelihood of the coincidence of the introduction of an individual of the same age and of the same species into a bottle in which the rightful individual died within a day is so slight as to arouse little concern. If contami- nation occurred in a prime bottle, to escape ready detection it would have to be an individual of the same species and of very nearly the same age as the individuals belonging in the bottle. Further, the 1 In case the No. 1 individual is lost, young are transferred from the No. 2 bottle, or if the No. 1 and No. 2 are both lost, the transfers are made from the prime bottle. In case the prime too is lost, extra transfers are made from the first or a later brood of the No. 1 or other surviving individual of the parent generation, and young from one of these transfers are used to start the new generation. A PHYSIOLOGICAL CHARACTER. 11 chances would be 3 to 1 (since there are three or more individuals belonging in the prime bottle) against young from the strange indi- vidual being used in propagating the line in the unlikely contingency of this bottle being used to continue the line. These facts, combined with the fact that the prime bottles were used only occasionally (when the No. 1 and No. 2 individuals had died) in propagating the strains, make the chances greatly against the perpetuation of a con- tamination of a pure lime, if such a contamination ever did occur. With a single exception,1 which was readily detected, we have every reason to believe that our selection cultures did not at any time be- come contaminated by the introduction of daphnids in the culture- water or by any other means.2 PEDIGREE RECORDS. All the descendants of each of the "wild" individuals originally taken into the laboratory are referred to as belonging to a single line, while any subdivisions of the stock within this line are designated as strains. Thus, of each of the lines in which selection on the basis of light reactions was made, there were plus and minus strains, while of certain lines there were several additional strains, all of the strains of any line of course having descended in the laboratory from a single "wild" individual. In the records each line is referred to by a certain arbitrary number. The generation of its descent since having been brought into the laboratory is indicated by a letter followed by a subscript. The plus and minus strains are indicated by the plus or minus signs. Thus 695 DH— indicates reference to the 264th generation (the letters of the alphabet having been used ten times over and to the letter D on the eleventh time) of the minus strain of Line 695. 3 Notes were made on loose-leaf sheets, a sheet being used for the notes for the three bottles (No. 1, No. 2, and "prime") of each gener- ation of each strain. Records on this sheet include the pedigree designation, the pedigree of the mother from which the young were derived, and the date of birth of the young, all made at the time of making the transfer. Later, the date of occurrence and (usually) 1 In one case, early in the course of these experiments, a strange daphnid did appear in one of the culture-bottles. It was a small, rapidly reproducing form, the eggs of which might conceivably be pressed through the straining-cloth, young developed to maturity, and eggs of the next generation produced before the second straining. It was readily recognized as an inter- loper. Such an appearance, even had it been that of a stray individual of the species under cul- tivation, would have been readily detected and leads to no great concern as to the purity of our cultures. 2 In a few instances two individuals were found in a No. 1 or a No. 2 bottle, due to the lifting out in the pipette of two (instead of one) individuals from the mother's bottle or from the expe- imental tank when making the transfers. The prompt finding of the extra individual added to our confidence in the purity of our cultures, but in every case the bottle in which an extra individual was found was discarded. Since in these cases the extra individual was not of another strain, but merely an extra individual of the same strain, its presence in the bottle did not represent a threatened contamination. 3 It is to be noted that the minus strains are strains selected for reduced reactiveness to light. No negatively reacting strains occur. 12 SELECTION IN CLADOCERA ON THE BASIS OF the number of young in the first brood and the date of occurrence of the second brood from the No. 1 mother were recorded. The transfer of young to the next descendent generation was noted for the appropriate mother, as was also the later discarding of the bottles and the preservation of individuals from the No. 1 bottle. The record of transfers made was given on the mother's as well as the daughter's sheet. This double checking served as a safeguard in case of any deficiency or discrepancy in the notes. Similar, but less complete, records were made for the No. 2 and prime bottles. The data obtained in conducting the light tests for determining the individuals to be selected was recorded in a separate note-book. Any discrepancy as to the number in a given brood, its pedigree designation, date of birth, etc., could again be checked by reference to this book, so that in reality a three-entry system of note-keeping was utilized. Very few discrepancies or deficiencies in the notes have been found, and these were readily cleared up.1 A sample sheet of the loose-leaf notes is appended, and also a page of the experimental-room notes. Copy of pedigree sheet of loose-leaf notes. Copy of page from experimental note-book. 695 F6- No. 1. 695 F6- No. 1. 10/20/14 From 695 E6- No. 1. 10/13/14. Temperature , 19.9°. Oct. 20, 14 young. Distributed to Time, 2 : 10 : 00. 695 Gb-. 1 2: 12:11, + Oct. 23, 2d brood. 2 12 : 15, + Oct. 31, preserved. 3 13 : 03, + 4 13 : 05, + 695 Fs- No. 2. 5 13 : 27, + Oct. 20, 13 young. 6 12 : 27, + Oct. 23, 2d brood. 7 14 : 00, + Oct. 27, discarded. 8 14 : 50, + 9 15:05, + 695 F6-. 10 15 : 50, + Oct. 20, young. 11 18 : 00, + Oct. 27, discarded. 12 19 : 55, + 13 over-time. 14 over-time. 1 However, for the sake of complete safety, it was considered wise to keep up the three- entry method of keeping notes. It did not prove unduly laborious. In any case, a new sheet must be made for each set of three new bottles of the new generation, and naturally the notes on the light tests required the entry of several of the items placed in the experimental note-book. A PHYSIOLOGICAL CHARACTER. 13 METHOD OF SELECTION. GENERAL PROCEDURE. Propagation of the older lines of D. pulex had progressed from 5 to 8 generations before the selection experiments were begun. Selection with lines later used was begun at once upon introduction into the laboratory. The culture-bottles were given uniform treatment so far as possible. The plus and minus strains of the same line were kept side by side on the table. The only obvious environmental differ- ences to which the two strains of the same line were subjected arose from the f act • that the young from the two strains were transferred to new food, usually on different days. While every effort was made to provide uniform culture-water, only a moderate degree of uni- formity was attainable, but this unavoidable difference in environ- mental treatment was not differential, and in the long run should have affected the two strains equally. Unlike the Cladocera material reared under laboratory con- ditions by many workers, the writer's material produced exclusively asexual young, there being with the material subjected to selection no discovered case in which males or sexual eggs were produced. The selection tests were conducted in a darkened basement room with dull-black walls. The tank was constructed of plate glass, the bottom being a heavy slab of smooth slate, grooved out to a depth of about 1.5 cm., the grooves being about 2 cm. wide. The plate-glass sides and ends were set in these grooves in an aqua- rium cement. The grooves were filled with the cement so as to leave a smooth surface continuous with the surface of the slab. The lines of contact of the glass sides and ends were cemented together with thick Canada balsam. After the aquarium cement and balsam had been given several days for hardening, the surface of the cement and the inside of the glass sides of the tank were coated with a mix- ture of lampblack, balsam, and turpentine. The result was a tank with uniform dull-black sides and bottom and with transparent ends. The tank measured inside 40 cm. by 26.6 cm. and was 7.2 cm. in depth. An improvement on this tank would be made by substituting thin slabs of smooth slate for the plate-glass sides. The source of light was at first a slender, cylindrical, carbon- filament, incandescent lamp. Later similar carbon-filament lamps could not be obtained and a tungsten lamp was used. The lamps used varied from 43.5 to 75.9 candle-power. They were placed at 14 SELECTION IN CLADOCERA ON THE BASIS OF the appropriate distances from the tank to obtain an illumination of 120 candle-meters, calculated to the middle of the tank.1 The heat-screen, 4.5 cm. thick, with parallel glass sides and filled with distilled water, was placed at the end of the tank through which the illumination was received. An upright black cardboard screen was placed between the heat-screen and the end of the tank, so as to permit rays of light to enter the end of the tank only in the area between the bottom of the tank and the level of the surface of the water. The water within the tank was filtered pond-water and was maintained at a depth of 1.8 cm. Fresh water was placed in the tank each day and a change of water was made after testing and selecting from each four broods. The young animals to be tested, consisting of entire first broods, were removed from the mothers' bottles by means of a small pipette, and taken to the experimental room in 10 mm. cylindrical vials. The vials were placed in a wire basket and immersed to about 3 cm. depth in a dish of water of the same temperature as the water in the experimental tank and left about an hour before the tests were begun. The temperature of the experimental room varied from 14° C. during the coldest weather in winter to sometimes as high as 22° C. in summer. Very rarely the temperature was as low as 10° C. or as high as 24° C. In preparation for the testing of each brood, the entire brood was (by means of the small pipette) placed in the center of the experimental tank within an upright cylinder of glass 1 cm. in diam- eter. The experimental light was turned on and all extraneous light was eliminated. The animals were then left undisturbed within this cylinder for 2 minutes, when the cylinder was lifted care- fully from the water, thus releasing the animals in the center of the tank; the temperature of water and time of release of the animals were recorded just before the release. The illumination within the water was sufficient to make the young daphnids just visible to the eye of one accustomed to working with them. As soon as a daphnid reached either end of the tank it was removed and the time of its arrival recorded, together with the proper sign to indicate to which end of the tank it had gone (see page 12). The interval between the release of the animal and its arrival at the end of the tank is referred 1 No allowance was made for the diminution in amount of light due to its passage through the heat-screen, the plate-glass end of the tank, and the water within, nor was allowance made for the somewhat different results in illumination due to using lamps of different candle- power. While the distances at which the lamps were used were such as to give a calculated illumination of 120 candle-meters at the middle of the tank, the illumination at the ends of the tank would of course be somewhat different with the lamps differing in candle-power. This fact was not taken into consideration in originally substituting a 60-watt Mazda lamp (about 75.3 candle-power) for the carbon-filament lamp of 43.5 candle-power. Having for a time used the lamp with the higher candle-power, it was considered wise to continue its use. Any variation in absolute intensity of the light at the different parts of the tank is regrettable, but it should of course be equally effective with the plus and minus strains and presumably could not disturb the course of the experiments. A PHYSIOLOGICAL CHARACTER. 15 to as its reaction-time.1 The test was continued for 15 minutes, unless all individuals had reached an end of the tank earlier; any remaining at the end of 15 minutes were removed and arbitrarily assigned a reaction-time of 15 minutes.2 Another brood was then tested in a like manner. In testing a brood of the plus strain, the individual first reaching the positive end of the tank was at once placed in a separate vial and later transferred to the No. 1 bottle of the new generation. The second one to reach the positive end was likewise placed in a separate vial, to be later transferred to the No. 2 bottle. The others were returned to the vial in which they had been conveyed to the experi- mental room. Three or more of these were later transferred to the "prime " bottle of the new generation. With the broods of the minus strains the procedure was the same, except that the two quickest to reach the negative end, the two which moved farthest toward the negative end, the two moving least toward the positive end of the tank, or the two slowest in reaching the positive end were selected for the No. 1 and No. 2 bottles of the new generation. Negatively reacting individuals did not occur in most of the broods, nor were there usually individuals which showed any tendency to react nega- tively to light. With D. pulex and D. longispina few individuals failed to move to the positive end within the limits of the experiment. Hence very frequently in the minus strains the No. 1 and No. 2 individuals selected were respectively the one slowest and the one next slowest in reaching the positive end of the tank. As with the data for the over-time individuals, the data for the negatively reacting individuals presented some difficulty. The occurrence of negatively reacting individuals was irregular and more or less spasmodic. Table 2 presents some data illustrating this point. Reference to any of the tables presenting the data by broods will show that when negatively reacting individuals occur they are frequently relatively numerous. The writer believes negatively re- acting individuals are (usually, at any rate) influenced by some unusual environmental factor and that an individual's swimming 1 The "reaction-time" as recorded in the notes and used in this paper indicates, in seconds, the interval between the release of the young daphnid from the glass cylinder in the middle of the experimental tank and the time at which it reached the end of the tank. Strictly speaking, this is the real reaction-time — the time consumed in the beginning of movement and estab- lishing orientation with reference to the light — plus the time consumed by the animal in swimming to the end of the tank. 2 The arbitrary assumption of 15 minutes as the reaction-time of individuals which as a matter of fact did not react within that length of time is open to criticism, but no better method of utilizing this significant portion of the data was ascertained. The " over-time " individuals can not be disregarded. The data for them are obviously very significant. In a slightly reactive strain, such as the minus strain of Line 757, they constitute a large portion of the individuals tested and represent a striking manifestation of the low responsiveness to light in this as in other strains. As pointed out in another connection, the arbitrary assumption of 900 seconds for their reaction-times greatly minimizes the rightful influence upon mean reaction-time of their slight or non-reactiveness to light. 16 SELECTION IN CLADOCERA ON THE BASIS OF to the negative end of the tank does not in most cases really indicate a negative phototropism. Such individuals, if tested a second time, do not ordinarily repeat the behavior.1 The best interpretation seemed to be that individuals which reacted negatively were stimu- lated by the light as their sisters were, but that a "negative" re- sponse was ordinarily called forth by some accessory influence. Occasionally this additional influence obviously was accidental mechanical stimulation; but usually negative responses could not be thus accounted for, and often they were certainly not due to this cause. The data for negatively reacting individuals were treated as though the individuals had reacted positively, except that the number of such occurrences was tabulated and is frequently referred to in the treatment of the data. It is questionable if, in making selections in the minus strains, one was justified in selecting individuals (where such a selection was possible) which went to the negative end of the tank. Cladocera are so generally positive that negative reactions are at once open to a question as to significance, but it is believed that the possibility of an actual change or mutation producing a really negatively reacting individual is sufficient justification for selecting these individuals in the minus strains. In case a daphnid remained unaccounted for at the end of the test of a brood, the tank was emptied, twice carefully rinsed, and filled to the proper depth with fresh water. At all times in the experiments every conceived precaution was taken to exclude extraneous light, to guard against reflection from any object without or small foreign body within the tank, to eliminate all mechanical stimulation, and particularly to guard against the possible mis- placing of an individual daphnid in the handling of the broods.2 In brief, in the selection experiments the entire first brood, soon after its release from the mother's brood-pouch, was removed from the culture-bottle, placed in a small vial, taken to a darkened room, and subjected to known and always uniform directive light- stimulation under carefully controlled conditions, the object being to attempt to procure (by selection through many generations) a strain more responsive to light than a second strain from the same *In the plus strain of Line 757 minus individuals occurred in 17 broods out of 172 broods from which selections were made. In 4 of these cases there were more than a single minus indi- vidual (19 individuals in 4 broods). In the minus strain of Line 757 minus individuals occurred in 13 of 175 actual selection tests, and in one of these cases there were 3 minus individuals (table 42). 2 The pipette in which the daphnids were handled was carefully rinsed after handling each brood. In conducting the selections the vials to receive the selected animals were arranged so that the one nearest at hand was the one in which the next individual removed from the tank was to be placed. A small opening was made in the cardboard screen in order to permit a small amount of light to fall upon the observer's watch and note-book. The pipette could readily be examined in this light and the animal seen within it, in case there was any doubt about the daphnid having been drawn into the pipette when its removal from the tank was attempted. The table on which the experiments were conducted was closed off from the remainder of the room by a black curtain. A PHYSIOLOGICAL CHARACTER. 17 line selected for its less responsiveness to light. In the beginning of this selection the most responsive individual from the first brood of the young mother was selected for the beginning of the plus strain and the least reactive one or, perchance, one reacting negatively and avoiding the light, for the beginning of the minus strain. In the next and later generations selections were made from the first broods (except in cases where the first brood was lost), the two most reactive individuals in the plus and the two least reactive individuals (or negatively reacting individuals if such occurred) in the minus strain being selected to propagate and continue their respective strains. At certain periods during the course of these experiments the rearing of the various strains was rendered difficult by poor food conditions. Sometimes during such periods selections were not made on the basis of reaction to light, but the individuals used to continue the strains were taken at random from the mother's bottle. In the tables giving the data by broods, such cases (and a few others in which for some reason the selection test was not con- ducted) are indicated as "random distributions." ENVIRONMENTAL INFLUENCES AND REACTION-TIME. The tables of data obtained in making the selections show that there were wide differences in the general reactiveness of different broods tested in making the selections (see tables 3 and 4). Some- times a brood had an unusually low or an unusually high mean re- action-time, when perhaps the immediately preceding and next succeeding broods of the same strain had a mean near that for the strain as a whole. Sometimes a considerable number of a single brood reacted negatively, although on the whole only a small percentage of the individuals reacted negatively. Occasionally it was noted that all the broods on a certain day responded slowly to light stimulation, while perhaps on the following day all responded promptly. These differences are due to environmental factors. Table 2 illustrates this point, as well as the spasmodic occurrence of negatively reacting individuals. Reference to this table shows that on August 29, 1913, the mean reaction-time of 87 individuals of Line 695 tested was 401 seconds, and 3.5 per cent reacted negatively; on the following day the mean reaction-time was 636 seconds and 1.6 per cent were negative; on the day following this the mean reaction- time was 398 seconds and 28 per cent were negative. Thus, on the second of these three days the individuals of the same strains were slower in their reactions, compared with the other two days, by approximately 60 per cent. And there were twice and 18 times as large percentages of negatively reacting individuals on the first and third as on the second of these days.1 1This is an extreme case, but it illustrates differences such as were repeatedly observed, though usually to a much less marked degree. 18 SELECTION IN CLADOCERA ON THE BASIS OF Such differences in response indicate environmental factors of great influence, but what these influences were was in most cases not determined. It was frequently noted, in the tests at the begin- ning of these experiments, that after the observer had tested several broods in succession the broods tested later reacted less quickly than the earlier ones. This may have been due to the accumulation in the water of C02 from the observer's breath. The difficulty was obviated by changing the water in the experimental tank after testing each four broods. These and other factors were much too influential for one to ignore them completely, but their influence was presumably a horizontal one, affecting all the individuals of a brood and of the different strains in the same manner and not in any way influencing selections, except as affecting the reaction-time and in causing a few TABLE 2. — Data from an early test series of Line 695 to illustrate differences in general reactiveness to light on successive days. Mother's pedigree. No. of young. No. of nega- tively react- ing individuals. Percentage of negative individuals. Mean reaction-time in seconds. By broods. By days. Aug. 29, 1913. Fresh water 695 Es - No. 9".. 695 Is + No. 5 695 Is + No. 6 Totals, etc 26 23 38 1 0 2 448 555 276 Fresh water Aug. 30, 1913. Fresh water 87 3 3.5 401 695 Is + No. 9 . . 695 E3 - No. 2 695 E3 - No. 10 695 Is + No. 12 695 E3 - No. 7 695 I3 + No. 11 695 Is -f No. 13 695 Es - No. 4 Totals, etc . . . 22 21 23 23 27 33 21 20 0 0 0 0 1 2 0 0 346 631 704 710 577 766 761 526 Fresh water Fresh water Aug. 31, 1913 Fresh water 190 3 1.6 636 695 Is + No. 1 . . 695 E3 - No. 9 .... 695 I3 + No. 8 Totals, etc 22 17 11 10 2 2 350 396 498 50 14 28.0 398 'This is mother No. 9 of the fifty-seventh generation of the minus strain of Line 695. individuals which otherwise would have been positive to be rendered negative in their reactions. It seems wise to postpone the detailed analysis of the effects of environment upon reaction-time and upon vigor until after the general analysis of the data has been presented (see pages 128-149). TEST SERIES. Because of the various factors instrumental in influencing the reactions of the different broods of Cladocera, and in order to obtain a critical measure of the effect of selection, if such occurred, so-called A PHYSIOLOGICAL CHARACTER. 19 test series were planned. These test series were conducted under a plan by which large numbers of broods of the plus and the minus strains of the same line were experimented upon 072 the same day and as nearly as was experimentally possible under precisely the same conditions. The plan was as follows: First, it was necessary to wait until such time as the plus and minus strains of the same line reproduced on the same day. A number of these young, from 12 to 20, of each of the strains were then transferred to individual bottles, given the same food, kept grouped together on the same table, and in every way treated alike. When their first broods appeared, four broods of equal or nearly equal size — two from the plus and two from the minus strain — were chosen, the choice being limited to broods released from the mother's brood-pouch within 2 or 3 hours of the same time. Thus we had broods containing equal or nearly equal numbers of individuals, of the same age (within 2 or 3 hours), and from mothers of the same age which had themselves received identical treatment from birth; other " quartettes " were selected in like manner. Further, the four broods, constituting a quartette, were handled in a definite order; a plus brood was experimented with first, then a minus brood, then the second minus brood of the quar- tette, and finally the second plus brood was tested. The tank was then emptied and replenished with fresh water. The second quar- tette was handled in reverse order: first a minus brood, then (in order) the two plus broods, and the second minus brood. The next quartette was then handled in the order indicated for the first quar- tette. Still other quartettes of broods were chosen from the second and later broods of the test-series mothers until the numbers of in- dividuals tested from each strain were quite large — in most cases larger than 600 and in several cases in excess of 1,000. The test-series, it was hoped, might serve as a means of eliminat- ing most of the disturbing factors unquestionably present during the reaction-tests, due to the fact that the selections in the plus strain and the minus strain of the same line were usually, through necessity, made on different days. SAME-DAY BROODS. The two strains of the same line only occasionally reproduced on the same day, so that consecutive reaction-tests of a brood of each of these two strains were not ordinarily possible; however, during the whole course of the selection experiments, there were a number of selections in the plus and minus strains of the same line on the same days. In addition to the complete tabulation, the data for these "same-day" broods are tabulated separately. Such a tabulation for the two strains of Line 695 is given in table 8. Such a comparison of the mean reaction-times of the same-day broods ought perhaps to afford a safer criterion of the effectiveness of 20 SELECTION IN CLADOCERA ON THE BASIS OF selection (if an effect be obtained) than a summary of the entire selection data. The difficulty is that the amount of this data is so limited. For example, there are only 41 of these same-day broods for Line 695, out of a total of 192 broods of the plus strain and 180 broods of the minus strain for which data were secured. POSSIBLE CRITICISMS OF METHODS OF SELECTION. Certain possible criticisms of the methods used in the selection experiments occurred to the writer and doubtless may occur to others. 1. Was there assurance that the individual daphnids selected were really the most and the least positively reacting individuals in their respective broods? (a) Was there reason to think that the individual daphnid first reaching the positive end of the tank was the one most influenced by the light? (b) May not this individual have been affected by a slight swirl in the water in being released from the cylinder or in some other manner subjected to mechanical stimu- lation? (c) Or may it have been affected differently from its fellows, due to some other condition external to itself? (d) Or may it have been influenced by some internal factor or factors and consequently its reaction determined by some " physiological state?" (e) Or may its reaction-time not have been influenced by its general vigor or by its ability as a swimmer? (/) Was the method of handling such as in any manner to unfavorably affect the animals and thus render their activities untrustworthy? 2. If the individual selected was really the most positively photo- tactic at the time of the selection, was it also the most positively phototactic an hour or a day later? 3. Was the selection method used calculated to produce a more vigorous race as compared with a less vigorous race? 4. Assuming real and lasting differences in light-reaction be- tween sibs, are these differences heritable anyway? If one accepted these possible criticisms in their fullest impli- cation it would at once be seen that the results here set forth were impossible of attainment, and the result with Line 757 is perhaps sufficient answer to these questions. The writer does not believe that these possible criticisms are at all damaging to or can be right- fully applied to these experiments; however, they will be discussed individually. (1) and (a) under it will be discussed after its sub- sidiary questions, (6) to (/), have been considered. (6) Observations directed upon the effect of mechanical stimu- lation show that a daphnid when mechanically stimulated usually moves promptly and rapidly; frequently the movement is from the source of mechanical stimulation if that is directive; but if non- directive, the movement is equally prompt and vigorous, though usually not different in direction from that in which the animal was A PHYSIOLOGICAL CHARACTER. 