UNIVERSITY OF CALIFORNIA
AT LOS ANGELES
GIFT OF
CARNEGIE INSTITUTION
OF WASHINGTON
SELECTION IN CLADOCERA ON THE
BASIS OF A PHYSIOLOGICAL
CHARACTER
BY ARTHUR M. BANTA
PUBLISHED BY THE CARNEGIE INSTITUTION OF WASHINGTON
WASHINGTON, 1921
74 3
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 OF GENETICS.
ANDREW B. GRAHAM COMPANY
WASHINGTON. D. C.
QL
•444
C6
$228$
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 70
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 .... 1 44
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
,. 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. pvlex 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-day 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 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.
1 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
last 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.
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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 Dii- 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 experimental 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 msrely 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 Gs-.
1
2:12:11, +
Oct. 23, 2d brood.
2
12 : 15, +
Oct. 31, preserved.
3
13 : 03, +
4
13 : 05, +
695 F«- 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 fact 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
'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
1In 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).
* 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-fo'rae 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
Fresh water
Aug. 30, 1913.
Fresh water
Fresh water
695 Es - No. 91....
695 Is + No. 5
695 I3 + No. 6
Totals, etc
C95 Is + No. 9...
695 E3 - No. 2...
695 E3 - No. 10
695 Is + No. 12
695 Es - No. 7...
695 Is + No. 11
695 Is + No. 13
695 Ej - No. 4
Totals, etc
695 Is + No. 1 ...
695 Es - No. 9. ..
695 I, + No. 8....
Totals, etc
26
23
38
1
0
2
448
555
276
87
3
3.5
401
22
21
23
23
27
33
21
20
0
0
0
0
2
0
0
346
631
704
710
577
766
761
526
Freshwater
Aug. 31, 1913
Freshwater
190
3
1.6
636
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 on 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 BBOODS.
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? (6) 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 tbo riinus 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-maintain ing" movements; and in its locomotion its move-
ments are somewhat less jerky, due to its relatively smaller antennae
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 CHARAC rER- 25
the side of the tank in the direction of its forr
sometimes it took a diagonal course away fro
er orientation, though
the side of the tank,
but in the general direction (as regards light) i»n which it was formerly
oriented. The individuals which were slow i£ reacting either moved
very slowly, scarcely more than executing " Tplace-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 nojt 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 indi -yiduals were not especially
common with D. pulex.
The statements regarding the behavic'r of D- pulex in the experi-
mental tank apply to D. longispina; this/species sometimes became
caught in the surface film and was unabl6 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 aj few strokes without orien-
tation and then settled to the bottom. Occasionally some individuals
just after their release swam upward ^nd attached to the surface
film. In many broods, some individuals, after settling to the bottom
or rising to the surface, made no furthf r movements during the 15
minutes of the test, while with some proods there was no further
movement on the part of any individual The majority of individuals,
however, began moving again within a V^w 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 mov<?ment after the first pause than
with the two species of Daphnia. Occasionally individuals, in their
movement toward the light, swam n^ar the surface, and when they
paused attached to the surface film. Sucn individuals frequently did
not move again. As with Daphnia, the orientation often was not
perfect and some individuals reacbed the sides of the tank, fre-
quently attaching there, and making no further movements; but more
generally they followed along the Pide of the tank with the same
general orientation as before. Orientation in a negative way was
uncommon with this species.
The frequency of occurrence <>f larSe 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. exspinosus vrith which selection was attempted
26 SELECTION IN CLADOCERA ON THE BASIS OF
were discarded as apparently unprofitable material for a selection
experiment on the basils 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.
I
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 even1 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 t|o 11 to 14 days for Simocephalus and Daphnia and 8 to 12 days
for A$oina. \
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. Fig are IB (for Line 695) and Figure 17s
(for Line 757) show two of these curves.
STATISTICAL TREATMENT OF DATA.
Irregularities in phototrofbic response are common. They are
noted in almost every paper dealing with the light-reactions of ani-
mals. Dependable results are 1 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 sq 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 giveni 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 ia
only a fraction of its probable error, though a difference less than 2^ 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.
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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
3108600
J
April 16
12
0
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
0 or 1
4
18.5
0
0
165
525
1280
320
485350
L
May 27
11
0 or 1
15
18.5
Selection
made but records incomplete.
M
June 3
1
0 or 1
7
21
1
1
180
900
3445
492
2080425
N
June 10
11
Oor 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
July 3
3
0
1
19.5
0
0
260
260
260
260
67600
Qo
July 5
3
0
2
21.7
0
0
430
730
1160
580
717800
R
July 15
11
0 or 1
1
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
615
1656250
u
Aug. 28
1
0
1
19.5
0
0
195
195
195
195
38025
v«
Sept. 9
2
0 or 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
]57
219504
E2
Dec. 20
1
0
9
18.8
0
0
50
270
1315
146
222825
F2
Dec. 30
11
Oor 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
S5S
684375
H2
Jan. 20
1
0 or 1
11
15.8
0
0
70
625
3259
296
1292531
II
Jan. 27
1
Oor 1
10
15.5
0
0
90
415
2140
214
566700
J2
Feb. 3
2
0 or 1
7
14.6
0
0
150
275
1535
205
305675
K,
Feb. 12
1
0
9
13
0
0
130
315
1895
211
432025
L.