21 previously oriented. In either case the effect is ordinarily very temporary. In the vast majority of cases, in making the selections, when the cylinder was lifted to release the animals the daphnids at first failed to move at all or moved very slowly and only later began swimming or increased the rate of movement. In spite of all precautions, doubtless some individuals were mechanically stimulated and thus influenced in their reactions; occasionally such was obviously the case. But while such stimulation might influence the rate of move- ment and occasionally the direction of movement, these effects were temporary and the occurrences of such influences were to all appear- ances infrequent, so that it is believed the aggregate effect of such influences upon the selections is of little or no significance. (c) Unquestionably it is true that even when every precaution is taken to secure uniformity of conditions and to eliminate all extraneous light, to eliminate disturbances of the direction of the incident rays of light, to prevent mechanical or other accessory stimulation, and to provide uniformity in handling the animals, etc., perfect conditions are not attainable. Imperfect experimental con- trol of environmental conditions is obvious enough and is referred to elsewhere in this paper (see pages 15, 17-20, and 128-149). But there are no grounds for assuming that these imperfectly controlled environ- mental conditions were differential or in any appreciable way affected the selections. To assume that in any considerable number of the selec- tions differential stimulation occurred would, we believe, go beyond the facts and be quite unwarranted. (d) It is obvious that conditions internal to the animals them- selves are not subject to complete control. The writer does not doubt that " physiological states" were occasionally a factor, and sometimes a determining factor, in individual selections, but he does not believe that internal conditions were frequently a determining factor. In cases of repeated tests of the reaction-times of a number of individual daphnids, sometimes several hours or even 2 or 3 days apart, the later results usually agreed with the earlier results — i. e., the animals which at the first test were most responsive to the light on subsequent tests were the most responsive or among the most responsive individuals. Hence there is excellent reason for believing that the individuals selected were actually the most responsive and the least responsive individuals of their respective broods. But assuming that temporary physiological states did sometimes influence the selections (as must occasionally have been the case), it would merely operate to render the selections less effective, and unless the internal temporary physiological factors were factors in a prevailing number of the selections, they could not neutralize the selections. (e) The individuals selected in the plus strains may have been merely the best swimmers, regardless of general vigor, or they may 22 SELECTION IN CLADOCERA ON THE BASIS OF have been the most vigorous individuals of their respective broods; but on the whole there seems no good reason for believing this to have been the case. If the differences were merely differences in quickness of movement or swimming ability, it would be quite as interesting to develop "hare" and " tortoise" strains of Cladocera as to develop strains more reactive and less reactive to light; and the result would have the same fundamental bearing on the problem of selection within the pure line. That the reaction-times of the vast majority of individuals were not influenced by differences in vigor is attested by the fact that there is no consistent relation between vigor (as measured by the mean reproductive index) and mean re- action-time. This point is discussed more fully in connection with the detailed analysis of the data for the various lines. (/) The strains subjected to selection did not show greater mortality nor less vigorous growth and reproductive ability than those lines not used in the selection experiments. The only difference in conditions through which those selected (as compared with those strains not subjected to selection) passed previous to being placed in the experimental tank was being placed for a time in a relatively small amount of the culture-water. There is no reason to think that this profoundly affected their light-reactions, or if it had, that the effect would not have been a horizontal rather than a differential one. (1) and (la) On numerous occasions the individuals of a brood were caused to react in the experimental tank a number of times. Of course, on second and subsequent trials only a single individual could be handled at one time in the tank because of the danger of mistaking the identity of the individuals. Of 10 such experiments, conducted within a few days and includ- ing all such experiments made within those days, there was com- plete agreement in reaction order in 6 experiments; in 3 there was fair agreement, and in 1 there was very poor agreement in reaction order. Even in the last-mentioned case, however, the individuals which reacted more quickly during the first test in general reacted more quickly on the average of all the trials, but the reaction order was greatly changed. The objection that the selections in general may have been made on the basis of vigor of individuals has been, we think, effectively discounted. In view of this fact, it is believed that such consistent results as those just indicated show conclusively that neither physiological states nor any other factor operated to obviate or hamper the selection on the basis of fundamental indi- vidual differences in reactiveness to light. It is believed that in the vast majority of cases the two selected in the plus strains were more reactive to the illumination to which they were subjected than their fellows, and that those selected in the minus strain were less positive than their fellows. A PHYSIOLOGICAL CHARACTER. 23 2. This point bears on the question of the occurrence of "physio- logical states" and their relative frequency. The discussion of (1) and (la) bears directly on this point. 3. To the possible objection that the method of selection was suited to the production of more vigorous strains in the plus and less vigorous strains in the minus strains, the answer is that such a result did occur to some extent in two of the lines, but it was not a cumu- lative result and the differences in reproductive vigor were not greater at the close than during the early part of the experiment. These cases are discussed in detail in the analysis of the data for the various lines. But, as is pointed out later, in those cases in which reduced reproductive vigor did occur it was not causally associated with differences in reaction-time. 4. This question, as applied to the selection problem, is a "leading" question. Its answer depends upon whether the differences are germinal or purely somatic. In the vast majority of cases such reaction differences as are encountered in these selection experiments are probably purely somatic, and the effect of selecting such variants is of course nil. Whether some of these reaction differences are due to germinal modifications will be taken up in connection with the experimental data considered in this publication. 24 SELECTION IN CLADOCERA ON THE BASIS OF NATURE OF SWIMMING MOVEMENTS OF CLADOCERA SPECIES USED IN SELECTION. Daphnia pulex is rather rotund in form and is only slightly heavier than water. It swims by vigorous strokes with perceptible intervals between, thus producing a very jerky movement. It is pelagic in its habits, and ordinarily its swimming strokes are repeated just frequently enough to maintain its level in the water. When loco- motion occurs the animal orients itself differently, i. e., instead of its antero-dorsal axis being approximately vertical, it changes to ap- proach a horizontal position with dorsal side uppermost; its swimming movements are more rapidly repeated and the animal's course may be fairly straight. The jerky character of movement is still quite in evidence. This species frequently attaches to the surface film of the water, but almost never to the sides of the container. A vigorous individual rarely or never rests upon the bottom. D. longispina is very similar to D. pulex in its habits and move- ments, but is more slender and somewhat less active; it is not quite so heavy; its axis does not so nearly assume a vertical position in its "place-maintaining" movements; and in its locomotion its move- ments are somewhat less jerky, due to its relatively smaller antennas and less vigorous strokes. Like D. pulex, it is distinctly pelagic and does not rest upon the bottom. It does not ordinarily attach to the surface film and never to the sides of a container. Simocephalus exspinosus is a rotund, rather bulky species, much heavier than the two species of Daphnia. It settles rather rapidly through the water, unless kept up by continued swimming movements. It lies upon and moves about over the bottom much of the time, fre- quently holds fast to the side of the container, and often attaches to and moves about at the surface film. It usually swims with ventral side uppermost. Its swimming movements are less jerky than in either species of Daphnia, due to its relatively smaller swimming antennae and more rapid antennal movements, though the jerky character is somewhat evident. In nature S. exspinosus spends most of its time upon the bottom and upon submerged aquatic vegetation. BEHAVIOR OF YOUNG OF DIFFERENT SPECIES WHEN RELEASED IN THE EXPERIMENTAL TANK. In the experimental tank the vast majority of individuals of D. pulex upon their release swam a few strokes without orientation, then oriented with reference to the light and swam toward the source of light. This orientation was not perfect in many cases, so that the animal sometimes swam somewhat diagonally. But even when the direction of movement was somewhat diagonal, the course was usually fairly straight. This diagonal course occasionally led to the side of the tank. In such cases the animal usually followed along A PHYSIOLOGICAL CHARACTER. 25 the side of the tank in the direction of its former orientation, though sometimes it took a diagonal course away from the side of the tank, but in the general direction (as regards light) in which it was formerly oriented. The individuals which were slow in reacting either moved very slowly, scarcely more than executing "place-maintaining move- ments," or they wandered about independently of the directive light stimulation for a time and later attained and maintained orien_ tation with reference to the light. Some individuals wandered about so long or swam so little that they did not reach either end of the tank in the 15 minutes of the experiment. These were the "over- time" individuals which were arbitrarily assigned a reaction-time of 900 seconds. D. pulex seldom attached to the surface film or to the sides of the tank. Negatively reacting individuals were not especially common with D. pulex. The statements regarding the behavior of D. pulex in the experi- mental tank apply to D. longispina; this species sometimes became caught in the surface film and was unable to free itself. Such indi- viduals were discarded and excluded from the record. S. exspinosus, on being released within the experimental tank, settled to the bottom at once or swam a few strokes without orien- tation and then settled to the bottom. Occasionally some individuals just after their release swam upward and attached to the surface film. In many broods, some individuals, after settling to the bottom or rising to the surface, made no further movements during the 15 minutes of the test, while with some broods there was no further movement on the part of any individual. The majority of individuals, however, began moving again within a few seconds, or at most within 2 or 3 minutes, assumed their orientation, and swam toward the light. Most individuals, after orienting, paused for a time, settled to the bottom, and afterward continued their movement toward the light; others, after the second pause, did not move again. With S. exspinosus there was less random movement after the first pause than with the two species of Daphnia. Occasionally individuals, in their movement toward the light, swam near the surface, and when they paused attached to the surface film. Such individuals frequently did not move again. As with Daphnia, the orientation often was not perfect and some individuals reached the sides of the tank, fre- quently attaching there, and making no further movements; but more generally they followed along the side of the tank with the same general orientation as before. Orientation in a negative way was uncommon with this species. The frequency of occurrence of large numbers of non-reactive individuals of S. exspinosus was a distinct difficulty to satisfactory experimental conditions. Because of this apparent difficulty, some of the earlier lines of S. exspi?iosus with which selection was attempted 26 SELECTION IN CLADOCERA ON THE BASIS OF were discarded as apparently unprofitable material for a selection experiment on the basis of reactiveness to light. The lines of S. ex- spinosus which were retained did not contain fewer non-reactive individuals during their early history, but the experiments were continued, and the results justify the conclusion that there was after all a sufficient basis for an experiment in selection. RELATION BETWEEN ENVIRONMENTAL CONDITIONS AND REPRODUCTIVE AGE. Reproductive age is very closely related to environmental con- ditions, principally to temperature. It is a matter of common obser- vation in the rearing of this material that the reproductive age is greatly affected by temperature and food conditions. During a local period of high temperature and warm nights in summer, a generation may be obtained in 6 to 7 days in S. exspinosus and in 5 to 6 or 7 days in Daphnia and even in 1 or 2 days in Moina, while during a period of continued cool days and nights in fall or spring, when the laboratory is not provided with heat, the reproductive age stretches out to 11 to 14 days for Simocephalus and Daphnia and 8 to 12 days for Moina. Curves were made for some of the lines, showing the mean re- productive ages for different two-month periods. At times they show remarkably close agreement in mean reproductive ages, not only between the two strains of the same line but also between the different lines of the same species and to a considerable degree between D. pulex and S. exspinosus. Figure IB (for Line 695) and Figure 17s (for Line 757) show two of these curves. STATISTICAL TREATMENT OF DATA. Irregularities in phototropic response are common. They are noted in almost every paper dealing with the light-reactions of ani- mals. Dependable results are generally obtained only by securing sufficient data to obtain averages from relatively large numbers. Like most other material, the Cladocera discussed in this paper showed considerable irregularities in light-reactions, and compari- sons of the two selected strains of the same line are made by con- sidering the data by longer periods, so as to avoid the extreme effect of local conditions and fluctuations in the behavior of individual broods. In the study of a character so complicated as a behavior charac- teristic it is essential to deal with relatively large numbers, and the safest way to deal with large series of data is by statistical treat- ment. Much time has been given to the statistical analysis of this data, although the treatment has been carried only so far as seemed A PHYSIOLOGICAL CHARACTER. 27 necessary in order to bring out the real significance of the material,1 and has been confined to the determination of standard deviations, probable errors, and a few correlations. Probable errors have been used in seeking to determine the significance of the reaction-time differences between the selected strains of the various lines. Many more probable errors have been determined (and included in the tables for the benefit of the reader) than have been referred to in the text discussions. On the other hand, probable errors have not been determined for many differences which were obviously too small to have statistical significance.2 For the benefit of anyone wishing to work over a portion of the data statistically, the sums of the individual reaction-times and the sums of the squares of the individual reaction-times are given in the tables which present the data for Lines 695 and 757 by broods. Hence, most of the statistical treatment for these two lines is capable of verification from the data presented in this paper. Likewise the data are available for any further statistical treatment. The standard deviations of the reaction- time means are large, whether the data is considered by shorter periods or by year periods. This is largely due to the over-time individuals, the data for which afforded considerable difficulty in statistical treatment. Other plans of handling the data for these individuals (other than assuming 900 seconds as their reaction-time) were considered, but they seemed to provide no better method of utilizing this very significant part of the data. 1 Such statistical treatment as has been accorded the data of this paper has been greatly facilitated by suggestions from Dr.'J. Arthur Harris, of the Station for Experimental Evolution. However, any errors or insufficient treatment must be ascribed to the writer, who has conducted the statistical analysis. 2 It is recognized, however, that the value of the probable error is not fully utilized when one fails to determine probable errors for all the differences in a series of data — for, obviously, a difference which is twice its probable error deserves more consideration than one which is only a fraction of its probable error, though a difference less than 2J^ times its probable error is considered of doubtful statistical validity. Nevertheless, to conserve the computer's time, it was felt advisable to omit the determination of probable errors for many of the differences which did not promise marked statistical value. 28 SELECTION IN CLADOCERA ON THE BASIS OF ANALYSIS OF DATA. LINE 695. PRESENTATION OF DATA. The data for Line 695 may be considered first. Line 695 is one of the original lines taken into the laboratory, the stock having been obtained from the spring-fed pond in the woods (Pond II), November 17, 1911. The selection was begun March 26, 1912. The stock had then descended 8 generations in the laboratory. By that time methods of rearing and handling the material had been worked out and the stock was well established. The manner of handling the young in making the selections has already been discussed. The selection data complete, summarized by broods, are given in the tables for the 695 plus and the 695 minus strains. Tables 3 and 4 contain in successive vertical columns, beginning at the left, the designation of the generation; the date of selection from within the brood; the hour of the day at which the experiment was conducted; the age of the young, whether a "new" brood or a brood released a day earlier (Sunday's broods and rarely others were tested when a day old); the number of individuals in the brood; the temperature of the water in the experimental tank at the time of the experiment; the number of individuals which went to the negative end of the tank; the number of individuals which failed to respond to the extent of reaching either end of the tank within the period allotted (15 minutes); the minimum reaction-time for an individual of each brood; the maximum reaction-time; the sum of the individual reaction-times for the brood; the mean reaction- time for the brood; and finally, the sum of the squares of the indi- vidual reaction-times (for computation purposes).1 Consultation of tables 3 and 4 shows that there are wide differ- ences in reactiveness of the various broods. These, in part at least, are due to the factors discussed on pages 17-20 and 128-149 of this paper. It is unfortunate, though not surprising, that these reaction differences are so large, but when all of the factors of the case are taken into account, it is believed that they are not unduly disturbing to the course of the experiment. Because of the relatively wide differences in reaction-time be- tween different broods and the*small numbers in many of the broods, 1 For the benefit of anyone wishing to go over the author's analysis or make further analysis, it was desired to present the complete data for all the lines discussed in this paper. But (in view of the present great cost of printing so many pages of tables) data are presented practically com- plete for Lines 695 and 757 only. It is quite impracticable to publish the individual reaction- times for all the individuals tested in making the selections, even for Lines 695 and 757. The sums of the squares of these reaction-times are given. This renders available all the data necessary for a critical analysis of the original data and should satisfy the most exacting reader, so far as Lines 695 and 757 are concerned. The detailed data for all the lines have been typed in tabular m, such as is used for the data for Lines 695 and 757, and are stored in duplicate at the Station Experimental Evolution, where they may be freely consulted by anyone interested. A PHYSIOLOGICAL CHARACTER. 29 TABLE 3. — Summary of selection data by broods for Line 695 plus. a 1 -j a o a a a CO a 3 8 a c CJ e 0> a o 0 o "•#3 o 1 a to •a o o a a o 0 a £ "1 1 a o '•+3 i a _0 i tn a > a B 3 0 "o 3 43 c8 0 . O..M a . bo m a J °l T3 a a "- 03 ll 1-3 e^ •3 o t4_, 0} O T3 a _ o oj S a £d £ IM O 05 Q> a 09 £ 0) 3 o «'g o 1 i . 08 O C2 a o M 0 ra QJ a o S 0) 03 «) S S O Q- wa ^ ** K h is * ^ CO m " S 3 *Ij 03 1912 H" Mar. 26 3 0 8 13.4°C. 0 0 90 295 1317 177 297039 I April 6 10 2 10 14.5 3 2 100 900 4850 b485 3108600 J April 16 12 6 9 18.2 0 0 130 390 2260 251 615700 J° April 23 4 0 5 16 0 1 360 900 3210 642 2261700 K May 11 5 Oor 1 4 18.5 0 0 165 525 1280 320 485350 L May 27 11 Oor 1 15 18.5 Selections made but records incomplete. M June 3 1 Oor 1 7 21 1 1 180 900 3445 492 2080425 N June 10 11 0 or 1 7 17 0 2 285 900 4420 631 3200350 O June 19 3 0 2 18.5 0 1 590 900 1490 745 1158100 P June 26 10 0 2 19.5 0 2 900 900 1800 900 1620000 Q July 3 3 0 1 19.5 0 0 260 260 260 260 67600 Q° July 5 3 0 2 21.7 0 0 430 730 1160 580 717800 R July 15 11 0 or 1 T 22 0 0 690 690 690 690 476100 S July 30 10 0 1 20 0 0 495 495 495 495 245025 s° Aug. 2 11 0 5 19.5 0 0 170 570 1880 376 841600 S« Aug. 5 3 0 or 1 2 18.7 0 0 280 310 590 295 174500 T Aug. 16 2 0 4 20.5 0 2 95 900 2060 515 1656250 U Aug. 28 1 0 1 19.5 0 0 195 195 195 195 38025 V" Sept. 9 2 Oor 1 1 20 0 0 140 140 140 140 19600 w Sept. 26 2 0 4 18 0 0 70 390 795 199 214625 X Oct. 7 1 Oor 1 3 15.7 0 0 192 296 696 232 167744 Y Oct. 18 2 0 2 18 0 0 180 315 495 248 131625 Z Oct. 29 1 0 3 17 0 0 88 210 458 153 77444 A2 Nov. 8 2 0 2 18 0 0 165 210 375 188 71325 B2 Nov. 20 1 0 2 16.4 0 0 244 375 619 310 200161 C2 Nov. 30 1 0 2 15.4 0 0 135 490 625 313 258325 D2 Dec. 12 1 0 8 16 0 0 90 235 1252 157 219504 E2 Dec. 20 1 0 9 18.8 0 0 50 270 1315 146 222825 F2 Dec. 30 11 0 or 1 16 16 0 0 80 455 3770 236 1169350 1913 G2 Jan. 6 11 0 or 1 8 16.4 0 0 80 570 2025 253 684375 H2 Jan. 20 1 0 or 1 11 15.8 0 0 70 625 3259 296 1292531 I2 Jan. 27 1 0 or 1 10 15.5 0 0 90 415 2140 214 566700 J,2 Feb. 3 2 Oor 1 7 14.6 0 0 150 275 1535 205 305675 Feb. 12 1 0 9 13 0 0 130 315 1895 211 432025 L2 Feb. 20 1 0 10 15.2 0 3 160 900 5180 518 3410200 M2 Feb. 28 2 0 11 16.2 0 0 160 530 3165 288 1100025 Nz Mar. 10 1 0 or 1 10 15 0 0 135 425 2600 260 764100 02 Mar. 19 2 0 3 13.1 0 0 110 260 500 167 96600 P2 Mar. 28 9 2 6 13.3 0 0 260 425 2155 359 797475 Q* April 4 2 0 7 20.5 0 0 110 215 1155 165 201275 R2 April 11 2 0 12 19 1 0 100 580 3065 255 1080975 S2 April 18 4 0 10 18.7 0 0 60 285 1680 168 334250 T2 April 25 3 0 11 18.4 0 0 125 450 2725 248 781175 U2 May 5 1 Oor 1 8 17.5 0 0 70 190 1010 126 141150 V2 May 12 1 Oor 1 4 14 0 0 100 230 635 159 112225 W2 May 20 2 0 7 14.5 0 0 135 450 1695 242 477225 Xt May 28 4 0 S 16 0 0 100 245 1380 173 254400 Ya June 5 1 0 2 18 0 0 110 125 235 118 27725 Z2 June 12 3 0 8 17.2 0 5 225 900 5265 658 4248225 A3 June 18 1 0 7 19.3 2 0 130 395 1410 201 341350 B3 June 27 10 0 5 20.5 0 0 80 170 630 126 85750 C3 July 3 2 0 4 22.3 1 0 140 870 1750 438 1134350 D3 July 10 3 2 2 22 0 0 190 400 590 295 196100 E3 July 19 2 0 2 22.3 0 0 520 575 1095 548 601025 F3 July 26 11 0 1 21.7 0 0 370 370 370 370 136900 F3« July 29 2 0 4 22.5 0 0 115 356 1010 253 285334 G3 Aug. 1 2 0 6 22.8 0 1 165 900 3255 543 2167925 H3 Aug. 8 3 0 13 21.9 0 0 90 750 4155 319 1699175 Is Aug. 14 1 0 4 o o 100 222 564 1 M.1 88638 J3 Aug. 20 2 0 7 20.4 0 0 153 543 2215 316 847305 Ks Aug. 26 3 0 10 20 1 0 75 497 2037 204 565827 L3 Sept. 3 10 1 4 22.5 1 0 80 215 603 151 100299 •This is the original brood from which selections were made for the beginning of both the plus and minus strains. b Mean reaction-times in italics are those for which there is a corresponding mean for a brood tested on the same day in the minus strain. 0 Selection was repeated from the later brood because of the loss of the earlier brood. 30 SELECTION IN CLADOCERA ON THE BASIS OF TABLE 3. — Summary of selection data by broods for Line 695 plus — Continued. B 1 A o 3 i o •*> B a .2 •§ a B 0 IM O B o M B 43 u fl 8 a I o OQ *•£ 1 B 0> •d "S O "fl3 «-^ '^3 .w *rt N o . .§ 8 a 0) **" S B B .S •M IM B O » cj o3 -3 B M V a f-g, 0 o 03 O '•S 0 a o o i i o B -.3 o Q) £ i . £J "" J3 gT3 11 °1 o M •5 ^ B s » g *o .5 0 g"s o .2 °3 •a 1 'S ° O B S || o c$ -— O M o d a> S o ^i o ^ i-H * *rH 2? 3 S) O Q~ w 4 Jan. 22 1 0 7 13.8 0 1 158 900 3486 498 2103652 Jan. 30 1 0 12 15.8 3 2 135 900 6451 538 4267309 F4 Feb. 5 2 0 7 14 1 0 183 617 2519 360 1025103 G4 Feb. 12 3 0 6 12.8 2 1 299 900 3347 558 2069435 Il4 Feb. 20 5 0 18 16.2 0 3 110 900 9963 554 6732879 Mar. 2 4 1 5 16.3 0 0 85 238 825 165 149961 J4 Mar. 9 5 0 12 18.3 1 0 53 250 1889 157 336795 K4 Mar. 21 3 0 8 17 0 6 219 900 5900 738 4986922 Li April 2 2 0 5 18.2 0 0 105 570 1345 269 495625 M4 April 9 2 0 9 18.5 0 2 120 900 3900 433 2334050 N4 April 16 3 0 10 19 0 2 85 900 3195 320 1906975 April 24 1 0 6 17 0 0 120 630 1455 243 539625 P4* May 1 2 0 12 19 4 2 183 900 6317 526 4182749 Q4 May 7 6 0 15 20.8 11 1 95 900 5536 369 3003036 May 14 1 0 1 21.2 0 0 297 297 297 297 88209 S4 May 20 4 0 9 21.2 3 1 135 900 3728 414 2125440 Ti May 27 4 0 8 19.9 0 7 645 900 6945 868 6086025 U4 June 3 2 0 5 18.6 1 0 220 850 2487 497 1484789 V4 June 27 1 0 3 21 1 1 130 900 1335 445 919925 W4 July 3 1 0 4 19 0 0 160 345 985 246 259825 July 10 1 0 6 21 0 0 170 735 3230 538 1925400 Y4* July 16 2 0 10 23 0 0 120 520 3020 302 1111700 Z4 July 22 5 0 7 21.1 0 0 75 604 2091 299 864455 As July 28 11 0 5 21 4 1 165 900 1885 377 1101575 Aug. 5 2 0 6 22 0 4 285 900 4355 726 3542125 cl Aug. 11 2 0 2 24.8 0 1 180 900 1080 540 842400 Ds Aug. 20 1 0 5 23 1 0 201 525 1633 327 592355 Es Aug. 26 7 0 8 21 0 2 125 900 3907 488 2562427 F6 Sept. 1 2 0 12 22 2 2 180 900 5780 482 3464606 Sept. 7 4 0 9 22 1 0 90 700 2737 304 1213419 Us Sept. 16 3 0 33 16.2 0 8 59 900 16589 503 11051527 Is Sept. 23 3 0 8 18.9 1 0 70 425 1398 175 389734 JD Sept. 30 2 0 14 20.1 3 1 178 900 5921 423 3114431 K6 Oct. 8 3 0 6 21.8 2 1 348 900 4046 674 2912988 Ls Oct. 15 1 0 15 20.1 2 0 70 745 4523 302 1951685 Ms Oct. 21 11 0 13 18.8 2 0 58 480 3054 235 917626 Oct. 28 4 0 15 17.5 4 2 125 900 6892 460 4112772 O« Nov. 4 6 0 14 18.2 2 1 82 900 3927 281 1759315 Ps Nov. 11 4 0 14 19.2 0 0 100 570 3050 218 904678 Q6 Nov. 18 4 0 15 17.3 6 0 115 875 4791 319 2206373 Its Nov. 26 2 0 14 18.5 9 0 100 840 5429 388 3134113 Ss Dec. 2 4 0 5 18.7 0 0 80 275 875 175 180975 Ts Dec. 8 3 0 8 16.7 1 0 56 263 1136 142 209724 Us Dec. 14 4 0 6 21 6 0 165 520 1943 324 740549 Vs Dec. 21 3 0 10 15.7 0 0 103 235 1388 139 205034 Ws Dec. 29 3 0 10 18.7 0 2 95 900 4178 418 2592880 1915 X5 Jan. 6 1 0 ' 9 19 0 1 90 900 2345 261 1088025 Ys Jan. 15 1 0 6 18.9 0 1 125 900 2725 454 1764225 Jan. 23 1 0 12 18.9 1 3 176 900 6136 511 4070576 A PHYSIOLOGICAL CHARACTER. 31 TABLE 3. — Summary of selection data by broods for Line 695 pliis — Continued. 0 1 "oj i o 43 c c a oj i •g V in O a O .2 '^ o d c .B g S i o o a) c o -d E o '3 -^ •£ •^3 '•5 CO

.2 a •S s G S 5 & 03 ^ ^ CQ 03 ^ co~ 1915 A6 Feb. 1 3 0 8 17.7°C. 0 0 192 465 2343 293 737297 Be Feb. 10 1 0 9 13.3 0 0 77 580 1648 183 492636 Ce Feb. 18 11 0 15 16.5 1 1 140 900 5746 383 2826876 D6 Feb. 26 1 0 12 19.2 0 0 58 267 1818 152 334294 Ee Mar. 6 6 0 10 19.3 3 0 118 633 3393 339 1397655 Fe Mar. 13 2 0 18 17 0 1 70 900 3387 188 1262947 G6 Mar. 22 4 0 13 17.6 1 2 140 DOC 7787 599 5350389 H6 April 1 3 0 8 20.8 1 1 72 900 2650 331 1381078 Ie April 8 9 0 10 18.5 5 2 417 900 6668 667 4665470 J« April 15 3 0 15 17.9 0 0 60 845 3485 232 1481175 K6 April 23 2 0 14 18 0 3 83 900 6573 470 4300045 L6 April 30 2 0 10 20.4 1 3 125 900 4271 427 2824692 Ms May 8 5 0 9 19.7 0 1 320 900 5634 626 3895706 Ne May 17 3 0 10 16.9 5 0 160 760 3300 330 1441050 Os May 26 3 0 5 17 1 1 205 900 2160 432 1251150 June 5 11 0 10 16.8 0 0 108 245 1523 152 248689 Q« June 12 3 0 13 19.4 1 1 82 900 3752 289 1877124 R« June 19 2 0 12 21.3 1 2 140 900 5340 445 3 195400 Ss June 28 3 0 4 18 1 2 245 900 2545 636 1930025 Te July 5 3 0 4 21 1 0 505 670 2401 600 1457641 Ue July 13 Random distribution. V« July 18 Random distribution. We July 27 Random distribution. V ,a Random distribution. Ye Aug. 16 5 1 13 23.5 8 3 90 900 5785 445 3766975 Z6 Aug. 26 Random distribution. A7 Sept. 6 5 0 2 20.8 0 0 180 335 515 258 144625 B7 Sept. 14 5 0 5 21.9 0 0 120 450 1160 232 336200 C7 Sept. 20 4 0 6 20.3 0 0 160 580 2070 390 823300 D7 Oct. 1 4 0 9 18.8 0 0 100 560 2150 239 713350 E7 Oct. 9 11 0 8 17 0 1 110 900 4080 610 2914000 F7 Oct. 20 5 0 12 18.1 0 0 70 855 2760 230 1322900 G7 Oct. 28 4 0 14 15.5 0 0 50 260 2030 145 347900 H7 Nov. 5 5 0 18 15 3 0 70 710 4610 256 1623150 I7 Nov. 15 4 1 18 15.9 1 1 60 900 4005 223 1855475 J7 Nov. 24 6 0 28 15.1 1 0 60 450 5665 202 1566775 K7 Dec. 2 4 0 17 14.5 0 0 50 760 4140 244 1597800 L7 Dec. 9 5 0 12 15.2 1 0 90 390 2160 180 508800 M7 Dec. 16 5 0 9 18.1 1 1 60 900 1935 215 954725 N7 Dec. 23 4 0 15 12.8 2 0 100 470 3720 248 1068550 1916 O7 Jan. 1 Random distribution. P7 Jan. 13 8 0 7 16 0 2 100 900 2875 411 1890875 Qv Jan. 23 8 1 7 18.2 4 0 75 180 820 117 103150 R7 Jan. 29 5 0 13 12 1 2 110 900 4060 312 2127250 S7 Feb. 7 6 0 15 14.7 2 0 60 635 3605 240 1236625 T7 Feb. 16 5 0 20 17 3 2 90 900 6205 310 2922075 U7 Feb. 23 3 0 9 11.4 0 0 100 570 2095 233 667125 V, Mar. 2 4 0 14 14.5 0 1 90 900 2795 200 1122575 W7 Mar. 9 5 0 9 17.3 0 2 145 900 3560 396 2175200 XT Mar. 18 7 1 12 Sel ctions made, but, lisrht current interrunted before experiment was completed Y7 Mar. 27 3 0 3 14.8 0 0 60 80 210 70 14900 Z7 April 8 3 0 13 14.1 5 0 95 690 3990 307 1701000 As April 19 3 0 17 15 0 4 90 900 6265 369 3947425 B8 May 1 4 1 6 15.7 1 0 80 430 1120 187 288000 Cs May 9 4 0 13 16.9 0 2 110 900 4560 351 2479600 Ds May 18 3 0 8 17.9 0 2 60 900 2620 328 1763400 E8 May 26 3 0 10 18.8 0 2 90 900 3535 354 2141575 F8 June 3 5 0 5 19.1 1 0 135 260 1065 213 235925 G» June 12 4 0 7 17.3 0 0 145 420 2185 312 735775 Hs June 20 Random distribution. Is July 8 3 1 8 20 3 4 270 900 4810 601 3607700 Js July 12 5 1 3 22.7 0 0 180 290 720 240 179000 K8 July 18 4 0 20 21.3 2 7 150 900 10600 530 7431800 Ls July 24 3 0 6 21.4 0 1 100 900 1820 303 1053000 Ms Aug. 1 2 1 14 20.9 0 9 250 900 9810 701 7912700 Ns Aug. 7 2 0 8 23.2 0 7 300 900 6600 825 5760000 08 Aug. 14 4 1 14 18.7 2 1 90 900 3670 262 1816506 Ps Aug. 19 2 0 8 20.3 0 4 100 900 4110 614 3308900 Qs Aug. 26 3 0 33 21 0 7 140 900 13320 404 8776400 Rs Sept. 2 2 0 12 20 2 4 2 160 900 4920 410 2854800 'All of the 695+ strain was lost except a substrain of 695+ which had been kept in a dark closet for 13 generations. Renewed from 695 Xs+ dark. 32 SELECTION IN CLADOCERA ON THE BASIS OF TABLE 4. — Summary of selection data by broods for Line 695 minus. a A "3 i o .3 a a ED a 5 41 *0 •*j a M a bo a 03 g i •a o C o o a 3-g 1 '•« •£ CO 2 43 a ** 01 o ft ht _ a -22 •*•* a o o • a £ o 03 a o ^ d .2 IN P. H a 0 «J >. a G 3 0 3 43 03 o! . bOjj ~o c a • •-* jj §« ** OT a> u >~c m g -*» § "o J "o O) •o.a IM O V . o! 3 -o .5 § !"§ '3 ^ 0 "a o a •M O m" V 01 5 i> cs J3 o M " 6 a a £** 05 -3 .a 09 •3 03 ,_ 3 O o U C3 a o a '•? 00 0) a 0> I f.2 O -*J a S O OS =3 ' > a o 1 a 1 a c S a 1 ° pj 0) U CO 1 a o o£ S ° T . c o o S e •a I '« E 03 4) |i E a 03 || o ° W J? >. I* S* ^2 « « • S 03 *" 1913 RJ Dec. 1 Random distribution. S, Dec. 8 11 0 5 15.5°C 0 0 188 380 1468 294 454376 T3 Dec. 16 1 0 5 17.5 0 0 210 430 1615 323 553075 U, Dec. 28 5 0 8 11.8 0 1 310 900 3756 470 2012560 1914 V, Jan. 4 10 1 8 12 0 2 280 900 4120 515 2628850 W, Jan. 12 1 0 8 11.8 0 0 223 665 3220 403 1452092 Xi Jan. 20 4 0 8 14.9 0 1 205 900 3894 487 2379668 Y> Jan. 28 2 0 3 13.4 0 1 339 900 1609 536 1061821 Z» Feb. 3 3 0 11 15.2 1 2 230 900 6082 553 4014612 A« Feb. 11 2 0 8 14.7 0 2 210 900 4843 605 3443159 B4 Feb. 19 1 0 14 14.9 1 0 102 740 4167 298 1651857 C4 Feb. 28 4 0 5 16.5 0 0 153 363 1405 281 426847 D4 Mar. 7 2 0 8 17.5 0 1 152 900 2963 370 1602127 E4 Mar. 16 3 0 2 17.3 0 0 152 221 373 187 71945 F4 Mar. 24 3 0 8 16 2 2 128 900 3512 439 2160278 G4 Mar. 31 3 0 4 16.3 0 2 430 900 3020 755 2429000 H4 April 8 1 0 4 18.6 0 0 145 340 915 229 233125 Ii April 15 5 0 8 19.7 0 0 120 820 3340 418 1937200 J4 May 2 3 0 1 19.7 1 0 490 490 490 490 240100 K4 May 8 4 0 11 19 6 1 88 900 3603 328 1704519 L4 May 14 2 0 8 21.2 0 0 100 360 1656 207 410010 M4 May 22 2 0 9 18.7 4 1 97 900 4224 469 2663016 N4 May 29 11 1 7 22 1 4 490 900 5608 801 4634574 04 June 6 2 0 2 17.9 0 0 535 610 1145 573 658325 P4 June 23 11 0 9 19.4 3 3 65 900 4325 481 2957225 Q4 June 29 1 1 3 19.3 2 1 555 900 2051 684 1473241 R4 July 8 2 0 7 19.6 1 1 305 900 3550 507 2022850 S4 July 14 2 0 6 21.6 0 0 115 340 1366 228 341958 T4 July 23 2 0 1 21.2 1 0 141 141 141 141 19881 U4 July 30 1 0 1 19.2 0 1 900 900 900 900 810000 V4 Aug. 8 3 0 9 22.1 0 2 190 900 5016 557 3356106 W4 Aug. 15 2 0 22 22 6 5 100 900 10052 457 6548080 X4 Aug. 22 5 0 2 22.9 0 1 630 900 1530 765 1206900 Y4 Aug. 31 2 0 8 21.1 4 2 123 900 3123 390 1939427 Zi Sept. 7 5 0 16 21.8 2 0 100 800 6383 399 3386309 As Sept. 17 11 0 6 16.3 0 0 85 395 1111 185 266303 Bs Sept. 23 3 0 11 19 8 1 100 900 3372 307 1670024 Cs Sept. 30 2 0 8 20.1 2 0 80 400 1701 SI 3 468945 Ds Oct. 7 5 0 6 20.1 3 0 60 370 1020 170 247950 E« Oct. 13 3 0 15 19 2 1 74 900 6135 409 3623985 F5 Oct. 20 2 0 14 19.9 0 2 131 900 5108 365 2750448 Gs Oct. 26 11 0 13 17.3 0 0 90 550 3285 253 1063923 Hs Nov. 2 3 0 12 16.2 4 0 80 590 3020 252 1119750 Is Nov. 9 5 0 7 16.7 2 0 150 620 2125 304 886375 Je Nov. 16 4 1 14 19 5 0 70 670 3123 223 1104069 K. Nov. 23 3 0 8 17.3 4 0 135 680 2005 251 722733 Ls Nov. 30 5 1 34 19 8 0 60 552 8346 246 2771592 Ms Dec. 5 1 0 5 18.4 0 0 60 188 613 123 85569 Ns Deo. 11 4 0 5 16 1 2 145 900 2435 487 1766693 Os Dec. 19 2 0 12 16 4 1 100 900 4350 363 2196550 Ps Dec. 26 2 0 11 12.5 0 0 72 280 2307 210 522109 1915 Qs Jan. 4 3 1 20 19.3 1 0 91 735 5364 268 1914546 Rs Jan. 11 3 0 14 20.7 3 1 205 900 6180 441 3273950 Ss Jan. 19 3 0 7 17.2 0 0 65 330 1140 163 252150 Ts Feb. 1 3 1 11 17.5 3 0 100 630 3439 313 1296451 Us Feb. 10 1 0 10 13.3 0 0 85 445 2340 334 678550 V6 Feb. 18 10 0 12 16.6 2 2 110 900 3977 331 2184369 Ws Feb. 24 3 0 10 20.2 1 0 105 338 1942 194 439790 Xs Mar. 5 3 0 13 16.3 0 0 55 395 2415 186 621425 Y6 Mar. 12 1 0 12 16 0 1 70 900 3052 254 1429104 Zs Mar. 20 4 0 8 18.8 0 1 92 900 2078 260 1068210 A, Mar. 30 1 0 8 16.5 0 0 90 516 1962 245 717912 Bj April 6 7 0 9 15 0 0 112 323 2092 232 529828 Cs April 13 11 0 10 17.5 2 0 150 515 3309 331 1258685 D, April 20 3 0 11 16 7 0 110 855 3373 307 1759685 E« April 28 2 0 8 19.3 1 1 90 900 2924 366 1754176 F6 May 6 1 0 13 17.9 0 0 75 261 1968 151 330808 34 SELECTION IN CLADOCERA ON THE BASIS OF TABLE 4. — Summary of selection data by broods for Line 695 minus — Continued. c ]c A o 3 a o •jS a c B CO •a i S M M CD m o *•£ 'fe O a ~ a 8 ^£ £ o CD 0 03 £ £ ^ .1 -+-> CO 1 0 LH at a, M 0 03 CD UN .2-2 .2 I a 0 i CO .S CD CD M "o CD J2 ^ C3 **- "^ r* .1 c o £ +> 0? a c "3 3 O T3 o a a O 1 CO CD *3 'S bO fl M CD .S ^ fl CD CD o c o a B o i"! 5, H CD C ••* g-g 13 3 o 3 43 "S . bfl cc CD ~- c a 2 CO g ^ ad a IH o d D* CO '•§ "B.S *S ^ c «*H t4 CD . fi § *"• 03 *O "^ 2 "5 £ "= £ *c °l O IH "B m o .5 o £- -C .S o *n o o C Q) .2 CD 03 X 3 o d £ C ffl « 6-~ O 0) B 0 S % x o TO1 ft) 00 O S £ 3 '-5 o w < ft £ * 25 u S ™ S M CO " S 02 1915 Gc May 13 4 0 11 17.8°C. 3 0 70 436 2083 189 563453 H6 May 22 1 0 8 17.3 0 1 90 900 2060 258 1114000 la May 31 3 0 6 15.3 0 0 246 642 2169 362 894821 Je June 8 3 0 10 18.8 0 0 220 792 4304 430 2259638 Ke June 15 2 0 11 20.6 7 0 171 395 3033 276 895395 Ls June 21 3 0 6 19.7 0 2 290 900 3355 559 2293575 Me June 30 2 0 6 19.8 1 2 330 900 3984 664 2920518 Ne July 7 2 0 3 21.9 0 0 620 665 1937 646 1251729 Oe July 14 Random distribution. P. July 20 Random distribution. Oe" July 28 Random distribution. P« Aug. 7 Random distribution. II Q» Aug. 13 Random distribution. Aug. 19 Random distribution. s66 Aug. 30 Random distribution. T6 Sept. 7 5 0 7 22.5 1 3 150 900 4670 667 3614500 Ue Sept. 13 4 0 7 20.9 0 0 150 630 2000 286 740650 V, Sept. 20 4 1 20 20.2 0 6 130 900 12635 632 9572475 We Sept. 29 5 0 11 18.2 1 2 125 900 5660 515 3778100 Xe Oct. 9 12 1 11 16.5 3 0 70 450 2070 1S8 514750 Ye Oct. 18 5 0 12 18.4 1 0 90 690 3445 287 132.3225 Z« Oct. 26 5 0 18 16.5 3 0 70 720 4835 269 2095125 A7 Nov. 2 4 0 12 14.9 0 0 60 220 1400 117 192650 B7 Nov. 9 4 0 16 14.8 0 0 90 435 3200 200 751700 C7 Nov. 17 4 0 11 12.6 2 1 100 900 3220 293 1479650 D7 Nov. 24 4 0 17 14.6 0 0 65 240 2140 1SS6 322050 E7 Dec. 3 5 0 12 12.3 2 0 75 510 2120 177 526650 F7 Dec. 10 5 0 11 12.1 0 0 65 405 1575 143 323025 G7 Dec. 17 4 0 21 20.2 6 1 80 900 7580 361 3569200 H7 Dec. 25 5 1 9 11.9 5 1 120 900 2820 313 1456550 1916 I7 Jan. 4 5 0 10 14.5 0 10 900 900 9000 900 8100000 J7 Jan. 13 8 1 8 18 0 1 80 900 2280 285 1279400 KT Jan. 23 3 1 9 19 1 0 105 600 2115 235 673725 LT Jan. 31 9 1 11 13.1 1 4 130 900 5870 534 4265500 M7 Feb. 7 4 1 17 15 0 1 50 900 3865 227 1580075 N7 Feb. 17 8 0 32 18 0 1 50 900 5230 163 1562800 07 Feb. 25 2 0 12 13.3 0 0 80 330 2065 172 445075 P7 Mar. 4 4 0 9 12.1 0 0 90 310 1705 189 373125 Q7 Mar. 13 3 1 11 18 6 0 90 750 3845 350 1912925 R7 Mar. 20 5 1 9 15 1 1 50 900 1865 207 1000125 S7 Mar. 27 4 1 10 15.2 2 0 170 840 5105 611 3218875 T7 Apr. 8 2 0 9 14 0 0 80 540 1840 204 624550 U7 Apr. 17 9 0 15 15.1 0 2 75 900 5145 343 2648775 V7 Apr. 29 4 0 5 15.2 0 2 180 900 2465 493 1771525 W7 May 9 4 1 13 16.7 3 0 70 500 3180 £45 1041000 X7 May 17 3 0 10 16.8 0 2 70 900 2800 280 1783150 Y7 June 5 5 0 5 19 0 0 145 720 2150 430 1141050 Z7 June 13 5 0 6 17.8 1 0 100 600 1390 232 493050 As June 20 4 0 13 17.8 1 0 75 420 2020 155 412700 Bs June 30 4 0 12 18.8 0 0 80 360 2010 168 403300 C9 Julv 7 6 0 7 19.5 0 3 180 900 3970 567 2889700 D8 July 14 3 1 6 20.7 2 0 135 470 1655 276 521025 E8 July 21 3 0 5 21.1 0 4 270 900 3870 774 3312900 F8 Aug. 3 Random distribution. G8 Aug. 14 5 1 26 19 0 0 110 460 4100 158 869400 H8 Aug. 19 12 0 7 19.8 0 0 120 360 1495 SIS 360025 Is Aug. 26 2 0 21 21 0 1 80 900 8540 407 4770700 Js Sept. 1 2 0 11 19 2 0 160 760 4420 402 2220000 •Strain lost July 22 and renewed from substrain of 695 - (695 M6 -) which had been kept in a dark closet for 37 generations. A PHYSIOLOGICAL CHARACTER. 35 a better treatment of the data is secured by considering it by unit groups larger than a single brood. Convenient units of comparison are month and two-month periods, and summaries of the data by such periods will be referred to rather than the data by broods. Tables 5 and 6 give, in successive columns, the two-month periods; the generations of descent during the periods; the number of broods used in selection; the total number of young tested; the average number of individuals per brood; the average age of the mothers at the time their first broods were produced; the average number of young per day of the mother's age at the time of produc- tion of the first brood — the reproductive index; the total number of individuals moving to the negative end of the tank; the number of individuals failing to reach either end of the tank during the time of the test (15 minutes); the average minimum reaction-time for the different broods; the average maximum reaction-time; the sum of the individual reaction-times (for computation purposes) ; the mean TABLE 5. — Selection data summarized by two-month periods for Line 695 plus. h V a •+•> 1 OJ 0 — >> ol -Q i o a o o i o a ^3 M ^* 3 M M *.* _^ rl _£j V m DO c fe aj ^ cp w a a a •3 * 0 a 0 o] a •+J g 1 o o j£ -3 '> .S o 03 cJ *" h £ BO M •M o cj 0> 8 on M a „ S ^3 "o 05 _2j •S a a .§ t Q a 0) 3 . •a *«-. a ^ "« ^ 3 ^ *? 0} •M •O O o "S 8 O "5 «4_l *3 3^ a a c 3 "M O Time period. *o OQ o w o 0) ,J3 0 ° 6 b <» .£ ">"a M a a C3 ^o 1 'I o a O o "o 2 M 03 "S ^ j2 tic Oi . M 'I: T3 ^ a a "a -o y o o "e! M J3 6 Z 05 *. tf-g M -d 03 o O §J S U *^ c o 1 2 CO S ^ ° ^ . cS O OJ s a > ".£ « a a J.| a si 43 0 0 X H <>a ^ «; ^ 2; " ^ ^ CO S^ CO £ Apr.-May 1912... 9-12 4 28 9.5 11.8 0.81 3 3 189 679 11600 414 244.04 31.11 June-July 1912.. . 13-19 8 23 3.0 8.7 .34 1 6 479 722 13760 598 240.92 33.88 Aug.-Sept. 1912.. 20-23 6 17 2.0 13.7 .15 0 2 158 418 5660 333 249.77 40.86 Oct.-Nov. 1912. . . 24-29 6 14 2.3 11.2 .21 0 0 167 316 3268 233 101.40 18.28 Dec. 1912-Jan. 1913 g 62 10.3 9.0 1. 14 o 0 77 428 13761 222 Feb.-Mar. 1913. 36—42 7 56 8.0 o 3 158 447 16930 302 Apr.-May 1913. 43—50 g 67 8.4 7.8 1^08 1 0 100 331 13345 199 June-July 1913. 51-58 g 35 3^9 7.2 .54 3 5 209 462 12355 353 Aug.-Sept. 1913... 59_g7 9 74 7.2 6.7 1.07 2 3 114 548 20737 280 Oct.-Nov. 1913. 68-75 g 66 8^4 75 1. 12 2 1 133 474 16019 243 Dec. 1913-Jan. 1914 76-83 7 50 7 .0 7.9 .89 4 7 253 822 25253 505 Feb.— Mar. 1914. 84—89 g 56 9.3 8.3 1.12 4 10 158 634 24443 436 Apr.— May 1914... 90—98 g 75 8.2 7 1 1 15 18 15 198 766 32718 436 June-July 1914.. . 99-105 7 40 6^3 .97 6 2 149 693 15033 376 224 . 64 23.96 Aug.-Sept. 1914. . 106-114 c 97 8.2 6.6 1 24 8 18 152 783 43400 447 Oct.-Nov. 1914 115—122 f 106 12^5 7^5 1.67 27 4 125 776 35712 337 Dec. 1914— Jan. 1915 c 66 8 4 7 1 1 18 8 7 111 612 20726 314 Feb.-Mar. 1915. . . . 131-138 7 85 n'e S'.3 l'.40 5 4 114 664 26122 307 Apr.-May 1915... 139-145 8 81 10.0 8.6 1.16 13 11 180 876 34741 429 279 '77 20^97 June-July 1915... 146-153 C 43 10.5 7.9 1.33 4 5 216 723 15561 362 267 . 64 27.53 Aug.-Sept. 1915... 154-160 4 26 6.6 9.2 .72 8 3 138 566 9530 367 246.41 32.60 Oct.-Nov. 1915. 161—166 •-• 107 17 7 8 4 2. 11 5 2 74 662 25300 236 Dec. 1915- Jan. 1916 167-174 7 80 12'.4 8.5 1.46 9 5 84 643 19710 246 206.09 15.54 Feb.— Mar. 1916. 175—18] L 70 10. c 7.9 1.30 5 5 91 664 18570 265 Apr.— May 1916... 182-18^ 67 11 .7 10 4 1 13 g 10 88 787 2209( 330 June-July 1916... 188-195 i 49 7^2 1.25 6 12 163 612 21200 433 288.31 27.78 Aug.-Sept. 1916... 196-200 ( 89 9.7 6.6 1.47 6 30 173 900 42430 477 338.61 24.21 • Including all the data available, some of which is additional to that for the broods tested. bAll available data is here used, including that for the No. 2 mothers whose broods were not ordinarily tested for use in selection. 36 SELECTION IN CLADOCERA ON THE BASIS OF individual reaction-time; the standard deviation of this mean; and its probable error. For the minus strain, table 6 gives three additional columns : the difference between the means for the plus and the minus strains; the probable error of this difference; and the quotient of this difference divided by the probable error. These tables are divided into sections, each containing the data for a period of a year, except the data obtained previous to August TABLE 6. — Selection data summarized by two-month periods for Line 695 minus. Time period. Generations of descent. > •4-1 O o S3 "3 49 o H 1 U! a o >> •o o £ 0 a. 11 1- j-. 1 « 4 £ft B . o m ^ .5 K ^ J 0) (4 00 -•— • a a ( , d Q a d •a T) 3 03 *«-. 0) IM CJ T3 OJ +? o *& ^ "fll 3 0 Q a 3 '•£ GJ o O T3 Time period. Strain. "o CO •a 6 "o fl bfl ^ C ^ •pn 'd 03 1 s IH ju m £ ^ d o o Z TJ O ^3 Cfl S,_2 • •-* ^ g a T> •S OJ « a B a .2 0 (O O ^ a el 2 d d « a • 2 •d |H OJ 0 '3 d ' i ° 03 M w ^H **H _< M . M . 03 3 S d 3 qj H *o c3 •" »— ( ^ *o 2 o ^ ^ QJ 03 o d °3 03 •H 0 03 M OJ OJ d 6 « fe c3 "* o-g. 6 Z3 d =3 «_a fe a M d OJ .^ d 03 | Q} .^ •§ ta ,2 O 55 03 "3 § 0 g 03 O S 3 T3 Time period. Strain. tn XI > .^ "O •p 0 o .> •O S 03 T3 0 0 03 . M w & S T3 c "D o 01 a Q O> C B S •^ 03 o 0) O) fi C _0) fi ° M-0 03 o **-< o 3 "8 "2 "s-S M a aj to . d OJ 03 •O S3 & a> 03 01 fc (- 0 fc o . C3 w S 03 g c o •§ ta ^J n O b V- 0 ^^ 55 Z ^ " ""*' ^ *" " 02 £ W K Q* Anr fi 1Q15> Plus 9 8 5 17 3 4 193 900 545 3 July 31 1912 a 7 5 IB 5 360 900 ess o - 88 Aug 1 1912 1 1 4 3 47 2 7 173 455 332 9 July 31 1913 1 i 5 6 6S / £0f SSS 7 ^ ( PlllQ Q 7 1 64 . 3 138 527 290 6 Tnlv ^1 1Q14 n K 1 73 / / 230 608 Qf)& Q — /^ Ana- 1 1Q14 ( Pliiq K 10 5 63 Q 2 125 662 314 2 + 2 July 31 1915 g // 3 68 17 3 679 311 9 A no- 1 1 Q1 £\— f P111C in 19 n 120 15 g 99 622 °84 0 223 11 13 74 July 31, 1916 Aug. 1, 1916- 1 Minus. . . ( Plus .... 70 S /4.6 18.3 .Uff 55 2 5 12 110 §77 900 277.1 383.6 £50.74 324 . 97 14.00 29.56 + 122 19.61 35.10 O.S5 3.47 IK n e i Q j 103 573 ^^/ O 206 33 ^S 9^ Figure 2c shows in diagrammatic form the mean reaction-times of the plus and minus strains of Line 695 by two-month periods. The distances from the axis of ordinates represent two-month periods of the selection data. The ordinates indicate the average reaction- times in seconds for the two-month periods. The solid line represents the course of the averages for the plus strain, the broken line for the minus strain. The roman numerals indicate the times at which the test series were conducted, and the plus and minus signs within the diagram indicate the positions of the means for these test series. ANALYSIS OF DATA FOR EFFECT OF SELECTION. On the whole, there is a marked coincidence of the fluctuations of the two strains, the means for the plus and minus strains in a general way following the same upward and downward trends. This A PHYSIOLOGICAL CHARACTER. 39 is obviously due to environmental factors. The data for the first 4 months previous to August 1, 1912, may be passed over with slight consideration, for (as stated above) the methods of handling the material were not sufficiently standardized at first. These first 2 two-month1 periods show rather widely different averages (see tables 6 and 7 and figure 2c), but from this time on less violent fluctuations in average reaction-times are encountered, except for very wide fluctuations in the average of the minus strain during the latter half of 1915, and an unusual upshoot in the plus strain during the last 4 months of the experiment.2 For the year, August 1, 1912- July 31, 1913, the mean reaction- time for the plus strain was 260.2 seconds (251 individual reaction- time records; see table 7) and for the minus strain 297.5 seconds (217 individual reaction-time records). The minus-strain mean was — 37.3 seconds greater than that for the plus strain. The difference was 3.2 times the statistical probable error (table 7). This large difference is due, of course, to the difference obtaining for the first 6 months of this year-period. This six-month period is discussed later. In the latter half of the year there was not a consistency in reaction-time differences, though on the average the plus was slightly the more reactive. The "same-day" broods for the year-period have a mean reaction-time for the minus strain only 0.8 second greater than for the plus strain (table 8). For the year, August 1913-July 1914, the curves show that the two strains differ little in mean reaction-time (plus mean 371.8 seconds with 361 individual reaction-times; minus mean 371.3 seconds with 304 individual reaction-times; see table 7) and in relative vigor (see figure 2s). For the entire year the mean reaction-times differ by only -f-0,5 second and the same-day broods by only —12.3 seconds (table 8). Two test series were conducted during this period. One in August 1913 consisted of 529 individuals in the plus strain and 467 individuals in the minus strain. The mean reaction- times were 522.2 seconds and 490.5 seconds for the plus and minus strains respectively, a difference of +31.7 seconds ±12.14 seconds. This difference is 2.61 times the probable error. The second test series, May 1914, consisted of 1,083 and 1,088 individuals and the means were 501 and 562.8 seconds. The difference in seconds was -61. 8 ±8. 88, a difference 6.96 times the probable error. The results of these two test series are contradictory and judgment will be reserved until the later data are considered. 1 The summaries of the data for the earlier portion of the experiments were made to include the first 4 months. Summaries thereafter were made covering year periods. Summaries for the lines later subjected to selection were made to conform to the summary periods already utilized for the older lines. 2 Actually this is only a trifle more than 3 months, as the selection was discontinued Septem- ber 2, 1916. 40 SELECTION IN CLADOCERA ON THE BASIS OF The year, August 1914-July 1915, gives as an average for the plus strain 368.7 seconds (478 individual records), 60 seconds greater than that for the minus strain (475 individual records), a difference 5.62 times the statistical probable error. For the first 10 months the plus strain consistently showed a higher reaction-time. For the 6 same-day broods the plus strain had a higher reaction-time by 2.3 seconds. A test series (May 1915, table 7) showed a higher reaction- time for the minus strain, but the difference was only —1.1 ±6. 73. Another test series (June 1915) gave a higher reaction-time by 51.1 ±14. 08 seconds for the plus strain. This difference was 3.63 times the probable error. During the remainder of the experiment with Line 695 (August 1915-September 1916), the minus strain fluctuated rather widely in mean reaction- time (figure 2c). For the year ending July 31, 1916 (399 and 439 individual reaction-time records), the minus strain had the higher reaction-time by 15.6 ±11. 77 seconds. The same-day broods gave a difference of —6. 9 ±19. 61 seconds. The remaining short period of the experiment, a trifle more than a month, shows a considerably higher reaction-time (higher by 150.8 seconds) for the plus strain. If the experiment with Line 695 had been discontinued in January 1913, one might have felt inclined to ascribe a possible effect of selection to the strains of this line (see figure 2c). But it is extremely improbable that such an effect really should be thus ascribed for that period and that the selective effect once obtained was later lost in some manner. If one were to interpret this as an effect of selection acquired through a mutation or acquired in some other manner, it seems quite difficult to account for its loss. The selections were not relaxed and there was no period of high mortality among the stock to account for the loss of a selective difference. In two portions of the experiment the plus strain consistently had a higher reaction-time for 8 months or more at a time. Considering the curves as a whole, there are three periods in which one or the other strain had a higher reaction-time for as long as 6 months at a time. In two of these periods the plus strain had the higher re- action-time and in one of the minus had the higher reaction-time A further examination of these data will be made in order to seek the explanation. RELATION BETWEEN REPRODUCTIVE VIGOR AND REACTIVENESS TO LIGHT. It would seem plausible to suppose that relatively slight differ- ences in the general vigor of the two strains might be a factor in determining the reaction-time, particularly in such periods as those just referred to. The rate of descent of a strain may be considered one measure of its general vigor. Figure 2A presents the relative rates of descent of the two strains of Line 695; the base-line represents equality of A PHYSIOLOGICAL CHARACTER. 41 descent, i. e., if mothers of the same generation of descent in the two strains produce their young on the same day the circle will fall upon the base-line with the figure 0 adjoining. If young from the same generation of descent appeared 13 days later in the plus than in the minus strain, the solid circle will fall on the base-line with a figure 13 placed above the line, while if the minus strain (as in the case of the seventh entry in this diagram) were 4 generations and a day behind the plus stram in producing young of a given generation, this is indicated by the open circle being placed 4 units below the base-line, with a figure 1 adjoining. By comparing the relative amounts of descent of the two strains from the end of one two-month period to the end of the following two-month period, one can determine whether the plus or the minus strain had during that interval descended the more rapidly, i. e., which strain was presumably the more vigorous of the two during that limited period. For example, from the end of May 1912 (first entry in this diagram) to the end of July 1912 (second two-month period and second entry in the diagram), the minus strain from having been 1 generation and 2 days in advance of the plus strainr had become 13 days ahead of the plus strain in its time of producing young of the same generation of descent. During the following two- month period (the third) the minus strain became just 1 generation (third entry in the diagram) behind the plus strain. Still one period later (the fourth) it was 1 generation and 7 days in arrears of the plus strain. Comparing rate of descent with mean reaction-time, the minus strain during the second two-month period descended the less rapidly, but -was the more reactive of the two strains. During the third period the minus strain descended the less of the two strains and was the less reactive. During the fourth period the minus strain again descended the less rapidly and was the less reactive. While the difference in reaction-time is frequently in the direction expected if influenced or determined by the relative rates of descent of the two strains, such is not true more often than the reverse is true. For the 27 two-month periods of selection with Line 695, the more rapidly descending of the two strains had the lower reaction-time in 10 cases, the higher reaction-time in 13 cases; for 3 two-month periods their rates of descent were identical; and for 1 period their mean reaction-times were identical. That is, the more vigorous strain, judged by its rate of descent, was the less reactive during 13 of the two-month periods, and in fewer cases (10) the more vigorous strain was the more reactive one. Hence, judged by this measure alone, the more vigorous strain was the more reactive strain less often than it was the less reactive strain. In a further endeavor to learn if relative vigor is a factor in influencing reaction-times, all of our data for Line 695 which seemed 42 SELECTION IN CLADOCERA ON THE BASIS OF to bear on the general vigor of the stock were tabulated. In con- sidering this point three criteria of the general vigor of the material were used, the age of the mother at the time her first brood was produced, the number of young in her first brood, and the interval between the first and second broods. From a general knowledge of the material and use in other connections of these measures of the vigor of the material, the writer can state that these are satisfactory criteria. Figure IA shows graphically the average number of young in the first brood for Line 695 by the two-month periods of the experiment. Figure IB shows the average age of mother for the Bame periods. 1912 1913 1914 1915 1916 4-5 8-9 12-1 4-5 8-9 12-1 4-5 8-9 12-1 4-5 8-9 12-1 4-5 8-9 10 00 t ! -,2.00 4-5 8-9 12-1 4-5 8-9 12-1 4-5 8-9 12-1 4-5 8-9 - 12-1 4-5 8-9 - 1.50 00 FIGURE 1. — Line 695. Reproductive vigor. A. Average number in the first brood by two-month periods. The vertical scaling above the base-line indicates the number of young; the horizontal scaling, the two-month periods of the experiment. Solid circles show positions for the plus strain; open circles for the minus strain. B. Average age of mothers at time first brood was produced. Vertical scaling below the base-line indicates the number of days. C. Actual values of reproductive indices (average number of young in first brood divided by average age of mother at time first brood was produced). Vertical scaling indicates the numerical values of the reproductive index. Each numerical value of the reproductive index (in C) is obtained by dividing the corresponding numerical value in A by the corresponding numerical value in B. A PHYSIOLOGICAL CHARACTER. 43 .3 O O O ° 7 ' ,°0 oo 4- ? 3 2 o o ,°o 6- 44 oo oooo oooooo 22O4985 9020 10 10 81- 0 O O — I 0 4-5 8-9 12-1 4-5 8-9 I2-| 4-5 8-9 12-1 4-5 8-9 12-1 4-5 8-9 1912 1913 1914 1915 1916 i ..til . B JLJJL f . T T . 7 I T T .50 .25 00 .25 .50 10 r >o - )0 4-5 8-9 1912 12-1 4-5 8-9 1913 12-1 4-5 8-9 1914 12-1 m rv 4-5 8-9 1915 12-1 4-5 8-9 1916 FIGURE 2. — Line 695. A. Relative rates of descent of the two strains. The base-line represents an equality of descent, i. e., if both strains produced young of the same generation of descent on the same day the circle would fall on the base-line with a figure "0" placed adjacent. Positions above the base-line (solid circles) indicate that the minus strain in a particular generation was lagging by as many generations as the point falls in units below the base-line and by as many additional days as are indicated by the adjacent numeral. The horizontal scaling indicates the two-month periods of the experiment. B. Reproductive indices plotted to show which strain is superior in vigor (and by how much it is superior) in the different two-month periods in experiment. Solid circles indicate that the plus strain is superior by the amount indicated by the distance the solid circle lies above the base- line; conversely, open circles below the base-line indicate the amount of superiority of the minus strain. The horizontal scale indicates, as always, the two-month periods of the experiment. C. Reaction-time curves. The plus strain in solid, the minus strain in broken line. The vertical scale indicates seconds of reaction-time, the horizontal scale two-month periods of the experiment. The points in the curves represent mean individual reaction-times by two-month periods. The roman numerals indicate times at which "test series" were conducted, and the plus and minus signs the positions for the test series means for the two strains. It seemed desirable to combine these measures so as to write the result in a single quantitative term. This was in part accomplished by dividing the mean number of young in the first brood by the mean age of the mothers at the time the first broods were produced. This gives the average number of young per day of the mother's age and may be called the reproductive index.1 This reproductive index is 1 Unfortunately this did not include data for the interval between the first and second broods. This, however, was the least important of the three available measures of vigor, and the data for it was less extensive than for the other two measures. 44 SELECTION IN CLADOCERA ON THE BASIS OF given in the seventh vertical column of the tables of data summarized by two-month periods (tables 5 and 6). The reproductive indices for the two strains are plotted in figure Ic, in which the actual values are represented by the amount of elevation above the base-line of the solid and open circles. The solid circles represent the reproductive indices for the plus strain, the open circles for the minus strain. The distance between the solid circle and the open circle for any period represents the difference in reproductive index, the solid circle or the open circle being elevated the more above the base-line as the plus or the minus strain has the larger reproductive index. In figure 2s the differences between the reproductive indices for the plus and minus strains are shown, a plus advantage being shown and its amount indicated by the eleva- tion of the solid circle above the base-line and a minus superiority by the amount of the depression of the open circle below the base-line. Throughout the first of the three periods of the curve for Line 695, during which there was a consistent difference between the mean reaction-times of the two strains for as great a period as 6 months (August 1912-January 1913), the plus strain had the lower reaction-time and was slightly, though only very slightly, the more vigorous. But during the succeeding four months (February-May 1913; see figures 2s and 2c) the plus strain had a considerably greater advantage over the minus strain in vigor and yet the mean reaction-times of the two strains were very near together. The fairly wide and rather consistent difference between the mean re- action-times of the plus and minus strains for 6 months during 1912- 13 is not accounted for on this basis. For the ten-month period (August-September 1914 to April- May 1915) during which time the plus strain continuously had a higher reaction-time than the minus strain, the plus strain was slightly the less vigorous of the two during 6 of the 10 months; but during the other 4 months the reverse was true, and for the 10 months as a whole the reproductive vigor of the two strains was practically the same. For the seven-month period at the close of the experi- ment, during which the plus strain had a considerably higher re- action-time (was less reactive) than the minus strain, the plus strain, judged by the measures applied to it, was actually considerably the more vigorous of the two strains. Hence, of the three periods of the curve during which the two strains differed consistently in reaction-time for as much as 6 months at a time, only one of these differences, that for the second period, and only a portion of it, could be accounted for on the basis of the more vigorous strain having been the more reactive. Comparing by two-month periods for the entire duration of the experiment, one finds that the strain of Line 695 which (on the basis of its reproductive index) was the more vigorous had the lower re- A PHYSIOLOGICAL CHARACTER. 45 action-time during 14 of these periods, while in 12 two-month periods the more vigorous strain had the higher reaction-time. For one period the average reaction-times were equal. Compared by periods of a single month, the strain having the greater vigor had a lower reaction-time in 23 cases and a higher reaction-time in 26 cases. For one month the vigor of the two strains measured the same, for one month the reaction-time was the same, and for three months the data were incomplete and a comparison could not be made. Hence in the selection experiment with Line 695 no relation is discovered between fluctuations in general vigor and mean re- action-time, either when the data are examined month by month or by longer periods during which there was a constant difference in reaction-time. Whether the measure of vigor is taken as the rate of descent or as the reproductive index, any reaction-time differ- ences which one might have expected to find explicable on the basis of differences in vigor between the two strains quite fail to appear so. The test series may again be referred to. Three of the four of these gave differences in mean reaction-time which are statistically significant;1 for one the difference was practically nil. Of the three differences of statistical significance, two were differences in which, in opposition to selection, the plus strain had the higher reaction- time and only one of the differences was in the direction of selection. Hence their net effect is to indicate a lack of effect of selection. Examined as a whole and on the points considered in detail, it is obvious that with Line 695 there was no effect of selection. This is by no means surprising, for if an effect of selection is obtainable with a given material it is not to be expected that such effect will necessarily be secured every time it is attempted. Selection can- not be effective unless germinal variation occurs which affects the character used as a basis for selection; and there seems no logical reason for supposing that germinal variation occurs in all, or to the same extent, or at the same time, in all material which is otherwise apparently entirely similar. It would be superfluous to make such an extended analysis of the data for each of the lines as has been attempted for Line 695. Tables of data and diagrams similar to those for Line 695 are given for all the lines (though some of these lines have their data presented somewhat less fully), but it has not seemed necessary to make the treatment nearly so complete. The tabulated summaries and the curves plotting reaction-times and the reproductive indices render the interpretation rather obvious after the full treatment accorded Line 695. Of the other lines, the data for Line 757 alone are examined rather extensively because of the interesting result within that line. 1 The times at which these test series were conducted are indicated by the roman numerals on figure 2c. 46 SELECTION IN CLADOCERA ON THE BASIS OF LINE 689. Line 689 was obtained from the surface-water pond in an open field (Pond I) and reared in the laboratory for 8 generations before being subjected to selection. Selection was continued 16 months, 44 generations in the plus strain and 42 generations in the minus strain, when the experiment was terminated by the accidental loss of the plus strain. The data are presented after the plan followed with the data for Line 695 in tables 9, 10, 11, and 12, and in figure 3, A, B, and c. For the first 4 months of selection the mean reaction-time for the plus strain was 498.4 seconds and for the minus strains 464.2 seconds. The difference was +34. 2 ±41.4 seconds (table 11). For the year (nearly), from August 1, 1912, to the close of the ex- TABLE 9. — Selection data summarized by tivo-month periods for Line 689 plus. 1 -*J 1 c3 O (H ,2 >> 03 J2 .i o 49 1 o o 1 O d •73 1 M d £ a OJ G) 3 M C .S "a V 03 '••3 o 03 -*- o 03 a d a QJ 1 o •*^ P 0 £ ^^Js 03 •*- tD V M 0 B V c MH O 03 0 to s. •S;s| O S a .1 ). t a a j . T3 C .^ "3 ,»« 3 3 I cfl o IM Time period. T3 "o ^ 0 o 13 2 0 -o ^H 0 0 * o "^ .6 .a 'R a o 1 ca O o § O !£ o £ _c *o d d fe f_, 4d tp_2 S 2 .S c 1 03 a o c 03 ^ a •a •o _4) ^H tt »Tl hp *d to ® E» M 3 14-1 _£3 fcfl to . "o o3 15 M c ,_ c3 X rt o