Feb. 20
1
0
10
15.2
0
3
160
900
5180
518
3410200
Mi
Feb. 28
2
0
11
16.2
0
0
160
530
3165
288
1100025
Nj
Mar. 10
1
0 or 1
10
15
0
0
135
425
2600
260
764100
0,
Mar. 19
2
0
3
13.1
0
0
110
260
500
167
96600
Pj
Mar. 28
9
2
6
13.3
0
0
260
425
2155
359
797475
April 4
2
0
7
20.5
0
0
110
215
1155
165
201275
Rt
April 11
2
0
12
19
1
0
100
580
3065
255
1080975
82
April 18
4
0
10
18.7
0
0
60
285
1680
168
334250
T»
April 25
3
0
11
18.4
0
0
125
450
2725
248
781175
U,
May 5
1
Oor 1
8
17.5
0
0
70
190
1010
126
141150
Vi
May 12
1
0 or 1
4
14
0
0
100
230
635
159
112225
w,
May 20
2
0
7
14.5
0
0
135
450
1695
S4S
477225
X,
May 28
4
0
8
16
0
0
100
245
1380
173
254400
Y»
June 5
1
0
2
18
0
0
110
125
235
118
27725
Zt
June 12
3
0
8
17.2
0
5
225
900
5265
658
4248225
Aa
June 18
1
0
7
19.3
2
0
130
395
1410
SOI
341350
B3
June 27
10
0
5
20.5
0
0
80
170
630
126
85750
Ca
July 3
2
0
4
22.3
1
0
140
870
1750
438
1134350
Dj
July 10
3
2
2
22
0
0
190
400
590
295
196100
Ei
July 19
2
0
2
22.3
0
0
520
575
1095
548
601025
F,
July 26
11
0
1
21.7
0
0
370
370
370
370
136900
Fj°
July 29
2
0
4
22.5
0
0
115
356
1010
2f3
285334
Gi
Aug. 1
2
0
6
22.8
0
1
165
900
3255
543
2167925
H,
Aug. 8
3
0
13
21.9
0
0
90
750
4155
319
1699175
I.
Aug. 14
1
0
4
0
0
100
222
564
141
88638
Jj
Aug. 20
2
0
7
26". 4'
0
0
153
543
2215
316
847305
K,
Aug. 26
3
0
10
20
1
0
75
497
2037
204
565827
L,
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.
*> 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.
« 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.
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M,
Sept. 9
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20.6»C.
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130
549
2475
876
892071
N»
Sept. 17
10
0
8
18.4
0
2
130
900
2759
545
1783651
0,
Sept. 27
10
0
13
16.8
0
0
106
352
2674
206
614380
Pi
Oct. 3
11
0
5
17.9
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156
455
1283
257
390103
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Oct. 9
10
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20.1
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110
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2813
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Oct. 16
9
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17.5
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173
405
2402
267
695916
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Oct. 23
11
0
7
15.3
0
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91
340
1184
169
242080
Ta
Oct. 30
11
0
6
16.7
0
0
113
435
1208
201
314160
U,
Nov. 13
11
0
5
15.1
0
0
91
318
778
156
158166
V,
Nov. 20
2
0
12
18.8
0
0
93
365
2276
190
505048
W»
Nov. 27
11
0
9
12.7
0
1
234
900
4075
453
2294323
X,
Dec. 5
11
0
5
13.8
0
0
358
691
2447
489
1281475
Y.
Dec. 13
10
0
4
12.5
0
2
308
900
2618
655
1974964
Zi
Dec. 22
11
8
12.3
0
1
320
900
4211
526
2450339
A.
Dec. 27
6
Se
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is mt
de but record incomplete.
1914
B«
Jan. 5
7
1
6
11.9
0
0
150
560
1840
307
669850
C4
Jan. 15
2
0
8
13
1
1
340
900
4200
525
2422408
D4
Jan. 22
1
0
7
13.8
0
1
158
900
3486
498
2103652
E«
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
II,
Feb. 20
5
0
18
16.2
0
3
110
900
9963
554
6732879
I,
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
Ki
Mar. 21
3
0
8
17
0
6
219
900
5900
738
4986922
I*
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
N«
April 16
3
0
10
19
0
2
85
900
3195
320
1906975
0.
P4
April 24
May 1
1
2
0
0
6
12
17
19
0
4
0
2
120
183
630
900
1455
6317
243
526
539625
4182749
Q«
May 7
6
0
15
20.8
11
1
95
900
5536
369
3003036
It.