. a O OJ IJ >l a 3 S.| fl 03 I O * H ^ <^-p'° 21 £ " ^^ ^^ 03 s*3 W fi Apr.-Mav 1912. 10-14 5 28 5.3 11. i 0.48 3 7 244 557 14090 503 June— July 1912... 15-19 g 26 5 2 9.0 .58 1 5 297 664 12825 493 Aug.-Sept. 1912. . . 20-23 15 2^3 0 3 315 464 5007 334 291 . 14 50.70 Oct.-Nov. 1912. . . 24-26 3 13 5.5 18> !29 1 1 226 566 4302 331 190.45 35.63 Dec. 1912— Jan. 1913 27-33 7 60 8.6 9.0 .96 o 4 112 531 19043 317 Feb.-Mar. 1913 34-40 g 69 8 3 8 3 1.00 1 3 144 476 20838 302 Apr.-May 1913..!. 41-47 7 60 7^8 7^9 2 0 117 436 13468 224 June-July 1913... . 48-52 5 42 7.2 8.2 '.88 3 1 187 688 13775 328 221.12 23.01 TABLE 10. — Selection data summarized by two-month periods for Line 689 minus. o *» i ^ >' I G o i 1 C i a rj g G) OJ a, (H M o o O cd **H J2 •a CO ^* ^rt ^ bO be -- •^ "-3 •^3 o 03 a o M fe o> ^ •§ 2 - Js § o 03 08 o 03 fl * 2 a a c? o3 3 O o ^ ^;> J o *2 o V a 00 S M *O 03 O > o ra M s . .3 "3 "o O co •£ a a a __ C a a -g •3 CJ 73 3 O t*H O •o "o S "o "o <- o 3 O c 3 43 c 3 o c6 "o GJ "-1 GJ W Es 3 IM O a> T3 Time period. "S tT.' w o o c S T3 M •53 .2 H2 'S s « ^ 0 •? TO a T; G IM O d 03 *> on Q? Z ^ 03 '-+-> bD tn > fi 1 03 a o c3 ••3 £ lH ja B ft y ^ o ^£ I* JO d a> £ m CD C C — o3 G) 03 FH •9 "2 _OJ a c _«j 1^ j^i bC ^^ i-j-r tf1 1£3 ^* G d ,[ f bO to **j a 15 G) rt 3 o t IH *S — " ^ C3 fll S c5 S «*H »^-J O ^^ O C Ci •a 03 L- CO t- O 13 . 03 fe 2 083 (U «- g . '> . 03 fe| o a a C3 C QJ S a C3 •8 to ^ o — O o O o a a u k| ^^ -^ O z H *" ^ < z^5 z *• < "3 ~d ^ d M £ c o IS . 03 ° 6 negatively m als. individuals f an end of th( e minimum e maximum individual rd deviation a IM O b O C o Qce between -times. •3 1*4 O s g JS ace divided b •or. "3 M *"* A« P ., , M . u 03 3 Q a 3 a. C y G CJ -^ — - ^ *o *^ ^ o M " S 0 d o •o d >- o 03 m ^ & a d >H ^ f "S . '> . 03 O Q l.§ S| d •§ Q) i r+ •g to £ a Z <0 H t5 £ * ^^ ^ s £ £ Q* * Q* Apr.- July 31, fPlus.. j 9-19 10 5.4 54 4 12 271 611 498.4 260.7 23.9 +34.2 41.4 0.83 1912 1 Q— 17 Q a P po j X „ .„ fiftfi ,o, a OfJQ J> S3. 7 Aug. 1, 1912- [ J\j. 1TLU8 , , fPlus.... o 1 i 20-52 (7 36 7.2 259 7 o 12 174 514 295.1 »OC7. 4 194.8 8.2 +40.0 10.5 3.71 July 16, 1913 I Minus . . 18-60 35 6.4 223 5 S zse 352 265.1 ^5.2 6.6 TABLE 12. — Same-day broods. Summary of data for Line 689. M o s'g £ V i w (_ gg 3 O 03 cj o M **H a s | a •-2 C3 S Jj d IM "cl 3 --a 2 * _a _a 3 1 « o fU O CD T3 M , Time period. Strain. -a o 0 M £< o <" 2 •3 d S]| ^ cfl C3 '^ Q) 5* •g M4S 'a .a '•*•* « 2 al m i 1| 0> U o C3 d g " fl •-on) bD C3 Si d d CD "o c3 •fl "o o "^ "o *" *j oJ ^O 03 0 o 2 ° d d o O d *X t* ^ 0) O •- a * * 2- »' "* < * § * Q H Apr. 9, 1912-July 31, f Plus . . 1 5.0 5 0 3 315 900 712 + 163 1912 \ Minus 1 4.0 0 0 •4-40 690 649 AUK. 1, 1912-July 31, [Plus. . J 10 8.9 89 4 2 125 464 295 + 3 1913 1 Hf ' 1 f\ SVA f f 9 i //?*y 0O€> ( Minus 10 7.2 7% J lol 40 / XftfiS periment with this line in July 1913, the mean for the plus strain was 295.1 seconds and for the minus 255.1 seconds. The difference was +40.0 ±10. 5 seconds, 3.71 times its probable error. There was only one same-day brood1 for the four-month period, but there were ten same-day broods during the later year-period. For these same-day broods likewise the reaction-time for the minus strain was lower than for the plus strain, but the difference was only 3 seconds (table 12). It is an interesting fact that in this line, in opposition to selec- tion, the minus strain persistently (except for 1 two-month period, see table 10) maintained a lower reaction-time than the plus strain. This difference, while not large for the greater part, is so general that it can scarcely seem to be merely coincidental. The same-day broods (table 12) show the minus broods with a smaller reaction- time, but for the only significant group of these the difference is so slight (3 seconds) as scarcely to be suggestive. 1For this pair of very small broods the mean of the plus strain was 163 seconds greater than that for the minus strain. 48 SELECTION IN CLADOCERA ON THE BASIS OF 1.00 - 50- 00 'I 1 A 8 J 1 i V .50 .25 00 .25 4-5 ^ 1912 8-9 T 12-1 B T i 4-5 1913 t \ 1 t I o A There were more over-time individuals in the plus strain, 24 as compared with 7 in the minus strain. The average minimum and maximum reaction-times likewise indicate greater reactiveness in the minus strain for the last 12 of the total 16 months of the experi- ment. The average minimum reaction-time for all the broods tested in the plus strain for this last year was 174 seconds (table 11), while the corresponding average for the minus strain was 136 seconds, indicating that the most reactive individuals in the minus strain had a lower reaction-time by 38 seconds than the corresponding individuals in the plus strain. A corresponding comparison for the maximum re- action-times for the two strains gives 514 seconds as the average maximum for the plus and 392 seconds as the average for the minus strain, the difference (122 seconds) indicating that the individuals with the highest reaction-times in the plus strain were 31 per cent slower in reacting than the corre- sponding individuals in the minus strains. These portions of data tend to confirm a greater re- activeness for the minus strain than for the plus strain. Comparison of the curves for the two strains of Line 689 may be made with the combined mean reaction-time curves for all the plus and for all the minus strains of those Daphnia pulex lines be- tween the two strains of which thprp WPrpnn PonsisfpntHiffprpTiPpc; P«sed curves (in faint lines) representing the ' WG.re /6S combined reaction-time means for the plus and in reactiveneSS Lines 691, 695, minus strains of all Daphniapulex lines in which m71/« -.,-1 TC1 fn, C.™ O~\ significant reaction-time differences did not , 714, and 751 (see figure 3c). ariSe-Lmes eoi, 695, 713, TH, and 751. Inspection of these curves shows that for 3 two-month periods (August 1912-January 1913) the plus strain of Line 689 was abnormally high, while the minus strain of Line 689 was abnormally low in reaction-time, and that aside from these 3 two-month periods the two strains of Line 689 did not differ so greatly in their reactiveness. It would seem possible that the pronounced divergence in reactiveness between the two strains of this line for the period indicated may have been due to the coincidence of unusual reactiveness of the minus strain and slight reactiveness of the plus strain brought about by differential 4-50 300 150 00 8-9 12-1 4-5 1912 1913 FIGURE 3. — Line 689. A. Reproductive indices, actual values. B. Reproductive indices, superiority. C. Reaction-time curves with superim- A PHYSIOLOGICAL CHARACTER. 49 environmental factors (see page 140). However, in 4 of the 5 other two-month periods of the experiment, the minus strain was the more reactive (the combined mean was 42 seconds greater for the plus strain), and it is possible that there really existed a genetic differ- ence between the two strains of this line. Perhaps the most that can be said regarding Line 689 is that the persistently lower reaction-time in the minus strain is suggestive of a genetic difference in the reverse of selection. It is much to be regretted that this line could not have been continued longer or that large test series were not conducted in order to secure better evidence concerning the real persistence of this apparent difference in reaction- time. With regard to any relation between mean reproductive vigor and mean reaction-time in Line 689, it may be said that no relation is discoverable. On the whole, the plus strain was somewhat the more vigorous (see figure 3, A and B) while the minus strain was the more reactive. Further, there is an entire lack of coincidence of lowered reaction-time means with higher reproductive indices. The influence of general environmental factors is seen in the somewhat coincident up-and-down movements of the curves for the two strains. LINE 691. The data for Line 691 are presented in tables 13 and 14 and in figure 4. This is one of the original lines of D. pulex. Selection was begun in the seventh generation and continued for 87 and 89 genera- tions in the two strains. The experiment was terminated by the loss of the plus strain after 27 months of selection. For the first 4 months of the experiment the mean reaction- times for the plus and minus strains (only 40 and 35 individuals respectively) were 480.9 and 434.9 seconds (table 13). The plus TABLE 13. — Selection summary for Line 691. 00 13 . J, rj .i o a o 1 O i B O d 03 M oi B J "O E . *S "en ff3 C !>-* c3 03 tf oi pj O a Oi d •3 -2 * "^ 0 '3 ^ O ED (LI I«H Q 03 a s x 9 o CO o "* M _ "\ u 't* 'QJ I "o J Q a fl s o •M •0 V "S o ^ *c3 *t < 3 "ol ^2 IM cu 0 V T3 O^g '•3 m ^« 3 0 a a 3 *^ O •a Time period. Strain. **-• o ED a en 1 o d "o 55-0 0 tt 'Ji fl "o '2. . a .iS ti en '> C •3 " minir maxil •3 p 03 O S § 0 CD "^ a S 1 o ^ .2 ^ o M h Itf O g a-s !-•— i & 3 09 O 00 — & a 3 "3 d _o S «•£ u IM o "O CJ Time period. Strain. IK *o a •O "3 T3 ® .1 '« 3 i *o o « 00 fe s •s.a S !° 73 o G9 a IH {J *o § X "i o! Mjn =3 « 'e S "a •a c s *^ C3 QJ 3 o> .• ,0 0> _o O g o c " 03 o ff> •- b — d o _o a £ M)T3 M . M . 03 fQ . 03 O 1> Cj Q_) rt 4) fl GJ g s C 03 1 Ql TO "H- 03 0 fe « ia « « 3 0 .1 1 M B O 3 _"S "o Q a _o -a 0 *o d M ai . c3 O Q) |.§ fc S S B •§ o * H fc a M ^^ a a 0> 0 02 to c o a 3 "" a O a CD '3 .2 •g g 11 o 'p a) a '8 o 03 . .!i c 1 g 0 08 'd OJ h O * a a =3 o d r_ — a a g T3 «"2 O *"* '•5 .0 >v- a S •" a d) QJ ^ a 3 a 2 03 ^ 3 tfl M 0 C 5 7 O ^"^ O o T3 03 ^* m 03 * <° a 0 O.p . 03 S fi S F a a (S 3 •§ V O d £ 0 H' ** 55 *• 3* 3 02 M 5* M Q3 Apr-May 1912... 7-12 6 25 1 14 458 900 17440 698 265.71 35.84 -244 52.27 4.66 June-July 1912 13-18 6 24 1 S 308 643 11381 474 + 63 Aug.-Sept. 1912... 18-19 3 5 2 0 348 572 2385 477 171.16 51.63 -303 52.89 5.33 Oct-Nov 1912 20-25 7 17 3 0 229 335 5019 295 -33 Dec 1912-Jan 1913 26-31 6 •11 1 2 113 380 12484 304 —28 Feb -Mar 1913 32-38 7 •n 2 0 123 418 12550 292 —21 Apr -May 1913 39-45 7 •Id 1 0 131 341 9515 238 —27 June-July 1913. .. 46-51 6 44 10 2 132 573 12275 279 201.87 20.53 -90 23.23 3.87 TABLE 17. — Selection summary for Line 711. OD ± i 3 a a a 2 k CQ Q Q Q H D JQ 3 TJ a) 03 O o •^ m DO '»-* S a fl g 43 •r* ^> > en m C o3.2 '-+3 5 § 773 •** 03 d a Q 03 O a d 03 01 i a g • •3 -2 5o C3 ^ GJ c *t R "o o3 O ta g S ^j .9 o .2 QJ t* 'Tt S M ^2 ** g S a a a fl) ^ T3 T3 CO •g ja •** *o O T3 Time period. Strain. «4-t O 3 o !S -*-> 00 1 0 ^> d "o ^T3 O o 0 a to ^ 9 1 ^ V > '^3 o3 . M m §"3 "O .9 G 1 a; •a 03 a o T3 ^3 .9 c3 I •73 "H S g o 5 . o m JO 0 « § g^ g o j» 1 M Genera u- 0 6 Z-8 !* "oi "" 3 o2 1* . d O QJ c3 QJ t- ^- 4= 60 03 m s a ^••3 a -" 11 a-3 03 •a 03 -*J 03 Probab Differei action Probab §s W^ S-0 ( Plus .... 6-17 12 4.3 52 3 18 322 727 488.7 327.7 30.7 Apr. 1912- July 31, 1912 I Minus . . 6-18 ;« 4-i -49 * SB sre 760 688. £ 315.1 SO. 4 -99.6 43.1 8.31 1 Plns i»_!;0 33 7. 1 235 5 4 129 397 241.2 163.3 7.2 Aug. 1. 1912- 1 I 1UD * • * . J.O »Jv July 31, 1913 ( Minus . . 18-51 S61 5.3 iSO 19 4 164 419 £86.9 193.8 9.6 -45.7 11.9 3.84 TABLE 18. — Same-day broods. Summary of data for Line 711. H a tn d "3 * i H a i * 3 O 3 >>*3 a a & .3 m M-l O 03 3 11 '£ §1 3 I . 03 g "3 J| 1 ;p || fl M (L 1 S 3 _9 ^d 2 Time period. Strain. O 55 "§ -3 o O ^» "^ a| v» i 0 •3 § OJ 03 JO C. O a f3 "^ OJ ^ fl ,y a 2 U "S M O ti a 60-2 *s 03 ^3 "o *o .S a 03 _O 'i "« c5 cfl 6 S3 53 6 i* o *c3 a j •" .^ Q P |g •J 03 a> 3 a Mar. 26, 1912-July 31, f Plus.. 3 4.0 12 0 7 477 900 680 +42 1912 1 fLf • ^ 4 w >j Q f- ay [ Minus 3 S. 7 JI i 6 ooS uzv 6aa r pius . g 8.3 50 i 6 128 398 224 Aug. 1, 1912-July 31, 1913.. _ (. Minus 6 5.5 33 6 0 132 348 £31 —7 A PHYSIOLOGICAL CHARACTER. 53 This is one of the original lines of D. pulex obtained from Pond II, the spring-fed pond in the woods, during November 1911. Se- lection was begun in the sixth laboratory generation and continued for 15 months, for 45 generations in the plus and 46 generations in the minus strains. For the first 4 months of the experiment with this line the means were, for the plus strain 488.7 seconds (52 individuals) and for the minus strain 588.2 seconds (49 individuals). The difference ( — 99. 5 ±43.1 seconds) was 2.31 times the probable error (table 17). The mean reaction-time for the plus strain for the first two-month period was 244 seconds less than that for the minus strain, but for the second two-month period the minus strain had the lower reaction- time by 63 seconds (tables 15 and 16). There were 3 same-day broods, for which those of the plus strain had a higher mean reaction-time by 42 seconds (table 18) . The data for this first 4 months of selection are not as satisfactory as that obtained later and wide fluctuations are less sur- prising. For the remaining year (nearly) of the experiment with this line, the mean for the plus strain was 241.2 seconds (235 individuals) ; that for the minus strain was 286.9 seconds (190 individuals). The same-day broods, of which there were 6, have means differing by only — 7 seconds. Exam- ination of the data (tables 15 and 16) and the curves (figure 5) for this line shows consistent differences in mean reaction-times during this year-period. The wide divergence in mean reaction-times for August-September is a chance result due to there being extremely few tested individuals during this period. But for the following 5 two-month periods the numbers of tested individuals were greater and the rather uniform courses of the curves are sugges- tive of a difference due to selection. Comparison of the curves for the means of the two strains of this line with the combined mean reaction-time curves for Lines 691, 695, 713, 714, and 751 (in which genetic differences presumably did not arise) shows (figure 5) that the plus strain of Line 711 after the first 4 months of the experiment was somewhat more reactive than the other plus strains. The means for the minus strain in general were higher than the combined means for the other minus 750 600 450 300 150 00 8-9 12-1 A-5 1912 1913 FIGURE 5. — Line 711. Reaction-time curves with superim- posed curves representing the combined reaction-time means for the plus and minus strains of all Daphnia pulex lines in which significant reaction-time differ- ences did not arise. 54 SELECTION IN CLADOCERA ON THE BASIS OF strains. Hence this comparison affords further evidence of an effect of selection in Line 711. An effect of selection within Line 711 is not certainly established, however, in view of the lack of sufficient confirmation from the same- day-brood data. But these data were not very numerous and it is TABLE 19. — Selection summary for Line 713. fj ^ h o 1 i a i a 4 OJ S3 i . . O o o 03 J4 CJ XI 3 -a •a oj "*^ en ,2 c M a S E '•+•» O CJ -t^ o 03 v> o 03 o a a a 03 a OJ OJ 0 O. •^ ^; S 03 .2 o ~ -*^ OJ fJ •M 03 2 ta OJ CJ "3 o •o •*> .2 « 2 1 03 "m^S >— a M G OJ a -3 XI Time period. Strain. (D OJ •M 0 ED -^ °J =3 s C3 fcC ** c "a! 3 "o 1 *s ximur "o3 3 •g .2 03 0 g £ O CO IM O S T> OJ CO a o 0 0 XI O O 55 -0 O OJ 0 O j»M . oi 43 03 . tt ai si > C :3 OJ ^C • ^ o3 'S OJ 03 S •3 OJ •o "2 OJ OJ .D CJ og flV OJ ^ OJ ^ "3 M fc XH 3 ., , M . M . 03 3 t a ^ (*l OJ ftl "o a) X! ^— « "S3 o-g OS oj 0) 1 a cj •o S3 S3 C. OJ a OJ 6 4> fc 03 O CJ i.i i.i 8.§ a 03 •§ ta '•§ •8 te^ O "jG H ** fc* ** ^ S •4* CO £ 3 * £ 3-° Apr. 1912- July 31, 1912 ( Plus .... ( Minus . . 9-18 10-19 10 12 5.4 5.7 54 67 3 15 *4 232 644 476.7 453.7 314.5 277.4 28.9 24.0 + 17.0 37.6 .45 Aug 1, 1912- i Plus . . . 19-53 37 6.4 236 3 5 139 401 244.6 163.0 7.2 July 31. 1913 I Minus . 20-54 36 5.6 203 7 733 255.4 755.5 8.9 -40.8 77.5 3.55 Aug. 1, 1913- ( Plus . . . 54-96 42 9.1 383 28 72 202 667 431.3 287.7 9.9 +27.0 13.9 1.94 July 31, 1914 I Minus . 55-93 40 8.2 327 20 A2 774 732 404.3 267.0 3.7 Test series, ( Plus . . . 80 15 17.9 268 31 12 113 708 265.9 217.1 8.9 + 18.6 11.6 1.58 Mar. 1914.. . ( Minus . 77 Ifi 18.2 273 32 6 720 6%9 247.3 181 .6 7.4 Feb. 1914- ( Plus . . . 76-84 9 8.3 75 4 15 149 758 425.3 Apr. 31, 1914 I Minus 73-83 7.7 77 4 9 245 722 457.6 -32.3 Aug. 1, 1914- t Plus . . . 97-136 40 11.9 477 43 39 145 633 306.0 241.3 7.5 July 7. 1915 I Minus . 94-139 46 72.0 550 85 43 349.6 236.9 6.5 -43.6 70.7 4.32 Tc st s 6 r i 6 s . 1 Plus 125 9 20 0 180 31 19 91 778 342.9 247.2 12.4 Mar. 1915... I Minus . 725 9 19.8 775 796 354.4 267.7 73.2 -47.5 18. S 2.25 Test series, ( Plus . . . 126 32 39.6 1266 66 161 96 817 372.7 265.2 5.0 + 1.0 6.9 .14 Mar. 1915. . . I Minus . 726 S2 39.6 7265 247 1 1 / 95 567 377.7 245 4 4.7 Feb. K 1, 1915- i Plus 120-130 11 14.3 157 19 17 104 678 335.8 + 19.3 Apr. 30, 1915 119-130 72 77.5 138 70 602 376.5 TABLE 20. — Same-day broods. Summary of data for Line 713. M OJ i S i £ a c g i S3 D. •3 .2 o o 03 •M o XI I BO C £3 c "-I3 o 03 %^> o c3 %_> v 03 OJ c3 ® C N O M O CJ C3 1 « £ 2 *o S3 OJ I'g 0) ta >. "3 .2 CJ CO "^ a £ 3 — a o a CM *M "2 ^t C3 I—I 3 *S 5 3 v^ O c> en O 3 Time period. Strain. (0 XI 3 T3 _> "S c 'x T) c3 o o - — S 13 a o3 0 h S fi "3 o 0 3 -3 M_ro C C 1 a •3 13 c3 -w g OJ i a d d 03 CD to o O Q. fl 0 '3 *° £ £ 3 •g 03 a *s >, 0 ~ 0 a FT7* 03 ^1 a a a o a 0 to T3 T) •C I o °5 .^H ^ "3 "§•0 3 | 3 _a B ff4 .55 "o I 0 O Time period. Strain. O _x 6 "o G bO .S !S ° 'd U o o tn 0 .B to fl C Q "o 2 'S . tfl CO 1 a ^ 73 B V 1 1 « • .2 ,£! 0 . oi s a S3 a 8| d ig '-^ XI O CD O C ^^ n"2 I13 |£ <* $'* t» 1 5* Q3 Si— 91 1 '^ 4 12 257 695 495 4 256.9 23.4 July 31, 1912 I Minus . . o ^X 8-21 JLO 77 6.5 77 777 618.9 272.6 21 .8 -123.5 32.0 S.S6 Aug. 1, 1912- j Plus .... 22-55 36 6^1 220 10 9 164 440 309.2 209.8 9.5 + 30.7 12.4 2.48 Tiilv "31 1 Q1 ^ <2>4> £ / 35 6.5 229 / o 753 446 278.5 175.0 7.9 uuiy ox, i . ' 1 1 > ' 7W mus , , { PlllO cc inn 11 6 9 31 39 197 *f'*f-1-' 658 406.4 268.4 10.3 July 31, 1914 J JrlUS .... I Minus . , OO XUU 55-707 t^ 7-4 349 SO 270 702 70.3 ^_ ffld) C 14.5 2 26 Aug. 1, 1914- | Plus .... 101-144 41 9.3 383 39 52 169 724 378^1 268^4 9.2 + is's 12.3 1^28 Tiilv "?1 1Q1 ^ 7/irfj < //v / e 70 3 465 79 47 770 697 362 3 259 4 8 1 v uiy 01, i.' i.' Test series, * jW 171 US . . j Plus .... 112 16 15^6 249 39 t1 8 105 738 344^8 222.9 9.5 + 13.0 12.9 1.01 Ont 1Q1.1 114 /fi 7 K 1 289 47 18 95 734 337 5 218.1 8 7 V-/L-L., xy x^t . . . Sept. 1, 1914- ( Plus .... 105-116 1 O 12 i o . i 10.4 125 *r* 17 13 167 t <-"^ 739 397 !l 258.9 15.6 + 95.2 18.9 5.03 Nn v ^ 1 1 Q 1 4 im—i 1 R 72 77 5 138 25 £> 707 609 307 9 185.2 70 6 \JV . OX, Xt7XT± Test series, * TVi mus . . j Plus .... i\J i 1 i O 113 35 30 '.1 1055 244 86 108 841 383.1 240.1 4.98 + 18.9 7.72 2.46 'M/^V 1 Q 1 A 7 7 % O/T 31 1 1089 277 80 96 o /y 364.2 243.5 4 98 X'lUv. xyxrr . , . Oct. 1 1914— ' M. inus . . L L if 109-121 13 11.2 145 24 9 130 O^f-t 691 352.5 *T * *'** DPP *?1 1Q14. j j. , . • • • • 111 —1 23 77 9 755 36 0 596 380.2 — 27.7 X_/cU. OX, 1 .' I I Test series, i yw 1 7i us . . ( Plus .... 129 36 3l'2 1123 242 195 96 804 421.9 287.3 5.78 + 25.3 8.0 3.16 A/Tar 1 Of £ 1 Qi 'c 37 .5 1133 193 181 93 832 396.6 275.3 5.52 ivx<*r.f iy xo . . "PVH i 1 Q 1 ^ ' Ai mus . . t T^lnc lot 10 10.2 102 g 9 101 711 317.5 f tJU. X , i • ' \ tj — Anr *}O 1 Ql ^ 1 i Ills .... 1 27—1 38 72 13 .2 159 20 415.8 — 95.3 xi. jji . o*J, xy x u Antr 1 1Q1 ^ ' W mUS . . 145—185 37 11 .5 427 36 41 107 662 301.1 250.6 8.2 il. Ug . X , 1 • ' 1 O — July 31, 1916 t Minus . . 148-195 39 S5 35 728 303.5 222 7 7.2 - 2.4 70.9 .SS Aug. 1, 1916- ( Plus 186-194 7 9^4 66 3 16 146 762 393.5 227.1 18.9 + 63.8 32.4 1.97 Sept.16, 1916 t Minus . . 196-202 7 7.6 53 S 5 S7 562 329.7 254-4 26.4 TABLE 22. — Same-day broods. Summary of data for Line 714- 1 i c o fl .2 _o i o fi hi _o d o k to e JN " 03 03 '^3 o 03 V a fi • fi . 03 » o fi fl 0 S3 a o 1 **~* d) M H M "o 03 3 fl •'•3 to ^ CD to ~fl a a fi o a LH fl -g •3 T3 "o •i O "* i! 3 o ^ a 3 03 3 03 (M 0 S ai 5 3 "o •§ Time period. Strain. QQ "C 6 1 > o I 03 ^ o * a 0 1 o Q £; •S "o3 . T3 w a a H3 •o M . MT3 MH ^ 3 — ' 03 S . to U) 03 . cj O tD i a S3 a g.a a 03 .Q 0 iSl Jl o &v •** n <-° Z"0 fc " ^ •" §~ £ £ Q D £ QJ A nr 1 ^ 1 Q 1 2— ( PlltQ o 2 "i 5 o 1 190 528 438 261 04 78 74 xljll. 1-1, l.'l.- July 31. 1912 1 Jt AUI9 • • . • t Minus . . £i 2 fl mV 6.6 73 7 7 290 790 669 274.24 57.30 -237 93.98 2.45 Aug. 1, 1912- ( Plus 3 6.0 18 1 4 340 667 429 289.06 17.05 + 226 27.62 8.18 Tiilv *%1 1 Ql ^ 0 2. 7 g o o 735 22S 203 97.72 27.73 U 1 1 l_> OX, l>M'i Aug 1, 1913- Pl*nu ' ' 5 3 32 3 4 275 600 292 250 . 26 29.84 July 31, 1914 t Minus . . 5.8 35 0 2SS 675 397 269.48 30.72 -105 42.82 2.45 Aiitr 1 1Q14— 1 ^ Q Q 129 19 5 117 657 292 203 . 96 12 12 llUJ... X, X>7X7t July 31. 1915 ( Minus . . 73 1 !>, t o i i i V o3 O a a •d a bo £ a is- M a d d o o o o _o o 03 09 13 .rt 2 — 03 ~ •»- 03 a e3 £ d ~a 4- 3 o "o fe i-i o 05 <2 0> H £ tn 0) £ MH O 03 d '3 •a cd o •£ 1 m a .2 0 o o 6 to ° *-s «o os o u ° "§ 03 c; 03 c. C 0 -o 03 i C3 *E S E 3 u . < £ o5 > - |> . S3 O ^ « E S S S 03 C o .£ a 03 •8 o o O H < * < *3 ^ _£ O. 03 fc fc " <* ^- 3 CO §- £ A »-ij> — TVTwv 1 Q 1 9 9-13 c 27 6.8 9.6 0.71 0 16 214 534 15738 583 rvjjr. iviay Ai7j.i0i . • Tim**— Tnlv 1 Q1 2 14-19 o R 51 11^4 4 3 240 635 19370 380 j iiiit^ j u i,y itJiAf . . Aitrr — Rpr^f 1 Ql 9 AT: i is 20-21 \J O 3 1.5 11.5 .13 0 0 288 295 825 275 - i uj,, i '' j M iyi^i. • . Oct.— Nov. 1912. 21-24 40 4 16 2^3 12^8 0 0 220 448 4138 259 Dec 1912— Jan 1913 25-31 7 67 10.2 9! 1 l! 12 2 6 115 476 18737 280 Feb.'-Mar. 1913... 32-38 7 51 7.3 8.5 1 5 147 437 16685 327 224.17 21.17 A r\r — fyT ft v 1 Q 1 *^ 39—45 7 47 6.7 8.3 !si 1 0 111 316 9910 211 . \|H . iVAtlJ li/AO. . . June— July 1913.. 46-54 g 57 7^2 2 12 173 595 22603 397 Aug.-Sept. 1913... 55-62 8 86 9.2 6.3 1.46 9 7 105 539 31176 363 251.87 18.32 £>0 f*Q c 17 6 3 7 5 84 2 o 141 394 9044 244 Dec.1913- Jan.1914 69-75 U 7 o * 48 6^2 8^4 '.74 1 9 319 725 22299 465 258.36 25.15 7fi— 81 g 32 5 6 8 8 64 0 3 233 626 14002 438 Apr.-May 1914..! June-July 1914... 1 \J O A 82-90 91-96 9 6 81 31 8.5 4.6 7^1 7.2 1^20 13 6 11 9 186 326 801 814 34353 18139 424 585 248.35 30.09 A « 1Q1/1 97-104 g 73 9.6 7. 1 1.35 11 13 165 834 33103 453 Au(5'~*^ept. iyi4. , . 105-113 g 108 12.2 7^4 1.65 24 2 87 517 28021 259 T^ 101HT 1C\\k m— 191 c 65 8. 1 7.9 1 .03 3 9 119 698 24354 375 j-Jec. Ay j.~r™ j an. i ./ 1«) mi m— 197 o g 59 12 5 91 l'37 9 7 137 623 20971 355 Apr. -May- June "15 iLl t 128-137 10 79 8'.4 8^2 6 18 315 768 33398 423 297.34 22.57 TABLE 24. — Selection data summarized by two-month periods for Line 719 minus. j 03 W £fl 1 d o i a a a g 01 i *_. •2 S 49 CO sl a3 c g o 03 v 03 2 S q a . c3 5 o i ja o J3 a> 5 5 2 a; d 9 a o o m v bO c 3 O O Ed u v 3 * 03 S »H 4) CO ^3 E M S *-«3 "ol o a o S si c •*-* £ «> ta j Time period. T3 •M O cn o M- o *O *- •s| O 3 0 S 'd _s "3 a o 3 3 ej o u o a; 02 k 3 5 a o 1 S 1 o o bO W °r £ 03 . _> C "1 OS g 0 03 •3 Tt b 0) J5H fe ^3 o t- o ^ td •-* S * 73 a a g •a O o^ a 8 o 33 oj -C S o**- fl § a> be o "n i ,0 gg & d M CD 6 ~ 4J n 03 u U eS o> 2§S 31 . 03 O to C3 CU S S G E 3 a « 03 a a> .c •a a 03 03 •8 II a] •8 W C z H fl ^^ 'A " ^ ' ** ' 02 14 ' co fl M _ M Apr —May 1912 Q 14. 7 T7 6 2 9 1 0 68 o 14 292 896 21155 579 + 11 TiinA— Tiilv 1 Q1 9 5 6 c 5 66 n 7 210 719 16320 418 -38 Aug Sept 1912 99 94. 9 ) 1 3 17 5 07 o o 200 251 854 +61 Oct Nov 191° ft 1 ^ 1 5 19 5 08 o 0 184 239 3809 254 +5 9qZ5fi CK 6 9 S 4 89 o Q 93 333 12566 228 + 52 Feb.-Mar. 1913.. 37-42 6 35 5.8 9.0 .64 0 0 126 353 7938 227 113.26 12.91 + 100 24.79 4.03 Apr May 1913 4? KA t;n 7 5 7 4 1 00 g o 124 423 15220 254 -43 C1 CO i (i _i i 4 6 7 9 64 A I 255 492 13543 330 + 67 Aug.-Sept. 1913.. 59-67 9 74 8.2 7.0 1.17 8 4 101 634 18502 250 196.73 15.43 + 113 23.94 4.72 Oct Nov 1913 AO_ 74. 7 nl' 8 0 8 1 99 o 3 149 403 14036 251 -7 Dec. 1913-Jan. 1914 75-81 7 58 8.3 8.3 1.00 4 5 179 701 21682 374 207.78 18.46 + 91 31.17 2.91 Feb Mar 1914 89 SO 7 4O 5 7 Q n 63 2 2 255 722 16S70 422 + 16 Apr May 1914 on— 07 o 5 7 6 9 83 4 22 229 837 36841 439 -15 June-July 1914.. . 98-105 8 63 7.8 7.8 1.00 10 9 210 791 29417 467 241.18 20.50 + 118 36.40 3.24 Allfr fipnt 1014 1Ofv-1 19 7 50 6 6 7 3 90 15 g 206 769 20071 401 + 52 Oct Nov 1914 ml 9O o 83 9 5 7 4 20 5 183 646 22045 266 -7 Dec 1914 Jan 1915 121 128 a 87 10 9 7 8 1 39 9 5 168 697 31633 364 + 11 Feb Mar 1915 1 9Q 1 "^ 7 9 5 S ^i 1 12 § 7 116 639 26135 373 -18 Apr., May .June, 1915 136-145 9 (35 8.1 7.8 1.04 19 6 147 606 22580 347 257.35 21.53 + 76 31.18 2.43 62 SELECTION IN CLADOCERA ON THE BASIS OF During the year (August 1912-July 1913) the means were 302.5 and 256.3 seconds. The difference (+46.2 ±12.1 seconds) is 3.81 times the probable error, a difference statistically significant. For the 12 same-day broods the means are 314 and 254 seconds— and the difference is +60 ± 21.68 seconds, 2.76 times the probable error. TABLE 25. — Selection summary for Line 719. "3 . i en i 3 h Q i Q i d Q a 1 O t-. a i 3 73 CJ vn M Vj d a ^H 32 *" CO m e »-; o .9 S a C3 § j3 c 03 OJ . d 03 o V S3 Q. 'a 13 o c3 'S 2 V K IM Q 03 a ta >, OJ o CO OJ 73 73 o .»_> a a ""* co o "o-S 73 K 3 QJ _to 73 "o3 — 3 0 a 3 a a a 3 d _o S «*-! 0 a OJ GJ S "o "S 73 Time period. Strain. "o m 73 d "o d ttc 'C. 2 '-+3 •f "2 03 1 "j o CJ Vj 0 hi •P 73 1 O 0 .Q £73 o IP O O — ^ . 03 0 £ tC tn § § 73 £ a a 1 a 73 73 CJ OJ o \3 CJ £ w s *• 03 hD ^* ^ M-l _, to M . C3 3 S a ~o s t h "o 03 *^ | fl *£? rT3 O ^ c3 aj fl cj 73 d hi O M ^ § 01 O 6 I fe 03 '" 1* 6 oj M — • o> g il |l 03 W gS 43 O S Is Apr 1912- ( Plus . . . 9-19 14 5.6 78 4 19 225 577 450.1 275.3 21.0 July 31, 1912 ( Minus . 9-21 15 5.1 76 0 25/ 493.1 308.4 -43.0 31.8 i.35 Aug. 1, 1912- j Plus . . . 20-54 36 6.7 241 6 23 156 454 302.5 232.3 ib!i +46.2 12.1 3.81 July 31, 1913 * Afinus 22-58 41 6.1 1810 /O 1 1 6k 372 256.3 142.9 6.6 Aug. 1, 1913- ( Plus . . . 55-96 41 7.7 315 31 39 215 664 409.6 257.6 9.8 +43.3 13.2 3.28 July 31, 1914 \ Minus . 69-105 46 8.2 375 25 45 755 685 366.3 255.8 8.9 Test series, ( Plus . . . 89 23 13.9 320 96 82 156 879 520.8 289.2 10.9 + 57.2 14.9 3.83 May 1914 . . ( Minus . 96 •23 * / / 33; 121 £0 1.5 b 808 463.6 271 .9 10.1 Apr. 1, 1914- ( Plus. 82-93 12 8.2 98 15 16 224 826 453.7 + 19.2 June 31, 1914 l Minus . 90-101 12 9.0 108 5 23 794 434.5 Aug. 1, 1914- ( Plus . . . 97-137 40 9.6 384 53 49 172 693 364.2 264.9 9.1 + 19.2 12.5 1.53 June 25, 1915 1 A firms 106-145 39 Q / 355 71 29 ;&4 668 345.0 239.8 8.6 Test series, j Plus . . . 110 43 22'8 979 189 46 95 688 310.9 217.2 4.7 +20.3 6.3 3.22 Nov. 1914. . \ Minus . 118 43 23.7 y Q1 Q ^/ 7* 35 103 697 290.6 197.0 4.2 Oct. 1, 1914- ( Plus. 105-117 13 9.9 129 27 3 93 511 261.1 Dec. 31, 1914 * Kfinus 113-124 12 11 .1 133 i£.Z 288.1 —27.0 TABLE 26. — Same-day broods. Summary of data for Line 719. g i o £ ^ c d g 03 p. o _o _o 03 ^-t OJ ^ D bo bfl — ^ OJ CO s O rt C C C 0 ^ o • e G . OJ 1* 3 ~ *j OJ 03 •M c g S hi A O cS >2 Q) hi 2 0 c3 a "3 i§ o "S 00 CJ ht ^5 s a ^ , a o a «-s 73 73 hi "oi _>> ~ ' — 3 O 3 3 09 3 v> "o OJ K O OJ 73 OJ •J Q M 03 Time period. Strain. 00 ,3 73 ^73 "S 'S ^S o -£.5 o '> 73 a •^ ^ > d 03 2 O bj S 3 H 73 O 0 a 73 fcO^oo d d 1 a 73 a 73 73 CJ CJ •" ~ O S s 0) o B O c £ M-rj 3 .^^ a 2 3 OJ t o go Q *o "^ ^ o 03 cy 03 c3 C cj 73 03 is E d t. OJ ^d « 2 d d •- . OJ O d> S 6 s a a 03 •g ia ^ 0 •~ o ^^ ^ £ * ^" ^^ S" OJ £ 5 a £ Q-0 Apr. 8, 1912- ( Plus . . 3 3.3 10 0 5 557 703 718 257.80 54.99 + 179 76.00 2.35 July 31, 1912 v Afinus 3 6 7 17 o e tfJipP 592 639 320.75 52 47 Aug. 1, 1912- ( Plus . . 12 7'.Z 88 2 12 141 465 314 262.11 18.85 + 60 21.68 2.76 July 31, 1913 ( Afimit 18 6.9 OQ 3 i 131 45 / 254 144.66 10.71 Aug. 1, 1913- 1 Plus. 11 7 .9 87 g 4 174 •r" •* 624 *"-* *T 340 207.54 15.01 July 31, 1914 ' Minus 11 88 9 204 665 394 249.62 17.95 -54 23.39 2. SO Aug. 1, 1914- 1 Plus. 10 11.3 113 14 1 -i 114 741 351 June25, 1915 ( Minus 10 7.8 78 '7 252 737 375 —27 For the following year (August 1913- July 1914) the means are 409.6 and 366.3 seconds. The difference is +43.3 ±13. 2 seconds and is 3.28 times the probable error. Again the plus mean is larger by a margin of statistical value. The 11 same-day broods have A PHYSIOLOGICAL CHARACTER. 63 means (340 and 394 seconds) differing by —54 ±23. 39 seconds, a result out of accord with that for the means for the complete data for this year and with that for the test series now to be considered. The test series conducted in May 1914 consisted of 23 broods from each strain (320 individuals of the plus and 331 individuals of the minus strain). Of the 23 pairs of broods, 17 broods from the plus strain had the higher reaction-time, while the minus brood had the higher reaction-time in only 6 cases. The reaction-time means for the entire test series were 520.8 and 463.6 seconds. The difference was +57.2 ±14.9 seconds, a difference 3.83 times the probable error (table 25). For the final period (11 months) of the experiment with Line 719, the mean for the plus strain was 364.2 seconds, for the minus strain 345.0 seconds. The difference was +19.2 ±12.5 seconds, a difference only 1.53 times the probable error. The ten same-day broods gave means of 351 and 378 seconds, and a difference of —27 seconds; again the same-day-brood data is out of accord with the data for the year-period. A test series conducted during November 1914 and consisting of nearly 1,000 individuals from each of the strains gave means of 310.9 and 290.6 seconds and a difference of +20.3 ±6.3 seconds (table 25). This difference, while small, is 3.22 times the statistical probable error. The course of the curves (figure 8c) throughout is rather irregu- lar, particularly for the plus strain, but the marked tendency for the curve for the plus strain to be higher than that for the minus strain is suggestive of a real difference in reactiveness between these two strains, with the minus strain generally, though slightly, the more reactive. In addition to the data for the entire course of the experi- ment when considered by longer periods, the two test series, and a minor portion of the data for the same-day broods, likewise tend to indicate that the minus strain was the more reactive. But the differences are less pronounced during the last year of the experiment than during the earlier two years. The minus strain was more re- active than the plus strain by only 19 seconds (a difference not of statistical value) during the last year-period. The differences for the earlier two years were 46 and 43 seconds and were 3.81 and 3.28 times their probable errors. However, the large test series conducted during this last year-period indicated that the minus strain was the more reactive by 20.3 seconds (a difference very near that for the last year's selection data) and because of the large number of indi- viduals used in this series and the consequently smaller probable error this difference was 3.22 times the probable error. The data for Line 719 as a whole and particularly the data of the 2 test series are very suggestive of a genetic difference in oppo- sition to selection between the two strains of Line 719, in spite of the lack of confirmation from most of the same-day-brood data and the 64 SELECTION IN CLADOCERA ON THE BASIS OF reduction in amount of this difference during the last year of the experiment. Additional evidence of a significantly lowered reaction-time for the minus strain of Line 719 is obtained by comparison with the minus strains of the other D. pulex lines which showed no evidence of modification in reactiveness during the selection experiments, Lines 691, 695, 713, 714, and 751. A composite curve for the re- action-time mean for the minus strains of these lines compared with 1.50 - 1.00 - .50 - 00 ;., Vi ' s I I [II!-: S i 4-5 8-9 12-1 4-5 8-9 12-1 1912 1913 4-5 8-9 12-1 4-6 1914 1915 '. . t 1 T B T I, < fi i/ .50 .25 00 .25 .50 o A I r i i 600 h 450 300 150 4-5 4-6 I9IZ 1915 FIGURE 8. — Line 719. A. Reproductive indices, actual values. B. Reproductive indices, superiority. C. Reaction-time curves with a superimposed curve (in faint broken line) representing the combined reaction-time means for the minus strains of all Daphnia pulex lines in which significant reaction-time differences probably did not arise. the mean for the minus strain of Line 719 (figure 8c) shows that beginning with August 1912 and continuing to January 1914 the mean for the minus strain of Line 719 was lower than that for the combined mean for the other minus strains for the same two-month periods in 8 of 9 two-month periods; 6 of these differences (ranging from 48 to 197 seconds) were of statistical value and one other just escaped statistical significance because of the small numbers of reaction-time records involved. From February 1914 the minus A PHYSIOLOGICAL CHARACTER. 65 strain of Line 719 continued more reactive than the combined means of the other lines for 10 months, but the differences were small and not of statistical value. For the remaining 7 months of the curve the differences were variable and the mean for the plus strain did not differ appreciably from the composite curve of means for the corre- sponding plus strains. The evidence seems suggestive of a mutation in the minus strain of Line 719, rendering it the more reactive to light. Since the differ- ence between the mean for the minus strain of Line 719 and the plus strain of the same line, as well as between the Line 719 minus strain and the combined means for the minus strains of the lines, which may fairly serve as checks, later decreased and was apparently practically lost, it seems probable that selection served to reduce or eliminate the effect of this apparent mutation. This reduction in (or possible elimination of) the reaction-time difference in Line 719 is worthy of consideration as contrasted with Line 757, in which the difference in reaction-time (which in that case is in the direction attempted in selection) became larger and larger in successive year-periods, until, instead of being a difference of 5.5 per cent, as in the last period of selection in Line 719, it was 99 per cent in the last year-period for Line 757. Line 719 affords a case parallel to that of Line 689, in which the plus strain likewise persistently had a higher reaction-time, contrary to any influence of selection. With Line 689 the period of the experi- ment was much shorter, but otherwise the results are very similar, except that in Line 719 the difference is not so large or so uniform, is not supported by most of the same-day-brood data, and became much smaller during the latter part of the experiment. It is to be noted with Line 719 that in reproductive vigor the minus strain was in general inferior to the plus strain. This was markedly true during the first and the last year-periods. The minus strain was slightly superior in vigor during the second year-period. Yet the minus strain was the more reactive throughout all these year-periods and was (absolutely) somewhat less reactive during its year of superior vigor (August 1913-July 1914) than during the other two years. The curves for the mean reaction-times by two-month periods (figure 8c) indicate an interesting correlation between the means for the two strains, the plus and minus curves, in spite of their fluctua- tions, following each other remarkably closely. This is again an expression of environmental effect. The small and variable, though fairly pronounced, superiority in reactiveness on the part of the minus strain for most of the period of selection is rendered even more interesting by the fact that the differences persist while the curves sweep up and down under environmental influences. 66 SELECTION IN CLADOCERA ON THE BASIS OF LINE 751. Line 751 was obtained in October 1912 from the surface-water pond on the upland (Pond I), from which Line 689 came a year earlier.1 Selection was begun at once. This is the only line of Daph- nia pulex with which selection was not begun during March 1912, after having been in the laboratory from 5 to 8 generations before TABLE 27. — Selection summary for Line 751. "3 . -i tn 0> i a o i a 0 i o _o a a t o o a i 'C "*"" _ro C E? fi C ^5 03 o 03 o 03 03 V a a a £ V a 43 a =5 .§ 3 'S o 03 3 t V h 2 "o a D a >> o v .S 4> "3. V £) t* ^5 a a a c T! S 43 X' _ o ,2 "o 3 c 3 "ol .2 o •M O "S T3 O ^•5 TJ K "3 3 "o | a 3 03 o •ff ^2 Time period. Strain. *o • . •— " bO '" C .£ 3 T) '3 'S 3 S <1> 05 0 > 01 a T3 O O o o t*-l - — O rM . 03 03 . ^ s 1 03 a •3 TJ S 11 M S ^ • o u o

S • 03 ^) C C3 o o e P d S* n"2 l^ O G^ ^" ^" S* 43 CO Q* 5s Nov. 13, 1912- ( Plus .... 1-31 32 8.2 261 10 8 139 450 282.2 179.6 7.73 + 11.4 10.28 1.10 July 31, 191J 1-31 S3 8.1 S67 15 c / A'i 459 270.8 164.1 6.78 Aug 1 1913— {PlllQ 32-71 40 8 0 321 14 31 177 657 375.4 252.7 9.51 June 11, 1914 i lllo .... Minus . . 32-70 40 s's 331 42 ;S7 400.7 256.7 9.62 -25. 3 JS.-4S ;.5« Test series i plus 54 17 19.3 328 19 41 140 752 416.1 260.0 9.7 Jan. 1914 > Minus . . SS /3^ 327 50 4SS.9 267.7 10.0 -17.8 13.9 1 .28 T-\ -I -I Qi 0 I Plus 48-58 1 ] 8 0 88 3 10 207 68£ 414.0 233.0 16.8 Feb. 28, 1914 1 i 1 US .... ( Minus . . 47-57 5!s 64 ;o 250 664 488.7 252.6 -74-7 27.10 2.76 TABLE 28. — Same-day broods. Summary of data for Line 751. u a g i a i a ^ c S rt a o o o 03 g j^ n a M • G rS 43 0 03 o 'C o 03 a s . Ol "^ a o M g D. o P g c3 tG ^ o 03 >2 03 ^ 3 hi a a , a o S c-fe m 01 •a •o Time period. Strain. 0 "o d individual _>, "w > '•*3 03 g>.2 s§ : individua an end of ge minimu ge maximu individua 49 03 0} T3 1 •s I 5 Jj 3 ence betwe and minus o I 2 rs •3 S »^ S 2 V C o d §1 •~ *" o d 55 P °-t . 03 O « ^ " 03 oj !* c3 aj 1^ s§ c .5 S~ T3 a 03 49 CQ 03 •§ £ || Qa 03 X) O M A S3 »• ta JJ S-0 Nov. 13, 1912- { Plus 16 7.4 119 8 5 166 489 325 204.06 12.62 +37 16.02 2.30 Tiilv ^11 1Q1 3 Iff c / 136 13 j2 165 476 288 170 04 5 87 Ana 1 lOI'l ( PlllR 13 6 8 88 7 10 165 565 350 . +5 June 11 1914 S 5 111 »/ g ^es 676 S^5 the beginning of selection. Selection was continued for almost 20 months, when both strains were lost (in the seventy-first and seven- tieth generations). The data for this line are given in tables 27 and 28 and figure 9. For the first longer period of selection (9 months) the mean reaction-time for the plus strain was 282.2 seconds, for the minus strain 270.8 seconds. The difference ( + 11. 4 ±10.28) was not significant (table 27). There was a large number (16) of same-day 1 All the lines of D. pulex except 689 and 751 came from Pond II, the spring-fed pond in the woods. A50 - 300 150 OO A PHYSIOLOGICAL CHARACTER. 67 broods. The mean reaction-time for those of the plus strain was 325 seconds, for the minus strain 288 seconds. The difference (+37 ±16. 02 seconds) is in the same direction as that for the entire data for this period. For the remaining period of selection with this line, 10J/4 months, the mean reaction-times were 375.4 seconds and 400.7 seconds for the plus and minus strains, respectively. The dif- ference was —25. 3 ±13. 46 seconds, a difference not of statistical sig- nificance. There was again a considerable number of same-day broods (13), for which the mean reaction-times were 350 and 345 seconds and the difference +5 seconds. A test series conducted in January 1914, consisting of 328 indi- viduals in the plus strain and 327 individuals in the minus strain, gave a mean reaction-time of 416.1 seconds for the plus strain and 433.9 seconds for the minus strain. The difference was —17. 8 ±13. 9 seconds. This difference like the others is not of statistical value. A curve for the reaction-time means of this line is shown in figure 9 and, like the numerical summaries just considered, shows that there is probably no effect of selection. There is just a possible suggestion of an effect of selection, however, in the last year's data, inasmuch as during this period the plus strain was continuously the more reactive, FIGURE 9.-Line 751. except for one two-month period. Reaction-time curves. -n L • • <• . i n But in view ol the great fluctuation in the reaction-time curves for the other lines subjected to selection, this suggestion has little weight. There is a feature of interest in the curves for this line, as with many other lines, the general close correspondence in the levels of the plus and minus curves in the different two-month periods; on the whole they sweep upward and downward together, an expression of a parallel effect of environment upon the two strains. It is a matter of interest and some importance to note that the means for the two strains of Line 751 start at approximately the same levels which were held by the other D. pulex selection lines (Lines 689, 691, 695, 711, 713, 714, and 719) during the same two- month period, although the other lines had been subjected to labora- tory culture for approximately a year and to selection for 8 months before Line 751 was brought into the laboratory. Comparison of the curves for Line 751 (figure 9) and for the other D. pulex lines (figures 2c, 3c, 4, 5, 6, 7c, and 8c) and with the composite curves for all the D. pulex plus and minus strains (figure 10o, on which the curves for Line 751 are superimposed), strikingly brings out this fact. This is important as showing that under laboratory conditions strains of 68 SELECTION IN CLADOCERA ON THE BASIS OF a given species tend to have approximately the same reaction-time means, regardless of the length of their previous laboratory history. LINE 762. Line 762 was one of the two lineally distinct lines of D. longispina subjected to selection. It was obtained October 19, 1912, from Pond I, one of the surface-water ponds on the upland, and was subjected to selection at once. This is a less favorable species for selection on the basis of reactiveness to light than the other species used. While it is moderately reactive, it does not so readily withstand the hand- ling, and individuals sometimes become disabled or become lodged in the surface film of the water. The summary of the data for Line 762 is found in table 29 and in figure 10A. The experiment with this line was of relatively short duration (10 months) as compared with the other lines, due to the TABLE 29. — Selection summary for Line 762. 01 ( o i i ( t — M .+-) d -fj a a • o 03 03 m o o o P H •§ ro (3 ^3 03 03 03 03 I pj- eg a> 2 a Q 03 .2 O "2 *^ ® V V IM 03 tu ^ O o m ffi TJ w .2 "£ o || d IH |l a 3 "ol O a _o a> a O *9 •3 •o *O GJ o »^ — - CO *V 3 O •3 "•^ "o SJ *o — ^ Time period. Strain. "o ^3 CO -o 6 "o a M ^ e (D *-4-> ]> e '3 03 3 .S s O 7] .n QJ S | a o o 1* o o 15 . 03 bfl^m a a 1 a •3 T3 "~! N O 8 j 63 "g ^ tj3 o) o *7 C cj •~ 03 O o •FH h ^ [3 ^" »— C3 2 03 to fc f-4 3 *+•" ^H bC . to . 03 .0 £ a ^f*^ 4) £* Q 03 -° , c 03 «; o! m fl GJ •"O 03 h O 9 a o 6 |l 03 '" O ;i2 . 03 O O *^ )H t. rM a^.§ 0 03 •§ tS '-3 33 O •§ taJJ O H° ^^ ** ^^ <~ s' +a VI £ 5 * £ S-0 Nov. 1912- ( Plus. . . . 1-30 31 6.9 215 i 21 168 573 366.2 229.2 10.5 July 31, 1913 ( Minus . . 1-28 31 0.4 3 168 eos 377.3 257.1 -11.1 16.19 .69 Aug. 1, 1913- ( Plus. 31-36 6 9.8 59 1 4 111 581 269 3 224 1 19^7 Sept. 8, 1913 ( Minus . . 29-35 7 n.s 79 8 aoo 805.6 -213.0 30.42 7.00 accidental loss of the plus strain in September 1913. Shortly after this loss the experiment was resumed by selecting in both directions from the minus strain. The line was henceforth designated Line 766. This was in effect a return selection within the minus strain.1 The data for the first nine-month period (November 1912- July 1913, table 29) gives reaction-time means of 366.2 and 377.3 seconds. The difference ( — 11.1 ±16. 19 seconds) is not significant. The 10 same-day broods for this line, comprised mostly of broods produced during June and July, gave the following means: 355 seconds for the plus strain (57 individuals) and 349 seconds for the minus strain (54 individuals). The same-day-brood data rather clearly substantiate the conclusion which the curves (figure 10 A) suggest, that until that time there was no effect of selection. For the final portion of the experiment, a little more than a month (until the plus strain was lost), the plus strain was markedly 1 It is unfortunate that return selections were not conducted in other lines, particularly in Line 757. The return selection in Line 762 (which then became Line 766) is not significant as a return selection, because presumably there was not an effect of selection within Line 762. A PHYSIOLOGICAL CHARACTER. 69 600 450 300 the more reactive, the means being 269 and 482 seconds and the difference —213 ± 30.42 seconds. There is, moreover, an interesting difference noted between every plus brood and every minus brood tested during this period. Without exception the plus broods were considerably more reactive than the minus broods tested at the nearest dates. Such a consistent difference in reactiveness involving every individual brood1 is not generally found in the data obtained from these selection experiments, except in portions of the data for Line 757, in which a pronounced effect of selection was obtained. 750 r 12-1 (913 4-5 8-9 12-1 1914 4-5 8-9 12-1 1915 4-5 8-9 12-1 1916 4-5 8 150 00 4-5 8-9 1912 12-1 4-5 8-9 1913 12-1 4-5 8-9 1914 12-1 4-5 8-9 1915 12-1 4-5 8-9 1316 FIGURE 10. A, B, C. Reaction-time curves for Daphnia longispina — Lines 762, 766, and 768, respectively. D. Composite reaction-time curves for all plus and all minus strains of Daphnia pulex with reaction-time curves for Line 751 superimposed. The divergence in reaction-time means for this last two-month period and particularly the complete and unusual consistency of differences in reaction-time, brood by brood, for this last period suggest an effect of selection. The amount of these data is too small to be more than slightly suggestive, but it seems quite possible that a mutation occurred in the plus strain of Line 762, thus producing 1 It will be remembered that temporary enviromental conditions are very potent factors in modifying the reaction-time means of individual broods and that consequently rather wide fluctuations occur in all the data. Hence, only in cases, in which the reactiveness of the two strains differs greatly do we fail to find the reaction-time means for different broods of the two strains of a line overlapping to a wide degree. 70 SELECTION IN CLADOCERA ON THE BASIS OF the great lowering of the plus mean and the marked differences in reactiveness between the two strains not present earlier. LINE 766. As noted above, the two strains of this line were derived from the minus strain of Line 762. The minus strain was a continuation of the minus strain of Line 762, the selection having been inter- rupted for three generations. The plus strain of Line 766 was de- rived from the minus strain of Line 762 by selecting the most reactive individual from the thirty-ninth generation of the minus strain of that line. The summary of the data is given in table 30 and figure 10s. Selection was continued for only 7 months, when it was interrupted by the loss of the plus strain. A rather wide difference in mean reaction-times for the first two-month period is not continued during the remainder of the ex- TABLE 30. — Selection summary for Line 766. '-• "4 ^ t ^ o ia i a h a i a! * 0i S *H o o o a M o 3*^ til b£ J4 C3 o ^ m oi a C g o ~£ o £ gj c u • "S "*^ '3 fl ~ cS OJ a 03 »ri V V D. a •^ ID a '3 *" 01 o o Q o E to ^ o •o (3 'C 'fcj ci **- OJ o •3 JQ o CO "^ CO •^ o> h 00 ^ S S ^ Q S a Q) IM T3 Time period. Strain. o •a o m a .2 '•Z3 CQ o 3g .2 M G d| negatively .als. individua an end of e minimu e maximu individua V> _cS s> T3 •o "o h O 0 Si O ^-> O en *° C « .S §^ 0 g B S o Q) "2 '> ^ " L S 2 Genera 'o ci l& _ a oj •— •s| °l »•§ . c3 O CJ Z " 03 flj |5 M a a, fe S ^'•s o CO "^ w _to C to c c c .2 *-5 o o3 ^o U^ o 03 _o tfa 03 p OB G- £3 OS u a •g a o> o at i ••3-S .S o> to 03 .2 aj o FH "« "C M S s O •M O a 03 S -a "S E "i d O OJ CO o CO a O O o o *o 15 c3 U3 05 > & '§ 03 E •3 o •d IH g O la B •3 o "-0 t- o ill O O g S1 c a 0 01 •S "8 §"•? _2 I i 03 . M b f-\ **^ M . til at 3 o C3 "3 s t o 03 -0 i-^ ~ *o *^3 O "^"^ 03 a) — cu "O OS fc .2 03 l- (U C Q <3 fe 03 '" . !> 6 1 c3 £ N SB C 03 •8 •8 a> O O fc < °" H« z^ *" S* ^'^ 43 CQ £ | § S3 Dec. 21. 1914- ( Plus .... 54-83 29 8.2 238 8 58 211 730 426.0 331.2 14.5 July 31, 1915 t Minus . . 54-82 S.6 7 772 299.7 -17.0 19.6 O.S7 Aug. 1. 1915- j Plus 84-125 39 9.7 376 7 31 111 657 315^7 242.1 8^4 July 31, 1916 ' Minus . . 83-124 38 7.7 t? ^4 647 328.0 230.8 9.1 -IS. 3 12.4 .S3 Aug. 1, 1916- ( Plus .... 126-128 3 9.7 29 0 3 143 845 350.0 268.5 33.6 +151.6 43.0 3.52 Aug. 22, 1916 t Minus . . 186-127 S 7.S %& 0 ^ 66 507 198 4 186.1 26.8 TABLE 32. — Same-day broods. Summary of data for Line 768. a ? ^ o O a o g . 3 o & "o3 SS C3 ^ 5 s a 3 a "3 ii ^ *• «*• >^ q H 3 m •o «s:y *o - 03 0 •d J3 O Time period. Strain. 1 o JD o . •" ?, > •3 OS > 11 |£l a. a '•^ 0 g ? c T1 S •2'-? «3 ss c £ O •M O •82 •— o -c O *i S a 03 0 I'l a c.2 «> s ** . v> . M - ^ O o o " ^; ^« 4£ o o S a Q Dec. 21, 1914-July 31, f Plus .... 17 10.6 180 2 52 212 80S 454 + 16 1915 Minus . . 17 9.9 169 Q 37 112 827 438 Aug. 1, 1915-July 31, 1 Plus .... 4 12.5 50 i 3 106 603 374 +38 1916 Minus . . 4 9 8 39 0 2 118 615 336 were a large number (17) of same-day broods for this period, for which the mean reaction-times were 454 and 438 seconds. The dif- ference is +16 seconds (table 32). For the year-period (August 1915-July 1916) there were 39 and 38 broods consisting of 376 and 294 individuals in the two strains. The mean reaction- times were 315.7 and 328 seconds. The difference is —12. 3 ±12. 4 seconds. There were only 4 same-day broods, for which the mean reaction-times were 374 and 336 seconds. The difference is +38 seconds. 72 SELECTION IN CLADOCERA ON THE BASIS OF This experiment was continued during the most of another month, but the data consists of the records for only 3 broods from each strain. The means are 350 and 198.4 seconds. The difference ( + 151.6 seconds), while large, is based upon entirely too few individual reaction-times to be assigned significance. It is obvious that with Line 768 there was no effect of selection. The reproductive indices for this line have not been worked out, but the plus strain was apparently somewhat the more vigorous. That this difference in vigor has not influenced reaction-time, to any considerable extent at any rate, is evident from the fact that the same-day broods of the plus strain were on the whole less reactive than those of the minus strain. There is again, as with many other lines, an obvious effect of environmental conditions upon reaction-time. In spite of rather wide local fluctuations, the means for the two strains, on the whole, follow each other rather closely, as is shown in figure lOc. Reference to figure 10 (in which are given, separately, the re- action-time curves for the three selection lines of D. longispina and composite curves for the reaction-times of all the D. pulex selection lines) shows that there is a striking, though somewhat rough, paral- lelism between the reaction-time curves for the D. longispina and the D. pulex lines. This is an expression of similar influences of the same environmental factors upon the reactiveness to light of the two species. A PHYSIOLOGICAL CHARACTER. 73 GENERAL INTRODUCTION FOR SIMOCEPHALUS EXSPINOSUS LINES. S. exspinosus in some regards seemed an unfavorable species for selection on the basis of its reaction to light. During the tests indi- viduals frequently attached to the surface film or to the sides of the experimental tank and rested there during the remainder of the test (see also page 25). Further, and to an even greater degree, the S. exspinosus young were unsatisfactory in that they so generally settled to the bottom of the tank and appeared non-reactive to light. In most broods there were several individuals which failed to reach either end of the tank during the 15 minutes of the experiment, while in some broods (particularly during the earlier course of the experi- ments) there was no response on the part of any individual of the brood. The reactiveness to light during the early course of the ex- periments was so slight that in many cases there were really no grounds for making a selection and there seemed little hope of a sufficient reactiveness to light to afford a basis for conducting an experiment in selection on this character. Several of the earlier S. exspinosus lines were discarded as un- profitable after a few generations of selection. Lines 740 and 757, while not promising at the start, were continued and subjected to selection for a time to see if it then seemed advisable to continue the selections. Even comparatively slightly reactive material might conceivably afford a basis for selection if there were enough reactive individuals to make a selection possible in a considerable percentage of cases. In addition to broods (particularly the earlier broods of these two lines) which showed absolutely no reaction to the light, there were other broods in which the only selection possible (in the plus strain) was from among individuals which had moved very slightly toward the light. In the minus strains the only choice in a vast majority of cases was an individual which showed no reaction to light. There were numbers of these in nearly every brood. Negatively reacting individuals were so rare with this species that it was seldom possible to make selections in the minus strains on the ground of a negative reaction to light1. But after a few generations it was con- sidered that the reactiveness of the strains of Lines 740 and 757 seemed sufficient to justify the continuation of these lines in the hope that there was sufficient reactiveness to test out the possibility of selection. The results justify the conclusion then reached, that after all there was sufficient basis for an experiment in selection. It is worthy of emphasis that the selections in the minus strains of lines of Simocephalus were rarely on the ground of a negative 1 However, as noted elsewhere (page 16), it is questionable if there is any real significance to be assigned to the negative reactions of the few individuals which went to the negative end of the experimental tank. 74 SELECTION IN CLADOCERA ON THE BASIS OF reaction to light; that the only selection possible in very many cases was from among individuals showing no reaction ; and that in the remaining cases the selections in the minus strains were postively reacting individuals with the highest reaction-times. Lines 740 and 757 were subjected to selection from August and November 1912 to May 1917. Lines 794, 795, and 796 were used in selection from December 1914 to May 1917. All these lines of S. expinosus, except Line 757, originated from mothers obtained from Pond IV, the larger surface-water pond on the hill, a mile from the laboratory of the Station for Experimental Evolution. TABLE 33. — Selection summary for Line 794- CQ -i i a o ^ a a i Oi 03 13 CQ O> "^ 00 a o a 03 0) a XI O, "a ^rt ^ - . i 3 -g O ._" £) h 2 S IM 0 2 a 1 >i o n O T3 .2 o> .S'S M IH OJ • Oi o a fe a a C3 •§ ta ^ X) o JQ _« 6 ci > -1 £ •"^ t^ •z •* ^ 2 h ^ •5*1 S^ £ £ Q £ Q-0 Dec. 23, 1914- ( Plus 7 13.1 92 1 22 155 876 475 276.95 19.48 + 116 25.78 4.49 July 31, 1915 7 81 2 s 173 681 359 225 . 50 16.90 Aug. 1, 'l915- ( Plus 5 16.2 81 o 28 259 900 463 July 31, 1916 g 13.8 69 / 23 900 490 —27 Aug. 1, 1916— < pius 2 12.0 24 *f o 4 105 645 323 270.21 37.20 May 1, 1917 ( Minut . . « 26 0 4 3*5 457 248.69 32.90 -134 49.66 2.C9 LINE 794. The selection data for Line 794 are given in tables 33 and 34 and figure 11. This line and Lines 795 and 796 originated from three mothers collected December 11, 1914. Selection was begun with the first A PHYSIOLOGICAL CHARACTER. 75 broods of young produced in the laboratory and continued for 29 months, 91 generations in the plus strain and 90 generations in the minus strain. For the first longer period (December 1914-July 1915) of the experiment the means for the plus and minus strains (358 and 288 individuals) were, respectively, 529.4 and 451.8 seconds (table 33). The difference (+77. 6 ±15. 6 seconds) was 4.97 times the probable error. For the first single month of selection (December 1914) the plus strain was more reactive by 40 seconds and again in June 1915 the plus strain was the more reactive strain, this time by a large margin, 274 seconds; but for the other 6 months of the first longer period the minus was more reactive by differences ranging from 88 to 219 seconds. For the same-day broods (table 34) the means were 475 and 359 seconds, the plus strain having a higher mean by 116 ± 25.78 seconds. Of the 7 same-day broods for this period, 5 occurred in succession in January and February 1915, almost im- mediately after the beginning of the experiment. The differences were +175, +207, +193, +140, and +152 seconds. Data for a pair of same-day broods occurring just before these five are incomplete, but the minus brood was the more reactive by at least 196 seconds, while the pair of broods just preceding those last mentioned were likewise same-day broods with the plus the more reactive by a small margin (27 seconds). Thus there were in effect 7 successive same-day broods, in all of which the minus brood was considerably the more reactive. These differences are so large and so persistent as to indi- cate for this early and limited period (January-February 1915) a real difference in reaction-time, with the minus strain the more reactive. In the last 2 months of this longer period (to August 1915) the minus strain had much the larger reaction-time, and this differ- ence was greatly increased in the next two-month period and con- tinued very large for another two-month period.1 The second longer period (the year August 1915-July 1916) gave averages of 483.8 and 589.1 seconds. The difference was — 105. 3 ±14.3 seconds, or 7.36 times the probable error. For the same period the 5 same-day broods had a difference of —27 seconds. This year's data, taken as a whole, might at first thought be con- sidered suggestive of an effect of selection ; but the great irregularities of the curves, the small difference between same-day-brood means, and the large differences in the opposite direction for the preceding year do not favor such an interpretation. A test series conducted during July 1916, consisting of nearly 1,000 individuals of each strain, gave means differing by only +0.7 ±9. 7 seconds. This test 1 The very great divergence in the two reaction-time curves, the minus strain being the less reactive of the two, for the six-month period June to November 1915 is no more easily accounted for than the earlier six-month period, during which the plus strain was markedly the less reactive. 76 SELECTION IN CLADOCERA ON THE BASIS OF series shows effectively that there was up to July 1916 no certain basis for assuming an effect of selection within this line. During the last longer period (August 1916-May 1917) of selection with Line 794 the means were 309.9 and 373.8 seconds. The difference (— 63.9±15.3 seconds) was 4.18 times its probable error. The 2 same-day broods gave means differing by —134 ±49. 66 seconds. As for the preceding year, the means are suggestive of an effect of selection, but the irregular course of the curves (figure lie), together with the result of the test series conducted in July 1916 (table 33) and the much higher reaction-time for the plus strain during 6 months of the experiment, make this interpretation ques- tionable. It is noteworthy, however, that in spite of considerable irregularities in the curves, the curve for the minus strain is appreci- ably lower than that for the plus strain in only one of the 9 two- month periods after the first 12 months of the experiment. After this early period all the data except the test-series data indicate that the minus strain was on the whole the less reactive throughout the experiment. Since the mothers from which the two strains of Line 794 originated were sisters from the same brood, it is hard to understand to what the difference in reactiveness (the minus strain being markedly the more reactive) during the period of the experiment (June- November 1915) can have been due. It is possible to think of it as a mutation in the minus strain, the effect of which was later eliminated by selection or by a second mutation in the same strain in the direction of less reactiveness to light; but these are mere sur- mises for which there is no real evidence and the differences them- selves are not pronounced and certain enough to be considered as of great significance. It is probable, however, that the rather wide differences in reactiveness in the minus strain of this line at different periods of the experiment are due to non-genetic factors, such as produced similar effects in Lines 695 and 740 (see figures 2c and 15) and elsewhere. Nevertheless, the data leave room for the suggestion that selection was possibly responsible for the elimination of the greater reactiveness on the part of the minus strain, however it may have occurred, and a development of a relatively greater re- activeness on the part of the plus strain. Figure HA indicates graphically the general reproductive levels for the two strains during the different two-month periods. There is no obvious relation between reproductive vigor and mean reaction- time unless the generally lower reproductive vigor of the minus strain be assumed to account for the generally higher reaction-time of the minus as compared with the plus strain; but with Line 795 (see figure 12B) the plus strain in general showed an even greater superiority in vigor, yet it was much less reactive than the minus A PHYSIOLOGICAL CHARACTER. 77 strain of the same line. Examined in detail, the data for reproductive vigor and reaction-time do not appear to be related.1 2.50 < 1 2.00 1.50 A o o O A I'M-: 1.00 _ 0 .50 _ 12-1 4-5 8-9 12-1 4-5 8-9 12-1 4 on 1915 1916 1917 750 600 450 300 150 11 B T T T 12-1 8-9 12-1 8-9 12-1 1915 1916 1917 1.00 .75 .50 .25 00 .2S .50 FIGURE 11. — Line 794. A. Reproductive indices, actual values. B. Reproductive indices, superiority. C. Reaction-time curves. LINE 795. Line 795 is a sister line of Lines 794 and 796 and was subjected to selection for a like period covering 94 generations for the plus and 95 generations for the minus strain. The data will be found in tables 35 and 36 and in figure 12. 1 For example, during the first 6 months the plus strain of Line 794 was persistently the more vigorous and generally the less reactive. The 7 high points in figure HA showing high reproductive indices (above 1.75) for the plus strain correspond 4 times with periods of greater reactiveness in the same strain, and 3 times with periods of less reactiveness; 5 similar high points of reproductive indices for the minus strain correspond with 1 period of relatively greater re- activeness in the minus strain and with 4 points of lesser reactiveness. The two-month periods of relatively low reproductive indices for the two strains Likewise fail to correspond with periods of lesser reactiveness to light. 78 SELECTION IN CLADOCERA ON THE BASIS OF Casual examination seems to reveal at once the fact that there is no effect of selection, and this is probably correct, but on careful analysis it is not altogether certain that an effect of selection is entirely lacking. The plus strain is generally the less reactive, but the curve for the minus strain is extremely fluctuating. For the first longer period (8 months) the mean for the plus strain was 703.3 seconds, for the minus strain 571.4 seconds (table TABLE 35. — Selection summary for Line 795. -jj o> M E S . a o S B •3 £ Time period. Strain. "o •*» -S d "S T3 .S M "oS 3 "S inimu I oJ 03 3 3 '— _c3 •M o S ^~ 0 lit _Q C_ "o III T3 Q o 0 * "g "o S ~S . • •* Q S g B •o V sl fe o) .; .49 _2 CJ O o g B "g •" C3 OJ o •E Si C i _o a £ a tc Jr ^•1 **"* f M . hfl 03 £1 o a o t-t C CJ »a — .a "S 32 o-g ej o B > •- > .§ 03 g OJ .g B 03 •8 »§'•§ •8 o •+a o ' OJ a DO B . B OJ o Cl ;r^ ^ a 03 03 C3 C3 g OJ P. 3 "u *S "*"* 2! "o C3 M .»« ta ^ O OJ fl °3 OJ 03 *_- a g B a "8 1 *} I'D M S "o3 3 O '-3 •3 g "S S 2 *o o 2 Time period. Strain. m •a & rs -a *Q 'x 12 i ^ (5 i-1 o -a d "C '^ .« a OJ ^ OJ C pO 3 Jj *3 o 0 •3 03 tr m T3 's B •3 Q "O C OJ T3 ^ g, • — B g in rt V M . M . E 03 3 (D 0] 3 § fc Q Q cS oj B OJ 03 g 03 t- V d !•§ d . '> . o! o o kt — at Q B •X' I |1 1 (D §3 Dec. 24, 1914- ( Plus. .. 12 11.6 139 8 70 363 900 661 277.77 15.89 + 63 23.89 2.63 July 31, 1915 ( Minus . 12 10 7" 128 S 57 354 57S 598 ^99 J7 17.84 Aug. 1, 1915- ( Plus . . . 12 13!5 162 3 88 334 725 617 323 . 84 17.16 + 138 26.74 5.16 July 31, 1916 ( Minus . 12 JO. 2 1 23 ^ 45 278 C1Q 479 337. 18 20.61 Aug. 1, 1916- ( Plus . . . 5 12.6 63 0 *t" 9 117 714 *T ' " 307 263 21 22 37 May 1, 1917 ( Minus . 6 73 SO 458 900 671 313.97 24.78 -364 33.38 JO. 90 35). The difference was +131. 9 ±15. 7 seconds, or 8.4 times the probable error. The 12 same-day broods for this period likewise gave a mean for the plus strain in excess of the minus strain (table 36). The difference was +63 ±23.89 seconds, 2. 63 times the probable error. For the second longer period (August 1915 — July 1916) of the data for this line the means were 625.0 and 530.7 seconds for the plus and minus strains. The difference was +94. 3 ±14. 2 seconds. This difference was 6.64 times the probable error. Twelve same- A PHYSIOLOGICAL CHARACTER. 79 day broods averaged 617 and 479 seconds, the difference being + 138 ±26.74 seconds. A test series conducted in June 1916, and consisting of more than 1,300 individuals of each strain, gave a plus mean of 615.4 seconds and a minus mean of 591.6 seconds. The difference (+23. 8 ±8. 9 seconds) was 2.67 times the probable error. Thus the data for this line to August 1916 indicates a signifi- 0 t < 2.00 1.50 A o ^ 1 11- s T ^ I 100 _ o O o .50 _ IZ-I 4-5 8-9 12-1 4-5 8-9 12-1 <* nn 1915 1916 1917 750 600 4-50 300 150 B T. 12-1 4-5 e-s 1915 12-1 4-5 8-9 1916 12-1 4 1917 LOO .75 .50 .25 00 .25 .50 FIGURE 12. — Line 795. A. Reproductive indices, actual values. B. Reproductive indices, superiority. C. Reaction-time curves. cantly lower reactiveness on the part of the plus strain, a result op- posite that of a selective effect. For the following and final 9 months of selection with this line the plus strain averaged the more reactive; the means were 457.2 and 502 seconds, and the difference was —44. 8 ±17.6 seconds, or 2.55 times the probable error. The 5 same-day broods gave as means 307 and 671 seconds. The minus same-day mean was more than double that for the plus strain, the difference being —364 ± 33.38 seconds, 10.9 times the probable error. Of these 5 same-day broods, • 80 SELECTION IN CLADOCERA ON THE BASIS OF 4 occurred in succession in April 1917 and constituted the final data for this line. The writer believes that considerable stress may be safely placed upon the same-day-brood data and is inclined to think that at the close of the experiment with Line 795 the plus strain was actually and significantly the more reactive. It is quite possible to suppose that the considerably higher reaction-time of the plus strain during the early part of this experi- ment (whatever may have been the cause and meaning) was gradually reduced by selection and that at the close of the experiment the plus strain was actually the more reactive, due to the influence of selec- tion. But in the absence of a test series at the close of the experiment, and without knowledge as to what the relative reaction-time means for the two strains would have been in later generations, with or without further selection, the supposition is not worth consideration. It is not at all improbable, however, that the differences in different parts of the curves for the two strains represent merely low and high points in the reactiveness of the two strains due to non- genetic influences, the plus strain being relatively slightly reactive and the minus strain unusually reactive during the early period, while the reverse was true during the later part of the experiment. Such fluctuations in the reactiveness of the two strains of the same line for considerable periods are seen in several cases, notably in Line 695 (figure 2c) and Line 740 (figure 15). In this line from June- July 1915 to April-May 1916 there was a general rise in the reproductive index for the plus strain and a general fall in the reproductive index for the minus strain. The result was a wide divergence in reproductive indices for the two strains culminating in April-May 1916, when the reproductive index for the plus strain was nearly twice that for the minus strain (figure 12A). This may be thought a result of selection acting in a cumulative way upon the vigor of the 2 strains, but there was no relaxation in or change in the method of selection, and yet the minus strain later reached as high a point in reproductive index as it had attained at any earlier time (though in general it remained lower than the plus strain). While the reproductive indices indicate (figure 12 A) that the plus strain was in general very much the more vigorous of the two strains, one fails to find (as always) any direct relation between reproductive index and reaction-time. The generally much higher reproductive index for the plus strain, associated as it is during most of the experi- ment with a considerably higher reaction- time mean for this strain, in itself serves as a general denial of an association in this line between vigor and reactiveness to light. LINE 796. This was a sister line to Lines 794 and 795. Selection was begun at the same time and continued for the same period as with those A PHYSIOLOGICAL CHARACTER. 81 lines (29 months), representing 95 generations in the plus and 91 generations in the minus strain. The data are given in tables 37 and 38 and figure 13. The courses of the reaction-time curves are extremely irregular, although in a general way the curves follow each other in a manner to suggest environmental influence upon reaction-time. It is obvious that there was no effect of selection. TABLE 37. — Selection summary for Line 796. ID 1 ( t i t t C H a a a * CD t oS c3 3 T3 J ^ U) M.*' '£ o a D fl o o •8 . _> CO jn C C £3 ^ 03 u 1 d 03 CD OJ S. a o 0 i lg -— CD 'S -S 03 CD .*• QJ a a N a 99 CD a a } 3 "3 3 3 = *oj •S "o V *o CD !S Time period. Strain. •M o GO 0 tj "" C _> .-, "^ •g •p •S O (U CO g m a o 1 o d o o Q ^^ . c3 O C 03 . Mjn > a "O w a a a 03 a •3 OJ •a ft O si s •3 ti ^ ^J3 _a oj 0 a § •- • 03 CD CD p g J-5 a ' ^ a £ oj ^~* -4 ., t M M . 03 f*» S a J3 LH OJ a o "o 6 OS J2 — . " S3 "" ~' ~~" o2 . > 6 cu 03 tu a -_• o3 a o .g •s 03 03 O fc. O 03 •8 § ^ O * * ° EH « £^ Z u ^ ^ §" 03 & Q S £ SX1 Dec. 25, 1914- ( Plus . . . 1-28 28 12.3 34.3 4 117 244 876 555.5 289.5 10.5 July 31, 1915 t Minus . 1-27 27 J3.0 35; Jo 147 604.8 290.9 10.5 -49.3 14.9 3.31 Aug. 1, 1915- ( Plus . . . 29-66 38 15.9 604 13 301 364 857 619.1 291.9 8.0 July 31, 1916 t Minus . 28-61 o'.-; 8.0 .J75 SS9 697.5 297.2 -78.4 14 7 5.33 Aug. 1, 1916- 1 Plus . . . 67-95 L'2 13.7 302 5 98 220 692 449.4 331.1 12^8 + 55.7 19^1 2.92 May 1, 1917 I Minus . 63-90 '( ) 9.£ .79.2 * 49 ;^4 7iO 393.7 311 .9 14.1 Test series, t Plus . . . 68 I'd 33.1 662 15 379 256 900 675.6 286.6 7.5 + 21.9 11.0 1.99 Aug. 1916 ( Minus 64 '*() 3.2 2 643 8 3<9S 189 900 653.7 302 8 S / July 1, 1916- ( Plus. .. 65-71 "r 16.6 116 1 76 346 828 662.4 201.3 12.6 +'208.6 25.1 8.31 Sept. 30, 1916 ( Minus . 61-68 7 /•4.1 99 3 28 ^43 849 453 . 8 319.9 21 .7 TABLE 38. — Same-day broods. Summary of data for Line 796. h OJ i a o i a a a d o f I 08 Q. O o _o 03 •M S _Q 60 60 a a O CD a a ^ o o y a • OJ M a ~ a 03 03 03 a S S Ck 3 o 03 o3 ^J £i b S "o 03 CD to J3 S 7) ^ a a a a •£ •3 CO "* ^ 3 M 3 "3 _o Mm S en IM 1 Q 08 g o &* W Time period. Strain. m 13 a> ^2-a ^a '3 _a 'S 03 'P S & 3 •g.a M O '? T3 O 03 OJ JH o C •3 C O Z ^ oS . c£ CO •3 « 1 a 1 •a S3 *LJ Q OJ oj .; u. ja CD a a "2 "" 03 CJ OJ £ S "^ fl M CJ a 2 ^ ~" 3 bC . 60 . ^ 0) cS J3 OJ fc "o d fc 8 d O .— . > "o •§ . 03 O CD " -^ CJ Q> II •a a oS 1 S to "* *Q. 03 £ C OJ OJ S- * •• o - O IZ-l 4-5 8-9 12-1 4-5 8-9 1M 4 1915 1916 1917 750 - 600 450 - 300 - 150 - FIGURE 13. — Line 796. A. Reproductive indices, actual values. B. Reproductive indices, superiority. C. Reaction-time curves. (The asterisk indicates a point established from insufficient data, only one brood.) 393.7 seconds. The minus strain was the more reactive by 55. 7 ±19.1 seconds, 2.92 times the probable error. The same-day-brood means (only 4 broods of each strain) differed by 220 seconds, the minus strain being the more reactive. A test series conducted during August 1916 consisted of 662 and 643 individuals in the two strains. A PHYSIOLOGICAL CHARACTER. 83 The means were 675.6 and 653.7 seconds. The difference was +21. 9 ±11.0 seconds, 1.99 times the probable error. Hence, for the first 2 longer periods the means for the entire data gave higher reaction- times for the minus than for the plus strain. These differences ( — 49.3 and —78.4 seconds) were 3.31 and 5.33 times their probable errors, but the lack of confirmation of these differences from the same-day-brood data at once throws doubt upon a real significance for these differences; while for the final long period of the experiment with Line 796, the test series, the selection data as a whole, and the same-day-brood data confirm each other in indicating a greater reactiveness on the part of the minus strain. The data for this line would seem to present these points of interest: a lack of selective effect, an effect of environmental in- fluences upon reaction-time as indicated by the courses of the curves, they following each other in a general way, and the lack of any marked evidence of a relation between vigor and reaction-time ID a case in which the two strains differed extremely in vigor. This last point may be worthy of a little further statement. The plus strain was very much the more vigorous during most of the experiment, yet the differences in mean reaction-time were not such as to indicate, everything considered, a significantly higher reaction-time for the minus strain than for the plus strain. Indeed, the same-day-brood data quite fail to confirm any significant dif- ference in reaction-time between the two strains for any part of the experiment. During the 3 two-month periods during which the differ- ence in mean reproductive indices were greatest (figure 13B), the minus strain was less reactive, but for 2 of the 3 other periods (in which the inferiority in reproductive index for the minus strain was almost as great) the minus strain was actually the more reactive. There were altogether 9 periods of very low reproductive indices (about 1.00 or lower) for the minus strain, and these were periods during which the plus strain, with two exceptions, ran fairly high in vigor (reproductive index over 1.50), and hence the difference in vigor was marked; yet the minus strain, far from being markedly less reactive, was during 4 of these periods actually the more reactive. It was the less re- active, however, during the other 5 periods and in 3 cases the differ- ences were large. Though in general the reproductive index for the minus strain of this line remained lower than that for the plus strain, it regained a high level (1.7) soon after this divergence in reproductive indices was most marked. Thus, while in both Lines 795 and 796 there was an apparent cumulative effect of selection upon the vigor of the minus strain, in both cases the effect was lost and the minus strain later attained relatively high reproductive indices, although the method and pro- cedure of selection were in no wise modified. 84 SELECTION IN CLADOCERA ON THE BASIS OF LINE 740. Line 740 was the first line of S. exspinosus subjected to selection. It was obtained in August 1912, from Pond IV, in which S. exspinosus occurs the year round. Selection was begun at once and continued for 181 and 184 generations, for a period of more than 56 months, TABLE 39. — Selection summary for Line 740. m "m ^ o • '- fl fl i o 03 03 . CO .*+ o _o .2 03 M w 43 T3 0 > en — ^ M || '•4J o 03 5 03 o 5 o a fl a 03 o fl 2 0. d « O 03 1 .9 "£ || a 03 a> ,y~ Q) to *-• S Q S o M » 03 Q> a a n o 5« i »*^ T3 Time period. Strain. 1 *o 3 •+d m °l " tfi "" G "3 3 *0 | 3 •a 1 03 "o o o O m 0 S Q> •o •p "3 6 o 03 O •3 1 O O fc-o o *^ . os M oi •'"' QJ 1 a •a T3 o « a 0< o o a) o o g fl "g G G 03 9) •2 •a « S "^ _0) li M^ £4 ... M . a rt S a 3 o t- "o "^ ° "o "d 03 u. o 03 L- 0) OJ fl • a £ 03 "" "S"B 6-5 . 03 O 0) * 1 fe a S " fl 03 O ta '•§ 1 0 z ^° *^ <~ **" ^^ m £ 3 « £ 5^ Aug. 27, 1912- ( Plus .... 1-34 37 11.5 427 o 291 384 872 735.2 266.0 8.7 July 31. 1913 t Minus . . 1-34 36 9.9 357 0 259 399 554 757.0 244-7 8.7 -21.5 12.3 i.77 Aug 1, 1913- i Plus .... 35-76 40 10.6 424 5 184 359 881 597.1 298 7 9 8 July 31, 1914 ( Minus . . 35-79 45 10.7 452 275 570 656.9 302.4 9.3 -59.8 13.5 4.43 Test series, ( Plus .... 65 23 28.4 653 1 358 173 900 666.4 294.6 7.8 Apr. 1914... t Minus . . 66 23 614 391 243 900 711.9 275.3 7.6 -45.6 10.9 4.17 Mar . 1914- i Plus .... 61-68 8 n'o 88 1 36 347 870 569.2 May 1914 . . * Minus . 62-71 10 12 6 126 o 56 269 534 596 S —27.1 Aug. 1, 1914- j Plus 77-118 42 10.8 452 16 244 305 '-"-''T 886 642.5 307.6 9.8 +34.8 is. 8 2.52 July 31, 1915 \ Minus . . 79-119 40 11.4 454 / 205 351 539 607.7 306.0 9.7 Aug. 1, 1915- i Plus .... 119-153 35 14.3 471 2 227 303 828 574.6 335.6 10 4 July 31. 1916 ( Minus . . 120-155 35 15.3 262 290 520 600.0 326.9 9.7 -25.4 14. « 1.79 Test series, j Plus 142 34 31.7 1078 5 628 202 900 641.6 324.6 6.7 + 63.7 9.5 6.71 Mar. 1916. . i Afinus 143 34 32.0 1089 7 523 175 572 577.9 331 .7 6.8 Feb. 1916- ( Plus .... 139-146 t'*T 8 17.9 143 2 82 359 900 653.8 + 12.0 Apr. 1916. . . t JMinus . 140-147 5 17.5 140 0 81 531 641 .5 Test series, i Plus .... •*• *r^* * *r ' 151 32 38 5 •* rr*^ 1231 4 199 113 817 420 2 318.8 6.1 June 1916. .. I Minus . . 152 32 40.1 1252 0 570 525.7 468.5 8.8 -105.5 10. S 10.24 May 1916- i Plus .... 147-153 6 14.3 86 o 45 234 725 574.7 July 31, 1916 I Hfinus 145-154 6 15.5 111 3 75 413 539 710 4 -135.7 Aug 1, 1916- i Plus .... 154-181 26 12.0 252 3 27 -ft u 113 631 r A w • «p 311 1 243.0 10.3 May 1, 1917 t Minus . . 155-154 24 13.5 324 0 153 669 395.5 334.0 12.5 -54.7 16.2 5.23 Test series , ( Plus .... 179 26 29.1 756 1 38 79 716 314.5 256.3 6.3 Apr. 1917... ( Minus . . 181 26 PQ 1 756 0 31 106 675 329.5 267.0 6.5 -15.3 9.1 1.65 Mar. 1917- ( Plus 175-181 6 \2.7 76 0 8 102 560 262.0 248.8 19.2 + 96.3 21.1 4.56 May 1, 1917 ( Minus . . 175-154 7 13.6 95 o y 59 421 165.7 124.6 5.6 •pw* >• TABLE 40. — Same-day broods. Summary of data for Line 740. S 2 o fl i fl fl fl hi £ a •o 03 S Q) fl a '3 ta ^ *o (D S "o3 14-1 | a s "3 fl o S IM O •o M "3 pj? 3 0 3 •** "o > 2 •S Time period. Strain. on o J 1 'fl •a "S 1 hi •g.2 S •g "§ 1 ;? ta . r* fl •g 03 •3 "0 E x> a o •3 o •3 bo n M fl « u £ « a s« •- 03 0 0 •s .2 O ^3 fl g — a S, M eg 01 03 *o g o 0 0^ 0 "S 03 oj 71 gj fl o5 •B 03 0> 00 03 u. m « 2 . > . 03 a; H o a 03 fl fl ^ iG to ® 6 d Is3 o • r* o ft a> XI d 1 .^4 •B a 09 h J* m f _O o d 09 S *o o 1 cj oj -a a d "3 3 *0 05 o eJ 0 m o S d •4 0 E H S M OJ 4_> '> a kl oj o d d o a E JS • S o Q ft H O "o s o »; >, a •8.B a o IM O 3 "8 03 . & i. QJ «—« ~" 73 « a a — C3 Ii 2-S _B c "8-3 '.3 1 d CT CO O 1 3 o '-' _j3 GO g Q o 3 03 *S CJ i O O . * o o. S ^f] £] a 1 *•"* I** •M a ** to ft &4 CO +J _o o a 2 o e 0) p ft 1 o H 03 a o 1 CJ . -3-0 CO O *~ £ a, M x S «•-> o o a a 3 O _3 'S i> "S . •3! 13 a a •" 03 **-" — 2"S § § 1- J| o-S a o o a V a P1 CO "o m a o o d > _o CJ L ""rf tu ' ~" *rj o H o a c a ~V 3 0 o c3 o 03 •43 O Y CO 9 '43 "E M ,_. "^ "* > 3. -a 0 03 d fe 0 ID a g bO o +3 > c CH H 09 o n a _o ii O. o 3 O -J a 3 0 M 3 49 o! 03 . CtO (7) "3 a a a „; a . **-" CO 0 03 cr CO "rf .t l o **-! 0 t*» C 2 03 3 -5 2 -S 0 t3 IM s ° J2 b "°.§ O C.J4 *o 12 "o -° .i § a a • r- 1 Q d o |H d 0 s a * 2 o3 .a 0 Si *> o a g o *** . 03 O OJ C3 O d S 3 « 03 03 a. a O B <; a H~ * £ ** S w ^5 °° ffl " * — +-• tn 1915 Z5 Nov. 29 4 0 21 15.5°C. 5 2 145 900 7895 376 3806975 A» Dec. 8 4 0 20 15.8 3 0 165 615 7115 356 2827225 B, Dec. 17 4 0 23 20.2 1 0 120 580 6145 267 1866475 C8 Dec. 27 5 1 21 14.9 5 1 95 900 5400 257 2031375 1916 Da Jan. 7 8 0 5 16.5 0 5 900 900 4500 900 4050000 Ea Jan. 17 9 0 6 16.8 0 0 140 240 1105 184 210725 F9 Jan. 26 5 0 20 15.1 0 1 110 900 5420 271 2113350 G« Feb. 4 9 0 17 12 0 4 60 900 7060 415 4527250 Ha Feb. 15 8 0 15 16.6 0 6 200 900 7750 517 5206900 la Mar. 1» 9 0 26 17 0 26 900 900 23400 900 21060000 J« Mar. 13 2 1 17 18 0 3 120 900 6225 366 3436175 Ka Mar. 22 4 0 19 15.9 0 3 90 900 5690 379 3423300 La Mar. 31 9 0 5 15.1 1 0 135 580 1770 354 756750 Meb May 4 7 0 4 14 0 3 190 900 2890 723 2466100 Na May 15 4 0 24 15.8 0 3 170 900 8625 359 2625625 Oa May 25 3 0 17 18.5 0 16 210 900 14610 859 13004100 Pa June 8 3 1 7 19.6 0 4 175 900 4370 624 3455450 Qa June 19 4 1 15 20.2 0 5 270 900 8805 587 6066825 Ra June 28 5 1 13 20.6 0 0 75 805 4483 345 2002129 S6 July 5 4 1 11 18.9 0 1 110 900 3560 324 1777550 Ta July 12 5 1 11 23.7 0 1 150 900 3520 320 1565800 Ua July 18 3 1 9 21.5 0 0 130 330 2060 229 506400 V. July 24 4 1 8 22 0 4 90 900 4390 549 3418900 W9 Aug. 1 3 0 19 20.6 0 6 85 900 10240 539 6987650 Xa Aug. 9 Selection made but record incomplete. Ya Aug. 16 11 0 14 19.9 0 1 120 900 4110 294 1737800 Z6 Aug. 22 3 0 8 21.8 0 8 90 900 7200 900 6480000 AT Aug. 28 3 1 14 18.5 0 1 90 900 3770 269 1671700 B7 Sept. 5 2 1 13 20.5 0 0 60 390 2360 182 559150 C7 Sept. 11 3 0 14 16.5 0 7 210 900 8780 627 6614000 D7 Sept. 30 Random distribution. E7 Oct. 21 3 Random distribution. F7 Oct. 31 22 Random distribution. G7 Nov. 10 4 0 11 17 || 0 0 70 390 2180 198 500200 H7 Nov. 20 14 Random distribution. I7 Nov. 29 9 Random distribute nn JT Dec. 11 2 1 12 11.5 H 0 1 130 900 4480 337 2272600 KT Dec. 20 3 0 6 14 II 0 1 190 900 2220 370 1173600 L7 Dec. 30 19 Random distribution. 1917 | M7 Jan. 10 2 0 12 18 0 5 130 900 7810 651 6008700 N7 Jan. 19 7 Random distribution. 07 Jan. 30 4 1 6 15 |l 0 0 40 180 560 93 67400 P7 Feb. 7 Random diRtriHntinn. Q7 Feb. 16 2 0 9 9.8 0 0 120 460 1895 211 509025 R7 Feb. 24 3 0 14 12 0 2 90 900 3730 266 2003100 S7 Mar. 4 2 0 15 11 0 3 100 900 5020 335 2947000 T7 Mar. 13 3 0 12 13 0 2 70 900 3946 329 2332716 U7 Mar. 21 3 0 16 14 0 0 60 270 2205 138 352225 V7 Mar. 29 2 0 8 14 0 0 80 170 1010 126 134300 W7 April 9 3 0 12 12 0 0 50 160 1310 109 161100 X7 April 18 2 1 10 15 0 0 40 525 1671 167 466759 Y7 April 25 2 0 20 14 0 0 60 185 2210 111 265216 •Second brood from this mother. b The Me generation was taken from a very late brood of the Ls generation which was used in con- ducting a test series. 94 SELECTION IN CLADOCERA ON THE BASIS OF TABLE 42. — Summary of selection data by broods for Line 757 minus. B A "3 c o 49 B a B 00 73 t B 13 V a O '1 "o o e M a 2 fl o e — C w Q) £H O w a W o 0> •a •c 0 *c3 "^ '^3 S • IP o S C3 **-• C ',Q O O .10 .S o o. 0 a a l •** •B a -fc^ CO M M OT •»- O o a G) ^> O _o a I * •§ a d "aJ a *o 0 05 5 a 2 '•3 S o "C a c w g .£: .H "d > G 0) a a) a | a o 11 D. H IS a 3

••> ,0 o ^ *03 'i '£ .2 J P .2 o 1 1? .2 W O £ "3 •s o 3 -a > a C) 03 o o a £ o 03 3 1 a .2 OB 03 3 d o 'u TJ 00 O 3 o •g' o 3 S^ "T3 e C ,-. o c3 g 1 S3 o * "8 h "o .5 0 03 l< si "33 •- 03 S ro 11 3 » I "^ a Q V IM 0 Z i i j O s| 6 I a . 03 o a> .a § M ^ 3 a a 3 T5 o Q~ w <3 ft * H~ ZT> Z * S « s • cc B 2 «•* 1916 c, Jan. 11 6 1 11 18 0 11 900 900 9900 900 8910000 D« Jan. 23 3 2 10 19 0 10 900 900 9000 900 8100000 E, Feb. 1 5 0 6 11.2 0 6 900 900 5400 900 4860000 F« Feb. 10 8 0 18 13.2 0 18 900 900 16200 900 14580000 G6 Feb. 19 3 0 14 15.5 0 4 140 900 6695 478 4428675 Ha Mar. 1 4 0 13 15 0 11 420 900 10920 840 9446400 la Mar. 11 4 0 12 16.4 0 7 90 900 7325 610 5909325 Jf Mar. 21 3 0 13 15.8 0 13 900 900 11700 900 10530000 K» Mar. 31 9 0 16 15.2 0 16 900 900 14400 900 12960000 Ls April 18 *7 1 Me May 2 3 0 15 16.5 0 13 220 900 12210 814 10662500 N« May 13 10 0 18 16 0 18 900 900 16200 900 14580000 O« May 24 4 0 22 18.3 0 21 150 900 19050 866 17032500 P. June 1 2 0 13 18.5 0 7 65 900 8435 649 6765375 Q« June 12 4 1 18 17 0 17 240 900 15540 863 13827600 R« June 22 4 0 12 18.6 0 0 60 450 3305 275 1141325 S, June 29 4 1 19 19 0 11 135 900 11825 622 9453525 T» July 6 8 0 11 19.2 0 11 900 900 9900 900 8910000 U« July 14 4 1 7 21 0 6 175 900 5575 796 4890625 V. July 22 1 0 6 21.1 0 4 80 900 3770 628 3254900 TY",b July 28 o 11 21 o 7 Xe Aug. 4 11 0 3 19.3 1 2 420 900 2220 740 1796400 Y« Aug. 11 2 0 7 19 0 3 50 900 4110 587 3169150 Z, Aug. 17 3 0 8 20.6 0 7 260 900 6560 820 5737600 A7 Aug. 24 9 1 8 21.3 0 8 900 900 7200 900 6480000 B7 Aug. 30 1 0 9 17 0 4 360 900 6470 719 4984500 C7 Sept. 6 3 0 12 19.5 0 9 220 900 9130 761 7702900 D7 Sept. 13 1 0 10 17 0 8 330 900 7880 780 6711400 E7 Oct. 20 Random distribution. F7 Oct. 29 21 Random HistrihntJon G7 Nov. 8 2 0 16 16 0 14 300 900 13200 825 11520000 H7 Nov. 17 2 0 14 13 0 12 64 900 11014 787 9746596 I7 Nov. 25 3 0 8 10.5 0 8 900 900 7200 900 6480000 J7 Dec. 7 2 0 7 13.2 0 1 50 900 2160 309 1208800 K7 Dec. 16 2 0 20 11.5 0 8 60 900 8990 450 6774100 L7 Dec. 25 Random distribution. 1 1917 M7 Jan. 4 2 0 6 12.5 0 0 100 210 795 133 113725 N7 Jan. 13 4 0 10 11.2 0 2 80 900 2670 267 1717750 07 Jan. 20 4 0 14 12 0 0 100 480 2970 212 735700 P7 Jan. 30 4 1 26 15.5 0 21 380 900 21150 814 18094900 Qi Feb. 10 3 1 4 10.5 0 2 450 900 2830 708 2158900 R7 Feb. 19 3 1 11 11 0 2 150 900 4560 415 2699800 S7 Feb. 28 2 1 7 10.5 0 3 110 900 3000 429 1996200 T7 Mar. 9 3 0 9 17 0 7 50 900 6440 716 5680600 U7 Mar. 17 Random distribution. V7 Mar. 26 3 1 10 15.5 0 7 110 900 7140 714 5948600 W7 April 3 2 0 16 15.2 0 9 130 900 9920 620 7822000 X7 April 11 3 0 15 13.5 0 4 270 900 8220 548 5382400 Y7 April 21 11 0 14 18 0 10 25 900 9419 673 8165063 •This selection made from a fourth brood of 71 individuals constituting a part of a test series. Data cot included here as not strictly comparable to other data of this table. b Selection made, but time of beginning of experiment inadvertently omitted. the plus was the more reactive during every month except for 4 months during the period of irregularity in reactiveness in 1914 and 1915 (see pages 115-116 and table 44). Averages of the data by two-month periods (tables 41 and 42), such as are given for Lines 695, 689, 711, and 719, serve still further to compensate for temporary irregularities in reactiveness and are used in the analysis of the data for line 757 and as a basis for the curves given in figure 18s. A PHYSIOLOGICAL CHARACTER. 97 DETAILED ANALYSIS OP DATA FOR LINE 757. The curve of the reaction-times for Line 757 (figure 18s) begins with the December 1912-January 1913 period, though it includes a few reaction-time records obtained in November.1 The means for this period are identical, but in the following period there is a slight divergence (the minus strain being the less reactive) followed by irregular but in general increased divergences in the later two-month periods, so that the course of the curves is from the start suggestive of an effect of selection. There is one period of two months (June-July 1914) for which the minus strain had the lower reaction-time and the difference was fairly large (132 seconds; table 44). In 3 other two-month periods the minus strains had mean reaction-times only 2, 20, and 48 seconds higher than those for the plus strains for the corresponding periods. But in all the other two-month periods of the experiment after the first 5 months of selection the mean reaction-time for the minus strain was from 55 to 629 seconds higher than that for the plus strain — e— 1 2 17 4 0 • 3 4 4 ll-l 1912 8 0 & 0 0 0 4-5 8-9 1913 7 10 0 O O O 1 2 1 12-1 4-5 8-9 1914 O O 0 0 12-1 o o 10 o 3 .3 3 0 4-5 8-9 1915 0 6 12-1 ' 999 0 ,4-5 8-9 1916 12-1 ' 9 4 1917 FIGURE 16. — Line 757. Relative rates of descent of the two strains. (See figure 2A for description.) (tables 43 and 44). Hence the mean reaction- time differences by two-month periods are nearly all in the direction sought in selection, and most of them are differences of statistical significance. Calculated by longer periods, all the differences are of large statistical value, the minus strains being the less reactive. This will be made clear in the following detailed analysis. REACTION-TIME MEANS COMPARED BY LONGER PERIODS. For the first 9 months of the experiment (November 1912- July 1913) the means for the plus and minus strains (29 generations and 320 individual reaction-time records, and 28 generations and 261 ^____ ^^______ 1 Data for two abnormally small broods of each strain were obtained during November. These data consist of the reaction-times of only 6 individuals of the plus strain and 8 individuals of the minus strain, and of course give averages of little value, so that it seems best to combine these data with the data for the following two months in constructing the reaction-time curves. The means (see tables 43 and 44) for the two strains for November alone are 685 seconds for the plus strain and 900 seconds for the minus strain. All 8 of the minus strain and 4 of the 6 of the plus strain were "over-time" individuals. These meager data have little significance, except to indicate that both strains were very slightly reactive at the beginning of the experi- ment. For the December 1912-January 1913 period alone (excluding the few November records) the means are 747 and 718 seconds. Hence the curves with the data for November separate would start with the minus strain, the less reactive by 215 seconds, to be followed by the December- January period, when the plus was the less reactive by 29 seconds. In tables 43 and 44 the data are presented with the November data separate from those for the December- January period. 98 SELECTION IN CLADOCERA ON THE BASIS OF individual reaction-time records, respectively) were: for the plus strain 788.3 seconds, and for the minus strains 838.9 seconds (table 45). The difference (— 50.6±11.18 seconds) is 4.53 times the statistical probable error. These data include those for the 5 months at the beginning of the experiment, when selection was only slightly if at all effective. For this nine-month period there are 9 same-day broods, containing 114 and 99 individuals, with the reaction-time for the minus strain averaging 58 ±16. 24 seconds greater than that TABLE 43. — Selection data summarized by two-month periods for Line 757 plus. •*J 1 (-. CJ t t i •d ft 03 0 ra a. £ bC a£ M M.W d ^o *^j a ^o '+i d o d 09 •fa CO CD M a OJ CD •S o M S §03 ^d '•3 o rr3 •*-> O 03 V O c3 O BQ g a d O3 CD •*• 0 o Ef O 03 *2 GJ ** *"* 0 "3 CD 0 M ••> e . ^.fl CD ,2 « g S _B a S £j .t t *O ,l_< ^ 03 •] i ^ 3 '.^ 03 o •M "O O o *o 2 O 13 oo ^-" 3 O § 13 3 *^> o Time period. •M O m a c |S M s .2 c3 "2 '•3 03 CD T3 S fe .2 .C 6 CD . GJ tg ^s 0) ~ C § .2 c CD Ui d •a 'oJ 65 "o _ 03 "§ ^ C) CJ [f'g'S "o 2 "o J3 M cs a5 M . "o d c 03 •d 2 03 d CD d o CD 0 l> *" ^ rt O Qj C 3 >^-o I £ S d *^ . e3 O CD OJ S fc g Q CD .§ a 03 •8 a X H < <•*« * 2 tH <^ ** aj S" CO £ Nov. 1912 1-2 2 6 3 0 13.7 0.22 o 4 495 900 318 01 87 *>7 Dec. 1912- Jan. 1913 3-9 7 82 11!? 0 59 245 900 61250 747 274 '. 69 Of . *J I 20.36 Feb.-Mar. 1913 . . . 10-15 6 72 12.0 9> l'24 0 55 407 900 57805 803 198.86 15.81 Apr.-May 1913... 16-22 c, 82 13.7 8.4 1.63 0 66 496 900 65632 800 208.50 15.53 June-July 1913. . . 23-29 8 78 9.6 7.4 1.30 1 63 679 900 63440 813 196.47 15.00 Aug.-Sept. 1913.. 30-38 9 120 12.3 6.4 1.92 1 75 389 900 81986 683 301.07 18.54 Oct.-Nov. 1913. . . 39-45 6 50 9.1 8.2 1.11 0 30 309 862 35109 702 270.01 25.76 Dec. 1913-Jan. 1914 46-52 8 82 9.7 8.8 1.10 0 33 279 811 48045 586 309.38 23.04 Feb.-Mar. 1914 . . . 53-59 7 104 14.9 8.3 1.80 2 57 348 900 67229 646 296.25 19.59 Apr.-May 1914... 60-65 6 72 12.0 8.2 1.46 0 46 423 900 51828 720 266 . 32 21.17 June-July 1914... 66-73 8 76 9.9 7.6 1.30 0 61 395 900 60373 794 227 . 95 17.64 Aug.-Sept. 1914.. 74-81 7 80 13.3 7.1 1.87 2 29 241 791 44890 561 298.18 22.47 Oct.-Nov. 1914. . . 82-89 8 119 15.0 8.3 1.81 2 69 243 859 78240 657 303.13 18.74 Dec. 1914-Jan. 1915 90-96 7 69 10.2 8.2 1.24 1 44 493 900 49288 714 269.78 21.91 Feb.-Mar. 1915... 97-102 6 66 11.4 9.5 1.20 0 16 121 742 27182 412 305.76 25.38 Apr.-May 1915... 103-108 5 53 11.0 9.7 1.13 1 24 261 863 34563 652 281.37 26.07 June-July 1915... 109-116 8 85 9.9 7.6 1.30 0 34 304 815 51316 604 281.06 20.56 Aug.-Sept. 1915.. 117-123 7 60 10.0 7.4 1.35 0 26 187 900 35235 587 323.40 28.16 Oct.-Nov. 1915. . . 124-130 7 132 17.8 9.2 1.93 5 31 132 763 57260 434 270.87 15.90 Dec. 1915-Jan. 1916 131-136 6 95 17.8 9.8 1.82 9 7 255 689 29685 312 200.72 13.89 Feb.-Mar. 1916. . . 137-142 6 99 15.2 10.3 1.48 1 42 251 847 51895 524 336.60 22.82 Apr.-May 1916... 143-145 3 45 17.8 9.8 1.82 0 22 190 900 26125 581 255.24 25.66 June-July 1916... 146-152 7 74 10.6 8.3 1.28 0 15 143 805 31188 421 276.37 21.67 Aug.-Sept. 1916.. 153-160 6 82 13.7 6.9 1.99 0 23 109 815 36460 445 309.23 23.03 Oct.-Nov. 1916. . . 161-165 1 11 12.8 11.0 1.16 0 0 70 390 2180 198 78.92 16.05 Dec. 1916-Jan. 1917 166-171 4 36 10.4 10.0 1.04 0 7 123 720 15070 419 298.80 33.59 Feb.-Mar. 1917. .. 172-178 6 74 12.3 8.8 1.40 0 7 87 600 17806 241 232.34 18.22 Apr. 1917 179-181 3 42 14.0 9.2 1.52 0 0 50 290 5191 124 77.53 8.07 for the plus strain. This difference is 3.59 times its probable error. Only one of these minus same-day broods had a mean reaction-time lower than that for the corresponding plus brood. For the year August 1913-July 1914 (44 generations and 504 individual reaction-time records, and 42 generations and 412 indi- viduals in the plus and minus strains, respectively), the mean re- action-times were 683.7 seconds and 783.0 seconds (table 45). The difference in mean reaction-time was -99.3 ±11. 77 seconds, or 8.44 times the probable error. The 1 1 same-day broods for this period (145 and 123 individuals, table 46) gave a difference in reaction- A PHYSIOLOGICAL CHARACTER. 99 time of —135.0 ±21. 74 seconds. Because of the smaller total numbers of individuals of the same-day broods, the statistical prob- able error was relatively large, but the difference was still 6.31 times the probable error. In only one of these same-day broods was the reaction-time for the brood of the minus strain smaller than that for the plus strain. However, during one period, the last two-month period of this year, the minus strain had a reaction-time 132 seconds lower than that for the plus strain. This was a period of rather TABLE 44. — Selection data summarized by two-month periods for Line 757 minus. m 5 o £ i. G o a i G i a S A 13 CD CO a M a CD CD 3 > 3 a M 3 a F-H Gj ^O si o 0 a Q a o e IM a . « 3 a a CD E o a *J S « 00 M G 3 O ii C t3 co O CS (O M a a CD M a B CD _a "o a a a> Q CD G S '3 a •£ CD •a Time period. V •H O en 3 O > "o Q «! °:>^ d"13 2 CD "3 ^, II nimu 3 a •* •*^ a _o '•£ 'a 3 O 'S '> *o IH o CD C" 0) to & 3 -^ a IM O M O § 0 0 "o = d £ !N CD cS • MI -a OS (D Q tp ° I s; a a s s a o Ij o to J8 o z; <-° « > a* z"° ** "ai o 03 1'S 3 a j) **— i o o "o Time period. Strain. "o « £ a _> '3 n M •g C3 g - m hi o •p m a "3 O C O O "o 13 . 03 ta . M <» -* Q T3 "a 03 a •3 •S g XI 4> o S o •3 o cS .0 o « g M fc 1 S 03 ••* "8-g o « . CJ O QJ || £.§ a .9 a 03 Xl O "*? Hi o> ^ 0 * ** EH ° z * ^ < * 03 PH Q * fi Q-0 Nov. 16, 1912- ( Plus. . . 1-29 29 11.0 320 1 247 468 900 788.3 226.8 8.57 July 31, 1913 t Minus . 7-25 25 9.0 267 7 227 639 900 535.9 171.9 7.75 -60.6 77.75 4.53 Aug 1 1913- ( Plus. . . 30-73 44 11.5 504 3 302 357 879 683.7 290.4 8.72 July 31, 1914 t Minus . 29-70 47 70.0 472 4 320 554 753.0 235.7 7.97 -99.3 77.77 5.44 Test s e n e s , ( Plus. . . 34 14 23.0 322 7 74 112 823 409.0 301 7 11.34 Sept. 1913. . ( Minus . 33 14 26.2 367 9 336 406 900 559.7 149.1 5.25 -450.7 72.49 36.05 Aug. 1, 1913- ( Plus. . . 30-42 13 12.1 157 1 98 376 882 691.2 292 0 15 72 Oct. 31, 1913 1 Minus . 29-47 IS 70.4 735 2 727 745 900 553.4 156.4 9.05 -162.2 75.76 5.93 Aug. 1, 1914- ( Plus. . . 74-116 41 11.5 472 6 206 282 829 604 8 305.4 9.48 July 31, 1915 t Minus . 77-774 43 77.5 507 4 334 350 554 706.8 290.5 5.70 -102.0 72.57 7.93 Aug. 1, 1915- ( Plus. . . 117-152 36 14.0 505 22 143 190 811 458.2 294.7 8.84 July 31, 191ti < Minus . 775-753 .37 456 5 376 467 770.2 260.7 7.95 -312.0 77.97 26.20 Test series, j Plus. . . 143 12 54^3 652 0 433 172 900 697.0 309.3 S 17 Apr. 1916. . . t Minus . 742 12 56.4 0 642 759 900 564.6 756.3 4.05 -167.6 9.72 75.35 Mar 1, 1916- ( Plus. . . 140-145 7 16.0 112 1 54 259 854 564.3 314.9 20.08 May31, 1916 t Minus . 735-745 7 75.6 109 0 577 900 542.2 186.9 72.07 -277.9 23.43 77.56 Aug 1, 1916- ( Plus. . 153-181 20 12 3 245 o 37 94 632 313.1 280.2 12.07 May 1, 1917. ( Minus . 754-757 24 77.0 264 0 757 245 554 625.9 333^7 73.55 -312.8 75.37 77.07 TABLE 46. — Same-day broods. Summary of data for Line 757. h. h o i a t a i a a 8 6 03 rv o O o oS IM o x> a 3-s 0 5 43 O a B S . p £ o s _j • •-« ^3 fl 03 03 03 os 2 o H o O g •^ "£ g •s o> a 8 '3 to •3 XI ^ £ M X J^ a a 3 "o3 a a IM a t: QJ OJ •M o •0 Time period. Strain. CD "S 03 "2 1 IJ a .a '3 3 •^ .3 O hi o o> m <£ 3 hi o 1 T3 6 p -*^ .^ C3 •H 03 0) r> '~ £j •3 0 ^ 1 M^ C] S a a "a TS o f^ w oj . 0) ® (-1 J3 CJ o 2 "^ c3 s> 0> •••• "H « a a _2 1 b MT3 bo M . s 2 V 03 3 *o 2 0 "S *o "^ "o-g 03 oj 03 oj a oJ T3 03 h 03 hi Q> d S o ^ d . ]> . rt O CJ fe 8 s a s.i a 03 •8 to|_ XI to ^ ^^ ^^ *" < ** s" DQ £ a B £ (5-° Nov. 16, 1912- ( Plus .... g 12.7 114 o 90 551 900 798.4 213.38 13.49 July 31, 1913 ( Minus . 9 99 0 50 725 900 556.5 733.47 9.05 -68 76.24 3.59 Aug 1, 1913- i Plus . 11 13.2 145 2 88 364 900 670 3 306 29 17.16 July 31, 1914 t Minus . 11 77.2 723 2 707 759 900 505.4 279.47 73.35 -136 27.74 6.37 Aug 1 1914- i Plus . g 12 9 103 0 62 329 859 676.1 299 . 79 19 92 July 31, 1915 < Minus . S 74.7 773 0 705 647 900 575.9 709.76 6.96 -200 21.11 9.46 Aug. 1, 1915- i Plus . 3 18. 1 145 g 57 233 804 537.8 300 . 89 16.85 July 31, 1916 I Minus . S 75^0 720 7 97 504 900 507.7 212.55 13.09 -270 27.34 72.65 Aug 1, 1916- I Plus . 1 6.0 6 0 0 40 180 93 3 50 28 13 85 May 1, 1917 I Minus . 1 26.0 26 0 27 350 900 573.5 754.56 24.45 -720 25.70 25.62 the mean reaction-times differed by —102.0 ±12. 87 seconds (table 45). This is a difference 7.93 times the probable error. None of the 8 minus same-day broods had a reaction-time lower than that for the corresponding plus brood. The mean reaction-time for these A PHYSIOLOGICAL CHARACTER. 101 minus broods was 200.0 ±21. 11 seconds greater than that for the plus broods (table 46). This difference was 9.46 times the large probable error. There were rather striking fluctuations in the mean reaction-times for both strains (and especially the minus) during this year-period.1 While for 10 months of this year the minus strain, judged by the reproductive index, was slightly, though very slightly, the more 1 5 10 : ii . ,i 1 Z* * » J A T ? >IrJ ••I«. l * M - I ° 6 o 66 5 - - It-l 4-5 8-9 12-1 4-5 8-9 12-1 4-5 8-9 12-1 4-5 8-9 I2~l 4 rin 1913 1914 1915 1916 UVJ 5 - <9 * S S B ft S * k dft ffl ffl O £ 10 s I s * 1 ' 5 * • * s 8 t Y I * *- O • ; . i • I Is< !! Sir i. 2.00 i h . i , m T ? T I ? • i|I sj^ l >> AUj _ 1.50 1.00 1 — - .50 1913 1914 1915 1916 ll-l 4-5 8-9 12-1 4-5 8-9 12-1 4-5 8-9 12-1 4-5 8-9 12-1 4 00 FIGURE 17. — Line 757. Reproductive vigor. A. Average number in first brood for the two strains. B. Average age of mother at time first brood was produced. C. Reproductive indices, actual values. vigorous of the two strains, it was the more reactive of the two during the only two-month period of this year in which it was the less vigorous of the two strains (see figures ISA and 18s). The mean reaction-time for the minus strain was only 20 seconds greater than that for the plus strain during the last two-month period of this year. 1 These fluctuations in the means for the different two-month periods, so far as coincident in the two strains, are due to the same environmental influences; and so far as they are not co- incident they are presumably due to the somewhat differential handling of the two strains (see pages 140-142). The material was handled with commendable care (mostly by an assistant) during this year-period. 102 SELECTION IN CLADOCERA ON THE BASIS OF During the year August 1915-July 1916 (36 generations and 505 individuals, and 39 generations and 486 individuals, respectively) the reaction-time means for the two strains showed considerably greater divergence (table 45). The difference was -312.0 ±11. 91 seconds, 26.2 times the statistical probable error. The 8 same-day broods for the same period (table 46) gave a mean difference of — 270 ±21. 34 seconds, 12.65 times the probable error. With one exception, each of the 8 same-day broods for the minus strain had a .75 .50 .25 00 .25 .50 900 750 600 450 300 150 T T 11 T I 00 ll-l 4-5 8-9 1912 1913 12-1 4-5 8-9 1914 12-1 4-5 8-9 1915 12-1 n 4-5 8-9 1916 12-1 4 1917 FIGURE 18. — Line 757. A. Reproductive indices, superiority. B. Reaction-time curves with composite curves for all the other Simocephalua plus and minus strains (from December 1914) superimposed. higher reaction-time by from 84 to 546 seconds. The test series conducted during April 1916 consisted of 652 individuals of the plus strain and 677 individuals of the minus strain (table 45). In every one of these 12 pairs of broods the brood from the minus strain had the higher reaction-time. The mean reaction-time for each strain was relatively high, presumably because of unusual environmental conditions. The means were 697.0 seconds for the plus strain and 864.6 seconds for the minus strain. The difference ( — 167. 6 ±9. 12 seconds) was 18.38 times the probable error, a difference unmistakably significant. It can scarcely be doubted that if this test series had A PHYSIOLOGICAL CHARACTER. 103 been conducted during any of the following months (under usual environmental conditions), when the plus strain was more reactive, the plus strain would have had a much lower mean and the minus strain possibly a somewhat, but probably not much, lower mean, so that the difference would have been much larger.1 It is interesting, however, to have this test series come at a time of relatively high 900 750 600 450 300 150 OO 5-10 5-10 5-IO 5-10 1913 1914 1915 1916 11-4 1917 * FIGURE 19. — Line 757. Reaction-time curves by six-month periods with similar curves for Line 740 superimposed . As noted on pages 87 and 88, the minus strain of Line 740 was exceptionally slightly reactive compared with the other Simocephalus minus strains (Line 757 excepted) during the period from April to September 1916, while the 740 plus strain was exceptionally reactive during Octo- ber 1916 to January 1917. Hence the point representing the mean|for the minus strain for the "5-10" 1916 period is exceptionally high, while the point for the^plus_mean for^the "11-4" period following is exceptionally low. reaction-time for the plus strain and yet find the difference in means so markedly significant. In the final nine-month period (August 1916-April 1917) of the experiment with Line 757, the mean for the minus strain was double 1 The mean for the minus strain in this test series was only 5 seconds higher than during the earlier test series conducted 2f years earlier (September 1913). On the other hand, the mean for the plus strain was 288 seconds higher in the later than in the earlier test series. Yet, in general, the plus strain during the latter part of the experiment was very much more reactive than during the earlier portion of the experiment. The explanation of these peculiar means compared with the means for the selection data lies in the following facts: (1) The plus strain of Line 757 was more reactive than normally during the first test series. This was, in part at least, due to local experimental conditions. Of the 15 broods tested in making selections (in the laboratory strains of the different species) during the period when this test series was conducted, the broods in 11 strains had lower reaction-time means than the combined means for the immedi- ately preceding and next succeeding broods of the same strains and the differences were much larger than in the 4 cases in which the reverse relation held. Hence it is clear that the first test series for Line 757 was conducted at a time when local environmental conditions induced ab- normally low reaction-time averages in most of the Cladocera strains. (2) During the second test series the plus strain of Line 757 was relatively slightly reactive. The test-series mean for the plus strain was 697 seconds, a larger mean than had occurred in the selection data for 14 months and larger than occurred at any later two-month period of the experiment. Two broods used in selection tests in 757 plus soon after this test series was conducted had similar averages, but of the other 27 broods of this strain tested during the remainder of the selection experiment only one had an average as high as that for this entire test series. It seems clear that the 757 plus strain was in a relatively slightly reactive condition at the time of the second test series. 104 SELECTION IN CLADOCERA ON THE BASIS OF that for the plus strain. The means for both strains were lower than for any other longer period of the experiment (313.1 seconds for the plus and 625.9 seconds for the minus strain, table 45). The difference was -312. 8 ±18. 37 seconds. The mean difference in reaction-time was slightly increased. Because of the much smaller total number of individuals, however, the probable error of the difference was larger, though the difference was still 17.01 times its probable error. There was only one same-day brood. Although from a single pair of broods little may wisely be assumed, it is interesting to note that the mean for the plus brood was 93 seconds, while that for the minus was 814 seconds, the latter mean being almost 9 times the former. While the reaction-time curves by two-month periods (figure 18B) clearly show the wide and increasing divergence in reactiveness between the two strains of Line 757, curves by six-month periods (November- April and May-October) are also given, together with similar curves for Line 740 for comparison with them (figure 19). Naturally these curves are less affected by local fluctuations and are smoother curves. OTHER FEATURES OF THE DATA. Examination of other features of the data for Line 757 brings out similar evidence of a marked effect of selection.1 The average minimum reaction-time2 for the two strains is a point on which their reactiveness may be compared. The average minimum reaction- time for the plus strain for the longer periods of the experiment was progressively lowered from 468 to 357, 282, 190, and 94 seconds (table 45). For the minus strain there was likewise a lowering, though not so great nor so consistent. The minus strain average minima were 639, 548, 380, 467, and 245 seconds (table 45). For the plus strain the average minimum for the last long period was only one-fifth that for the first of the 5 longer periods; for the minus strain the minimum for the last period was two-fifths that for the earliest period. The average minima for the two strains show wide differences throughout the experiment, the differences being 171, 191, 98, 277, and 151 seconds for the 5 longer periods. These differ- 1 In a single minor detail the data for a small portion of the time do not seem consistent with an effect of selection. This point is the relative numbers of negatively reacting individuals in the two strains. From August to December 1915 there were considerably more of the nega- tively reacting individuals in the plus (21) than in the minus (5) strain. This is of course con- trary to one's expectation in a less-reactive strain. However, 19 of the 21 negatively reacting individuals in the plus strain for this period occurred in four broods. Since in all strains negatively reacting individuals are extremely irregular in their occurrence, and when they do occur are often relatively numerous within a single brood, it seems probable that these reactions are in large measure determined by environmental conditions and hence are of little, if any, real significance (cf. page 15). If, however, they were to be ascribed an important significance here, this data alone (and only a very limited portion of it) runs counter to the plain implication of all the rest of the data for Line 757. During the remainder of the experiment the plus and minus strains of Line 757 had approximately equal numbers of negatively reacting individuals (tables 43 and 44). 2 This, of course, includes the reaction-time of only a single individual (the most reactive one) of each brood. Like the data for the average maximum reaction- time, it has much less value as a measure of reactiveness than the mean reaction-time for all the individuals tested; nevertheless, it is highly significant. A PHYSIOLOGICAL CHARACTER. 105 ences are more instructive if stated in percentages. The average minima are 37, 54, 35, 146, and 161 per cent higher for the minus than for the plus strain. These differences clearly point to a marked and cumulative di- vergence in reactiveness of the most reactive individuals of the two strains, a divergence which became in the final period more than 3 times as great as the average minimum for the plus strain. It is a striking fact that for the last 9 months of the experiment the mean reaction-time for the plus strain was considerably lower than the average minimum reaction-time for the broods of the same strain for the first 21 months of the experiment. A comparison of the average maximum reaction-times is less instructive, for a large percentage of the broods of S. exspinosus contain over-time individuals; but even here comparison indicates increasingly greater reactiveness for the least reactive individuals of the broods of the plus strain toward the end of the experiment. For the first 9 months every brood in the plus strain as well as the minus strain contained over-time individuals and the average maximum reaction-time for each strain is (the arbitrary) 900 seconds. For the longer periods of the entire experiment the maxima for the plus strain declined from 900 to 879, 829, 811, and 632 seconds (table 45). There is a slight decline for the minus strain, the averages being 900, 884, 854, 888, and 854 seconds (table 45). The difference in average maximum reaction-time for the two strains increased from 0 to 5, 25, 77, and 222 seconds. The average maximum reaction-times are 0, 1, 3, 10, and 35 per cent higher for the minus than for the plus strain. For the minus strain the total lowering of the average maxi- mum reaction-time was only 46 seconds, or 5.1 per cent as compared with 268 seconds, or 29.8 per cent, for the plus strain. The data for the same-day broods may be referred to again. They, too, show an interesting decline in the mean for the plus strain through the various periods (from 798 to 670, 676, 538, and 93 seconds, table 46). The average for the last period is for a single brood and is abnormally low. The general trend is unmistakable, however. There is no marked lowering for the minus strain, the means being 857, 805, 876, 808, and 814 seconds (table 46). The in- creased divergence between the means for the two strains is striking— from 58 to 135, 200, 270, and 720 seconds. Stated in percentages, these differences for the several longer periods of the experiment, compared with the mean for the plus strain, are 7.3, 20.2, 29.6, 50.1, and 774.2 per cent. Again, although allowing that the difference for the last period was obtained from too meager data, the divergence is marked and continuous, and these differences arise from the most strictly comparable data obtainable. Considering the number of "over-time" individuals for the two strains in the different periods of the selection experiment is another 106 SELECTION IN CLADOCERA ON THE BASIS OF method of checking up the data (table 47). This is a test of vital significance. The number of over-time individuals is extremely significant of the reactiveness of a strain. For the 5 longer periods of the experiment the figures for the plus strain are 247, 302, 216, 143, and 37 over-time individuals; for the minus strain 227, 320, 334, 376, and 151 individuals. These can be compared better on a per- centage basis. The percentages of over-time individuals for the plus strain for the 5 longer periods of the experiment are 77.2, 59.9, 45.8, 28.3, and 15.1 per cent; for the minus strain 87, 77.7, 65.9, 77.4, and 57.2 per cent. Starting near equality (though the plus strain almost from the start had significantly fewer over-time indi- viduals), the differences are 9.8, 17.8, 20.1, 49.1, and 42.1 per cent. Hence the percentage of over-time individuals in the plus strain decreased throughout the experiment and particularly during the last 21 months of the experiment. The final percentage was less than one-fifth that for the earliest longer period. For the minus strain there was also a decrease, but it was less consistent, and in the end the percentage of over-time individuals was two-thirds as large as at the beginning (see table 47). It is possibly worth while to consider the mean-reaction times of the more reactive individuals of the two strains of Line 757, omitting from consideration for a moment the over-time individuals. While this is a point of interest, the fact remains that the relative number of over-time individuals is in itself as good a measure of reactiveness as might be desired. The large influence of the arbitrary reaction-times of 900 seconds, assumed for the over-time indi- viduals, does not ascribe too much influence in determining the mean reaction-time. In point of fact, the reverse is true. This arbitrary method of assuming 900 seconds as the reaction-time for the over- time individuals minimizes the rightful influence upon mean re- action-time of these relatively non-reactive individuals. The data are given in table 47 and show that there is a progressive reduction in the mean reaction-time for the more reactive individuals of both strains, considered by the longer periods of the experiment. This reduction for the minus strain is from 431 to 376, 334, 326, and 260 seconds; but for the plus strain the reduction is larger, from 410 to 360, 356, 284, and 209 seconds. The differences between the means for the two strains, with some irregularity, became larger, the plus strain having the lower reaction-time. These differences were as follows: —21, —16, +22, —43, and —51 seconds. The reduction in mean reaction-time for the plus strain in the final period as com- pared with the first period of the experiment was 201 seconds; for the minus strain, 171 seconds. The differences for the last two longer periods of the experiment have statistical significance, being respectively 2.97 and 4.2 times their probable errors. 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Lft CO CO t*~ CO CO ^ ^O ^* IOOO 00 1OCOOO-*M< CO >> : h "3 : o> CO ^ N^ "3 •s r* 1 i-J 1 05 « S CO •c 1-1 S rH i-1 5 I"! ^.§ 1*1 §| ^.2 5 II Is" Islli f" .A PHYSIOLOGICAL CHARACTER. 123 general trend is very nearly that of a straight line. This general form constitutes intrinsic evidence of the reliability of the curves as a whole. The curve for Line 757 plus (figure 15) starts higher than the level attained by the Line 740 plus strain for the same period. This is probably significant. The two-month period curve for the Line 757 plus strain held very closely to a level near 800 seconds for the first 9 months of selection. The curve for the plus strain of Line 740, while prevailingly at a lower level for these periods, fluctuated from 672 seconds to 876 seconds. The mean difference between the plus strains of Lines 757 and 740 for this nine-month period is 39 ±12. 36 seconds, 3.15 times the probable error. Hence there is evidence that at the start the plus strain of Line 757 was less re- active than the plus strain of Line 740. During the first 31 months (to February 1915) of selection with Line 757 the plus strain in general had a higher reaction-time than the plus strain of Line 740 (table 48 and figure 15), which tends to strengthen one's conviction that at the start the plus strain of Line 757 was less reactive than the plus strain of Line 740. But from February 1915 the fairly consistent differences between the plus strains of Lines 757 and 740 are striking and indicate that the plus strain of Line 757 had become the more reactive of the two. Of 14 two-month periods the plus strain of Line 740 was the more reactive only twice. One of these differences is slight (16 ±41. 5 seconds), the other is large (246. 6 ±34. 2 seconds); but this difference arises from the coincidence of an exceptionally low average for Line 740 plus (the lowest, by 104 seconds, of any two-month average for this strain) and an exceptionally high average for Line 757 plus.1 Aver- ages by two-month periods are obtained from too few individual reaction-time records to be highty reliable when considered singly, so, while this difference is large, it must be considered in relation to the other averages and evaluated accordingly. For the other 12 two-month periods of the latter half of the experiment with those two lines, the plus strain of Line 757 was the more reactive as com- pared with the plus strain of Line 740; the differences were from 22 to 240 seconds, and were respectively 3.6, 1.0, 0.7, 9.2, 1.6, 2.7, 3.1, 2.6, 1.7, 3.7, 1.4, and 6.4 times their probable errors (see table 45). That is to say, even when considered by shorter (two-month) periods, 12 of 14 differences indicate that the plus strain of Line 757 was more reactive than the plus strain of Line 740, although, with their large probable errors due to relatively small numbers, only 7 of these differences are statistically significant. 1 The mean for the December 1916-January 1917 period for the plus strain of Line 757 was obtained from the reaction-time records of only 36 individuals of 3 broods. The corre- sponding mean for the 740 plus strain resulted from 41 individual reaction-time records. These numbers are scarcely more than half the usual numbers for two-month periods. 124 SELECTION IN CLADOCERA ON THE BASIS OF Considered by longer periods, it is found that during the first (9 months) and second (12 months) longer periods of the experiment the plus strain of Line 757 was less reactive than the Line 740 pus strain by 39. 0± 12. 36 seconds and 86. 6± 13.11 seconds, differences of statistical value, 3.15 and 6.6 times the probable errors. During the following year the plus strain of Line 757 became the more re- active, though the difference for the entire year-period was only 37 ±13. 61 seconds, or 2.76 times the probable error. During the next year, August 1915-July 1916, the difference is 116.4±13.67 seconds, 8.51 times the probable error — the plus strain of Line 757 being the more reactive. The averages for the final nine- month period of selection differ by 1.9 ±15. 89 seconds, the plus strain of Line 740 having the lower reaction-time. This on its face distinctly controverts the existence of a significant difference in reactiveness between the plus strains of lines 757 and 740. But, as already noted, in one two-month period during this longer period there was an exceptionally low average for the plus strain of Line 740 and an exceptionally high average for the plus strain of Line 757, and the striking influence of these two aberrant averages pro- duces the difference noted above, a difference which is quite out of line with the general behavior for the latter part of this experiment of the two strains concerned. The curves for Lines 740 and 757 by six-month periods (figure 19) show the plus strain of Line 757 the less reactive until the May- October 1915 periods, from which time the 757 plus strain is shown the more reactive for the remainder of the experiment. The ex- ceptional reaction- time periods (December 1916-January 1917) for the two strains compared serve to cause the curves to approach for the last six-month period, but the plus strain of Line 757 maintains the lower level. Hence it is seen that, whereas on the start the plus strain of Line 757 was fundamentally less reactive than the plus strain of Line 740, during the latter half (nearly) of the experiment there is fairly conclusive evidence that the plus strain of Line 757 was more reactive than the plus strain of Line 740, which may fairly be con- sidered as a check, since it was subjected to the same treatment as Line 757 and throughout a contemporaneous period of time. Comparing the mean reaction-times by two-month periods for the plus strain of Line 757 with the means for the plus strains of the remaining lines of S. exspinosus subjected to selection, Lines 794, 795, and 796, the result is still more suggestive of a changed reactive- ness of the plus strain of Line 757. The remaining 3 lines are lumped in this comparison, a procedure justified only in the interest of brevity and the securing of less fluctuating averages. The plus strain of Line 757 was less reactive than the combined plus strains of Lines 794, 795, and 796 in only 3 out of 15 two-month periods, the A PHYSIOLOGICAL CHARACTER. 125 means for Line 757 plus being exceptionally high, in each of these periods. For the other 12 periods Line 757 plus was the more re- active, the differences varying from 25 to 269 seconds and being 4.1, 1.1, 1.8, 5.1, 14.0, 1.0, 2.0, 4.6, 3.1, 9.3, 5.6, and 8.1 times the large statistical probable errors (table 48). Considered by longer periods, it is seen (table 48) that the differences are — 5.1 ±13. 97 seconds, +123.1 ±10.24 seconds, and +89. 5± 13. 86 seconds, the plus sign indicating greater reactiveness on the part of the plus strain of Line 757. Reference to figure ISA, which shows the re- action-time curves for Line 757 and combined reaction-time curves for all other S. exspinosus lines, Lines 740 (in part), 794, 795, and 796, shows clearly to what extent the mean reaction-time for the plus strain of Line 757 is prevailingly lower than the combined means for all the other S. exspinosus plus strains. These curves show at a glance that the plus strain of Line 757 was essentially lower in its reaction-time than the other plus strains of the same species. The 3 two-month periods in which the reverse exists are there seen in their true light — as exceptionally high points in the reaction-time curve for 757 plus. Thus, from examination of the data for mean reaction-times for contemporaneous periods, it is found that during the latter half of the experiment the plus strain of Line 757 is more reactive than the plus strains of the other S. exspinosus lines. That this is a result of selection is indicated by the fact that in the beginning of the experiments both strains of Line 757 were appreciably less reactive than the two strains of Line 740, with which Line 757 may fairly be compared. Comparison of the reaction-time means for the minus strain of Line 757 with the 740 minus strain (table 49 and figures 15 and 19) shows at once that the minus strain of Line 757 was from the be- ginning the less reactive of the two, and that as the experiment progressed the difference in reactiveness between the minus strains of the two lines increased until at the close of the experiment the divergence was fairly large.1 Of 27 two-month periods the minus strain of Line 757 had a lower reaction-time than Line 740 minus only 2 times, a remarkable uniformity of differences when the irregularities of the curves are considered. Of the 25 times when the minus strain of Line 757 was the less reactive, the differences were of statistical value 18 times, being 4.5, 5.1, 3.4, 10.1, 3.0, 5.7, 11.5, 6.5, 7.2, 3.3, 12.8, 9.7, 6.5, 4.4, 5.4, 5.7, 8.6, and 12.3 times their probable errors (see table 49). Seven of the differences by shorter periods and the 2 differences in the reverse direction were not of statistical significance. Considered by longer periods, the mean 1 As is general, when comparing reaction-time means of two strains for contemporaneous periods, there is seen an obvious rough parallelism between the two curves; but the uniformity with which these two curves parallel each other is rather exceptional. 126 SELECTION IN CLADOCERA ON THE BASIS OF differences were 77, 126, 99, 170, and 230 seconds. The difference for the middle period is influenced by the exceptionally low means for the Line 757 minus strain during 2 two-month periods. Other- wise the divergence consistently increased. When the minus strain of 757 is compared with the shorter S. exspinosus lines (Lines 794, 795, and 796) the divergence between the minus strain of Line 757 and the minus strains of other lines which may serve as checks is even more marked (table 49 and figure ISfi). The minus strain of Line 757 had a higher average than the combined averages for the minus strains of Lines 794, 795, 796 in 14 of 15 two- month periods, the differences ranging from 51 to 319 seconds and all but two of them being from 4 to 13 times their probable errors. The one exceptional difference was a difference of only 1.1 seconds. CONCLUSION REGARDING EFFECT OF SELECTION IN LINE 757. It is seen, then, in Line 757, that by all the tests applied and in spite of all the safeguards used in connection with these data, a marked effect of selection is indicated. This is shown by the course of the curves plotting the mean reaction-times of the two strains; by a progressively increased (though somewhat irregular and once inter- rupted) difference in mean reaction-time between the two strains, the plus strain having the lower reaction-time; by a decrease to 94 seconds in the average minimum reaction-time of the plus strain as compared with a decrease to 245 seconds in the minus strain; by a decrease of approximately one-third in the average maximum reac- tion-time in the plus strain,1 while at the same time the minus strain decreased only 5 per cent; by a decrease of four-fifths in the number of individuals of the plus strain which failed to respond to the light, while the decrease in this number in the minus strain was only one- third; by the results of the two test series applied to the strains of this line; by the data for the same-day broods; by every statistical test which was applied to the data; by a comparison with similarly treated strains of other lines of the same species; and, to the eyes of the observer, most of all by the greatly changed behavior in the experimental tank of the plus strain as compared with that of the minus strain. There seems no escape from the conclusion that in Line 757 there is a marked effect of selection. This effect appeared in the early period of the experiment and with some fluctuation increased throughout its duration. The form of the curves would seem to indicate that the effect of selection was continuous and was still 1 This decrease in the maximum reaction-time means more than a decrease of one-third suggests. At the beginning of selection with Line 757 some broods were left in the experimental tank for an hour or more, and those individuals which did not respond to the light within the period allotted for the experiment (15 minutes) often did not move at all within the much longer period. It seems probable that the maximum reaction-times for this early period might readily have been near 3,600, or even 10,800 seconds in many cases, if each of the selection tests had been continued for an hour or 3 hours. A PHYSIOLOGICAL CHARACTER. 127 cumulative at the close of the experiment. The divergence in mean reaction-times is neither controlled nor noticeably influenced by the relative vigor of the two strains, nor does it appear to be explicable on any other grounds than as a divergence coming about through selection. It is to be regretted that no return selection was attempted with Line 757. The selection experiments were very laborious, and at the time when it was decided that they had been carried far enough, there did not seem sufficient justification for prolonging the experiment. Though it was then realized that the two strains of Line 757 were pronouncedly different in their reactiveness to light, the full significance of this difference, particularly as involving a changed reactiveness in both strains of Line 757 as compared with the corresponding strains of the other S. exspinosus lines, was not brought out until months later, when the extensive data were fully worked over. Time was not then available for again taking up the selection experiment. A further fact of importance is that the effect of selection was permanent, or at any rate persisted, through a long series of genera- tions. In December 1919 (32 months after selection had been dis- continued) the selective effect still persisted and probably to as marked a degree as at the termination of selection. The two strains, meantime, had been propagated for 112 generations without any regard to their past history. The persistence of the effect of selection was demonstrated by testing a large number of broods of both strains. In extremely few cases did a brood of the plus strain fail to show more reactiveness to light than the corresponding brood of the minus strain, while in most cases the broods of the plus strain were markedly the more reactive. 128 SELECTION IN CLADOCERA ON THE BASIS OF ENVIRONMENTAL CONDITIONS AS AFFECTING REACTION- TIME AND VIGOR OF STOCK. Any relation between environmental influences and reactive- ness to light is a fact of general biological significance and is a matter of importance in the present connection. In the light tests with Cladocera little-understood environmental influences were important factors in influencing reaction-time in all three species which were subjected to selection. These environmental influences are dis- cussed under the following heads: (1) temperature influences; (2) influences of exhaled substances from the observer's breath; (3) relatively temporary chemical (?) differences in the water used in the experimental tank; (4) occurrence of negatively reacting indi- viduals; (5) general influences operative through longer periods of time; (6) influence of vigor. 1. TEMPERATURE INFLUENCES. The temperature of the water in the experimental tank was a minor factor influencing reaction-time. This influence was not marked enough to be evident to the observer in the tests day by day, but the somewhat lower reaction means during the winter periods indicate and correlation between temperature and reaction- time demonstrates that lower temperatures are to some extent associated with greater reactiveness and a lower reaction-time mean. The temperature of the water in which the animals were tested was always near that of the basement room in which the tests were conducted. Fortunately this room was less affected by fluctuations in summer temperatures than the remainder of the building, but in winter temperatures ran fairly low, —10° to 15° C., inasmuch as this room was not provided with much artificial heat. Correlations between individual reaction-times (50-second classes) and temperature (0.5° C. classes) of the water for all the tested individuals of strains of D. pulex cover the following periods and give the following values : Time period. No. of tested young. r E r/W Time period. No. of tested young. r E* r/ET Apr. 1914 194 + 0.0071 0 . 0484 0.15 Feb. 1915... 292 +0.0142 0.0395 0.36 May 1914 317 - .2107 . 0362 5.82 Mar. 1915. . 347 - .0532 .0361 1.47 June 1914 175 - .0019 .0510 0.04 (Summer) July 1914 197 - .0009 .0481 0.02 Apr.— Sept. Aug. 1914 205 + . 1238 .0464 2.67 1914 1,474 - .0193 .0175 1.10 Sept. 1914 386 - . 2482 .0322 7.70 (Winter) Oct. 1914 385 + . 2742 .0318 8.62 Oct. 1914- Nov. 1914 438 + . 0693 .0321 2.16 Mar. 1915. 2,025 + .0442 .0150 2.95 Dec. 1914 301 + .0851 . 0386 2.20 (Year) Jan. 1915 262 + . 0969 .0413 2.35 Apr. 1914- Mar. 1915. 3,499 + . 1369 .0112 12.22 A PHYSIOLOGICAL CHARACTER. 129 These correlations are small and of those for single month-periods only three or four can be credited with statistical value, those for the May, August, September, and October 1914 periods. Two of these are positive and two are negative correlations. Five of the correla- tions from March to September are negative and two of these are significant, both in numerical value and in comparison with their probable errors. One of the two positive correlations for the spring and summer period is very small, but the other is larger and of possible significance. The correlations for October-February are all positive, but only one is significant. The correlation for the six- month spring and summer period is -0.0193 ±0.0175; that for the six-month winter period is +0.0442 ±0.0150. These very limited data suggest that negative correlations are possibly general for the spring and summer months, while positive correlations are general for the late fall and winter months. The correlation for the year April 1914-March 1915, while only +0.1369, is 12.2 times its prob- able error. These data cover the year-period of most consistent shifts in the composite reaction-time curves for the D. pulex strains (figure lOo) and probably show the changes in reaction-time associated with season and temperature at its maximum for D. pulex. Correlations between temperature and reaction-time for S. exspinosus were confined to separate correlations for the strains of Line 757 covering in each case the selection data for 19 months from October 1915 to April 1917, the period during which occurred the greatest differences in reaction-time between the two strains. The correlations were, for the plus strain (689 individuals) , +0.2428 ±0.0242 and for the minus strain (683 individuals), +0.2363 ±0.0244. Each value is large enough to have a real meaning and is approximately 10 times its probable error. Hence there is shown for both species, and particularly for S. exspinosus, Line 757, some significant correlation between reaction- time and the temperature of the water in the experimental tank. It is a question how much of this correlation, if any, represents a real direct temperature influence. If the temperature influence were direct, one would perhaps expect within the limited range of tempera- ture occurring in the tests that higher temperatures would be asso- ciated with greater reactiveness and a lowered mean reaction-time, which would be indicated by negative correlations. Since the significant correlations are mostly positive, higher temperatures tend to be associated with less reactiveness and a higher reaction-time. It is possible that the lowered reactiveness with higher tempera- tures is really due to reduced oxygen in the water used in the experi- mental tank. The water, when brought in the day it was to be used, was always at a lower temperature than the experimental room. As it warmed up, the excess gas in solution appeared as bubbles. The 130 SELECTION IN CLADOCERA ON THE BASIS OF water was ordinarily used soon after it reached approximately the room temperature; hence it was probably near saturation for the principal gases which had been thrown out of solution. Whether the gases retained in solution were equally oxygen and other gases would of course depend upon the actual amounts of individual gases within the water when taken from the pond. Assuming that the pond-water was ordinarily near saturation for oxygen when brought into the laboratory, after it had given up a portion of its contained gases (as it always did) it would presumably be saturated for oxygen. Since the amount of oxygen at saturation at 25° C. is about 26 per cent less than at 10° C., this is a basis for the suggestion of an ex- planation for the greater reactiveness at the lower temperatures. No specific tests have been made to determine if this is the correct interpretation, and the results of the summer correlations for D. pulex do not fit in with this explanation. There is also the possibility that temperature influenced reaction- time indirectly through its effect upon the food in the culture-water. The water used as culture-water went through the changes in tem- perature (and chemical constitutents) to which a shallow out-door pond is subject in the climate of Cold Spring Harbor and the develop- ment of the protista upon which the Cladocera feed is naturally influenced by temperature as well as other environmental factors. Before being used the culture-water (during its successive strainings) was kept for approximately 6 days indoors in jars surrounded by running water. The temperature of this running water varied with out-door temperatures, but was much less extreme. Hence there is also room for the assumption of influences of temperature upon the Cladocera stock, not directly as a temperature influence per se, but indirectly through the culture medium, which is a veritable micro- cosm. 2. INFLUENCE OF SUBSTANCES EXHALED FROM OBSERVER'S BREATH. That the amount of carbon dioxide was actually an important factor in influencing reaction-time is suggested by the fact that if several broods were tested successively in the experimental tank without change of water the later broods reacted markedly less promptly than the ones tested in the same water an hour earlier.2 The young Cladocera are so small as to be just visible in the illumina- tion in the experimental tank, so that one must get the eyes at about reading distance from the water of the tank in order to observe the young animals. As a consequence, the exhaled air very largely 1 The spring water is known to contain relatively a large amount of COj. 2 This was observed a number of times, after which the plan of frequently changing the water in the experimental tank was adopted to obviate this disturbing influence. It seemed impracticable to change the water oftener than after testing each four broods, although it was sometimes noted that the last brood or two of the four tested between changes of water was less reactive than broods tested just after the change. A PHYSIOLOGICAL CHARACTER. 131 struck the surface of the water. These differences in reactiveness were not due to temperature differences, since they occurred in cases where there were no temperature differences in the water. It is assumed that this difference in reactiveness was due to the accumu- lation of carbon dioxide from the observer's breath in the water contained in the tank, though exhaled volatile organic substances may have been wholly or partially responsible. 3. RELATIVELY TEMPORARY CHEMICAL DIFFERENCES IN WATER USED IN EXPERIMENTAL TANK. Probably the environmental factor of greatest importance con- sisted in differences in the chemical content of the water used in the experimental tank, although this water was always obtained from the same pond and handled in the same fashion. Such chemical differ- ences in the water would account for the pecularities in reaction-time which were so apparent in the reactions of all the broods tested on certain days. Not infrequently, between successive days or be- tween periods of a few days' duration, considerable difference was noted in the general reactiveness of the stock as a whole. On one day the individuals tested would react fairly promptly, while on the following day they might react conspicuously less promptly. This clearly points to environmental influences operative temporarily but pronouncedly. These temporary differences in reactiveness, pre- sumably due to differences in the water, are well illustrated in table 2 (page 18), showing in abbreviated form the data for the broods of Line 695, tested on August 29, 30, and 31, 1913. 1 The data given in table 2 present an extreme case selected to show the environmental influence upon reaction-time (and upon the occurrence of negatively reacting individuals) at its maximum. The averages for the data for the two strains of Line 695 for the 3 days shown in table 2 are 401, 636, and 398 seconds, based upon 87, 190, and 50 individual reaction-times. The mean for the second of these days is 235 seconds or 58.6 per cent greater than that for the first day and 238 seconds or 59.8 per cent greater than that for the third day. The standard deviations and probable errors for these means were not determined, but obviously (from other experience in deter- mining probable errors with such data) these differences are of large statistical value. As already noted, this is an extreme case (and it is doubtful if many as extreme could be found in any portion of the later data), but the factor operative here presumably was also operative to some extent in many of the tests conducted in making 1 These data were obtained with due regard to the necessity for changing the water in the experimental tank at frequent intervals and they fit the requirements of the present purpose very well. They are part of the data of the first test series conducted. As a test series this was not satisfactory, as the numbers of individuals in the corresponding plus and minus broods were not as nearly exactly the same as in later test series. Also, the broods were not always paired here, i. e., in the usual course a plus brood was always tested just before or immediately after a minus brood of the same age and containing the same number (within one or two) of individuals. 132 SELECTION IN CLADOCERA ON THE BASIS OF selections; but this effect was probably the same with all the indi- viduals of the plus and minus strains and could not have interfered with the choosing of the proper individuals or affected the experiment as a whole, except that it added to the irregularity of the reaction- time records. If, however, this disturbing influence did sometimes prevent the making of the proper selections, that would only render the selections less effective and would not confuse any results ob- tained. 4. OCCURRENCE or NEGATIVELY REACTING INDIVIDUALS. The data shown in table 2 also serve to illustrate the irregular occurrence of negatively reacting individuals. There were 3 negative ones of a total of 87 individuals, or 3.5 per cent, August 29; 3 out of 190, or 1.6 per cent, August 30; and 14 of 50, or 28 per cent, August 31. While these numbers are not large, the widely differing percentages indicate very well the relatively spasmodic occurrence of negatively reacting individuals on August 31 (see also page 15). There is great irregularity in the occurrence of negatively reacting individuals in general, so that while the illustration taken from table 2 is somehwat extreme, it is not unique.1 These illustrations, (1) to (4), bring to notice relatively tempo- rary disturbing factors which are a matter of concern, but for which no way of complete elimination was found; yet they do not serve to discount the selection data as a whole. This is clear for two reasons: (1) these factors, which in the long run presumably must have affected the plus and minus strains in a similar way, could not often have interfered with making the proper selections and could contribute only to the fluctuations in reaction-time; (2) the consistent and con- vincing results with Line 757 could not have come about if these dis- turbing factors had seriously interfered with the course of the selec- tion experiment as a whole. 5. GENERAL INFLUENCES OPERATIVE THROUGH LONGER PERIODS OF TIME. The general relation between mean reaction-time and environ- mental conditions is obvious in the curves plotting mean reaction- times by two-month periods. This relation is seen in that reaction- time curves for the plus and minus strains of the same line in a general way move upward and downward together to some extent and at times the nearly coincidental courses of the curves for the two strains are very striking. This was not due to any large extent to tempera- ture differences in the water used in the experimental tank, although temperature differences in this water were sometimes a factor (see pages 128-130). Temperature influences fail to explain many of the broad fluctuations of the reaction-time curves, e. g., the high levels obtaining for the curves for D. pulex (figure lOo) during December 1 It chances in this case that the lowest percentage of negatively reacting individuals falls upon the day of highest mean reaction-time. This is a mere coincidence. A PHYSIOLOGICAL CHARACTER. 133 1913-March 1914, a winter period during which relatively low levels are generally seen. It is not difficult to believe that these environmental factors may have been in part chemical differences in the pond-water, gradual changes in carbon-dioxide or oxygen content, or other chemical differences, since the effects observed are operative through longer periods of time than the temporary environmental effects already considered. Such gradual changes in the water of a pond are well known, and the water used for culture-water and in the experimental tank during these tests was pond-water. We know nothing about the precise specific character of the changes involved in the present case. COINCIDENT CHANGES IN REACTIVENESS. The curves for Line 695 (figure 2c) show these general environ- mental effects to a marked degree, particularly in the relatively low levels of the curves for October 1912-November 1913 and in the higher level from December 1913-September 1914. The general coincidence of level of the two curves does not end here, but it is not so marked through the iater course of the experiment because of the relatively wide fluctuations of the curve for the minus strain. The correlation between the mean reaction-times of the plus and minus strains of this line by two-month periods is 0.4962 ±0.0979. With Line 689 (figure 3c) the environmental effect is observable throughout the course of the curves. In Line 691 (figure 4), while the curve for the minus strain is quite irregular, the general course of the curves is very similar. The .same may be said for Line 711 (figure 5). In Line 713 (figure 6), in spite of the very irregular curve for the plus strain, the curves in general follow each other remark- ably closely. The correlation for the two strains of this line is 0.6982 ±0.0773. With Line 714 (figure 7c) the curves are unusually irregular for strains of Daphnia pulex, yet the environmental influence is obvious from the general course of the curves. The correlation is 0.5027 ±0.0971. In Line 719 (figure 8c), again, the curve for the plus strain is very irregular, yet the general coincidence of the curves is obvious. Line 751 (figure 9) shows the same general coincidence of the curves. This line, within which selection was begun in November 1912, is the only D. pulex line with which selection was begun later than March 1912. It is interesting to note that the curves for this line start at virtually the same level as was attained by the other D. pulex strains during the same two-month period (December 1912- January 1913) and closely follow the composite curves for the other D. pulex strains (see figure lOo). The curves (figure 10, A and B) for the relatively short exper- iments with Lines 762 and 766, both Daphnia longispina, show the same general indication of a parallel effect of environment upon the 134 SELECTION IN CLADOCERA ON THE BASIS OF mean reaction-times of the plus and minus strains. Likewise, with the other D. longispina line, Line 768, the curves (figure lOc) for the two strains, despite irregularities, follow the same general course. With the S. exspinosus material the general influence of environ- ment upon the mean reaction-times of the two strains of the same line is less conspicuous than with the lines of the two species of Daphnia, but it is evident in most of the curves and is particularly clear for the curves for the final several months of the experiments. The curves for Line 794 (figure lie) show this influence to a marked degree, in spite of the irregular portions of the curve for the minus strain. Because of the unusually irregular curve in Line 795 (figure 12c), the environmental influence is less in evidence, but there probably is a small positive correlation between the reaction- time means for the two strains by the two-month periods of the experiment. In Line 796 (figure 13c) the curves follow each other in a general way. With Line 740 (figure 15) the curves on the whole follow each other to a remarkable degree. A clearer demonstration of the effect of environmental influence working through relatively long periods would be hard to find. The correlation between the mean reac- tion-times for the plus and minus strain by two-month periods is 0.6437 ±0.0733. The relatively shorter range of the up-and-down movements of the curves for this line renders this correlation smaller than one would otherwise expect from the relatively close parallelism of the two curves, but, considering the limited num- ber of terms involved in the correlation, it is very large. In Line 757 (figure 18e) the environmental effect is somewhat in evidence in the courses of the two curves, but naturally it is obscured by the marked effect of selection, particularly during the last 21 months of the experiment; yet, even in this latter period, in spite of the wide divergence of the curves, there is an evident rela- tion between the two which seems intelligible only as an effect of environmental influence. The correlation for the reaction-time means for the two strains is 0.4283 ±0.1041. A composite curve plotting the mean reaction-times for all the plus strains and another for all the minus strains of D. pulex is shown in figure 10D. The courses of the curves for the strains of any single D. pulex line, despite irregularities, strikingly resemble the corre- sponding portions of this composite curve. Likewise, the few re- action-time curves for strains of D. longispina (see figure 10, A, B, and c) to some extent follow the same course as the composite re- action-time curve (figure 10o) for the D. pulex strains. A composite curve of the reaction-time means for all the plus strains and one for all the minus strains of S. exspinosus (except Line 757) is shown in figure 18B. The environmental influence is well shown in the generally fairly coincidental courses of the two curves, A PHYSIOLOGICAL CHARACTER. 135 and, as with the lines of D. pulex, there is a striking resemblance between the curves for any one of the strains and the composite curve for all the corresponding strains of S. exspinosus. There is some tendency for the composite reaction-time curves for strains of D. pulex to follow the same course as the curves for S. exspinosus, but this is seen to a limited extent and in only a few places, such as in a general rise for both species for June-July 1913 (figures 10D and 15) and in a general rise followed by a pronounced drop in the period from April to November 1915 (figures lOo and 18). While in general the physiology of these two species is much the same, there are still such specific physiological differences that any coinci- dental changes in reaction-times bespeak pronounced contempo- raneous effects of environment. SEASONAL CHANGES IN REACTIVENESS. There is some tendency observable in the reaction-time curves for a drop in the curves, indicating greater reactiveness during the winter than the summer months. The composite curves for the D. pulex lines (figure K)D) in general show lower portions of the curves for the October-March periods, but this does not hold for the winter period for 1913-1914. Seasonal changes are seen to some extent for the winter and summer periods in the composite reaction-time curves for the S. exspinosus lines (figure 18s) and in the six-month curves for Lines 740 and 757 (figure 19). The seasonal (winter and summer) differences are possibly due largely to temperature influences, but there is no certainty that this seasonal shift is due to temperature alone or even to temperature as indirectly working through the oxygen or other content of the water used in the experimental tank and through the culture medium, yet it seems probable that these influences may be a large factor in these seasonal effects. CONTEMPORANEOUS SHIFTS IN REACTION-TIME MEANS. Contemporaneous shifts in the reaction-time curves for the strains of different lines of the same species are seen. There are a few shorter time-periods in the data for which there are really re- markable contemporaneous changes in the curves for the different strains of the same species. For the August-September 1912 period the 14 D. pulex strains, with two exceptions, showed drops in the reaction-time curves. All but one of these drops were considerable in extent. The two strains which did not show drops in this period and the one which showed a slight drop in this period showed pro- found drops in the next two-month period, while most of the strains showed further drops in this later two-month period. Temperature influences1 were not the factors determining these drops in the 1 Temperature is referred to frequently because the data include temperature records, whils for the other environmental factors no definite measurements are available. 136 SELECTION IN CLADOCERA ON THE BASIS OF curves. In general, the temperature of the water in the experimental tank was very much the same for August-September 1912 as for June- July preceding. It is interesting to note that this August- September period, because of poor food conditions, was a period of great difficulty in maintaining our stock; mortality was high; most broods were excessively small, usually 1 to 3 individuals; and sterility was relatively common. Yet this is one of the periods of the greatest drops in the curves (decrease in reaction-time) which occurred at any part of the experiment with D. pulex. A second interesting contemporaneous change in the reaction- time curves was a marked rise for all of the ten strains of D. pulex for December 1913 to January 1914. Though the temperatures were somewhat lower for this period than for the preceding period, it should be noted that the reaction-time curves for D. pulex strains remained very close to this high level for 8 months, namely, from December 1913 to July 1914 (from early winter to midsummer). Hence it seems impossible to attribute this fluctuation in the D. pulex curves for the December 1913-January 1914 period to tem- perature influences. Another local drop is seen in every one of the 10 curves for S. exspinosus strains for the February-March 1915 period. This drop is very large for 5 of these strains. Temperatures for this two- month period were in general slightly lower than during the preceding and succeeding periods, but as compared with other periods these are comparatively slight temperature differences and the drops in the curves can not be considerd primarily, if at all, due to the slight temperature differences between these two periods. It is worthy of note that the contemporaneous shift in the re- action-time curves affecting every one of the 10 strains of D. pulex for December 1913-January 1914 period is not markedly reflected in the 4 S. exspinosus strains (see figures 15 and 18s) and that the profound drop in the curves for strains of Simocephalus for the last period mentioned above is not in evidence for the strains of D. pulex. This in itself would seem to indicate clearly enough that the effect is not due to temperature influences per se.1 One might be tempted to assume that the first and third of these profound shifts in the reaction-time curves were due to some improvement in Cladocera stock due to favorable laboratory condi- tions, inasmuch as the first appears after 4 months of selection in the D. pulex lines and the third after 2 months of selection in 3 of the S. exspinosus lines, except for the facts that (1) the D. pulex lines had been under laboratory conditions for 33/2 months before selection began and hence a total of 7% months before these drops occurred, and further, that they did not maintain these lower levels; (2) that 1 The lack of contemporaneous shifts in the curves for strains of D. pulex and