May 14
1
0
1
21.2
0
0
297
297
297
£97
88209
84
May 20
4
0
9
21.2
3
1
135
900
3728
414
2125440
T«
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
v«
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
x«
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
Z,
July 22
5
0
7
21.1
0
0
75
604
2091
299
864455
A,
July 28
11
0
5
21
4
1
165
900
1885
377
1101575
B.
Aug. 5
2
0
6
22
0
4
285
900
4355
726
3542125
C.
Aug. 11
2
0
2
24.8
0
1
180
900
1080
540
842400
D,
Aug. 20
1
0
5
23
1
0
201
525
1633
327
592355
E.
AUK. 26
7
0
8
21
0
2
125
900
3907
488
2562427
F.
Sept. 1
2
0
12
22
2
2
180
900
5780
482
3464606
G,
Sept. 7
4
0
9
22
1
0
90
700
2737
304
1213419
Hi
Sept. 16
3
0
33
16.2
0
8
59
900
16589
503
11051527
It
Sept. 23
3
0
8
18.9
1
0
70
425
1398
176
389734
J.
Sept. 30
2
0
14
20.1
3
1
178
900
5921
4S3
3114431
K,
Oct. 8
3
0
6
21.8
2
1
348
900
4046
674
2912988
Lt
Oct. 15
1
0
15
20.1
2
0
70
745
4523
302
1951685
M.
Oct. 21
11
0
13
18.8
2
0
58
480
3054
235
917626
N,
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
P.
Nov. 11
4
0
14
19.2
0
0
100
570
3050
218
904678
Q*
Nov. 18
4
0
15
17.3
6
0
115
875
4791
319
2206373
itft
Nov. 26
2
0
14
18.5
9
0
100
840
5429
388
3134113
Si
Dec. 2
4
0
5
18.7
0
0
80
275
875
175
180975
7"1
Dec. 8
3
0
8
16.7
1
0
56
263
1136
142
209724
u.
Dec. 14
4
0
6
21
6
0
165
520
1943
324
740549
Vt
Dec. 21
3
0
10
15.7
0
0
103
235
1388
139
205034
w,
Dec. 29
3
0
10
18.7
0
2
95
900
4178
418
2592880
1915
x,
Jan. 6
1
0
9
19
0
1
90
900
2345
261
1088025
Yi
Jan. 15
1
0
6
18.9
0
1
125
900
2725
454
1764225
Zi
Jan. 23
1
0
12
18.9
3
176
900
6136
511
4070576
A PHYSIOLOGICAL CHARACTER.
31
TABLE 3. — Summary of selection data by broods for Line 695 plus — Continued.
a
g
1
i
3
.a
.a
.a
Jj
0
1
|
I
I
I
1
1
I
-.3
1
s
.
I
"o
S
|
3 ^
'•?
A
"**
1
£
1
1
-4
c
1
1
2 T
I
I
0
'•S
"o
•3
I
1
O
Date of select
the brood.
8
•3
H
a a
I
a
3
f,
•3
Temperature
tank.
.^ C
5 c
|1
1 Minimum r<
seconds.
1 Maximum n
seconds.
2
OQ ffl
1
•8 j
ij
O)
1915
A.
Feb. 1
3
0
8
17.7°C
0
0
192
465
2343
£93
737297
B<
Feb. 10
1
0
9
13.3
0
0
7
580
1648
183
492636
C.
Feb. 18
11
0
15
16.5
1
1
140
900
5746
383
2826876
D,
Feb. 26
1
0
12
19.2
0
0
58
267
1818
152
334294
Mar. 6
6
0
10
19.3
3
0
118
633
3393
339
1397655
F«'
Mar. 13
2
0
18
17
0
1
70
900
3387
188
1262947
G6
Mar. 22
4
0
13
17.6
1
2
140
900
7787
599
5350389
H«
April 1
3
0
8
20.8
1
1
72
900
2650
331
1381078
I.
April 8
9
0
10
18.5
5
2
417
900
6668
667
4665470
it
April 15
3
0
15
17.9
0
0
60
845
3485
232
1481175
K«
April 23
2
0
14
18
0
3
83
900
6573
470
4300045
L«
April 30
2
0
10
20.4
1
3
125
900
4271
427
2824692
M«
May 8
5
0
9
19.7
0
1
320
900
5634
626
3895706
N,
May 17
3
0
10
16.9
5
0
160
760
3300
330
1441050
0,
May 26
3
0
5
17
1
1
205
900
2160
432
1251150
P.
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
3195400
S*
June 28
3
0
4
18
1
2
245
900
2545
636
1930025
T«
July 5
3
0
4
21
1
0
505
670
2401
600
1457641
V,
July 13
Random distribution.
vt
July 18
Random distribution.
w«
July 27
Random distribution.
Xe*
Random distribution.
Y,
' Aug.' "16 ' '
5 1 13
23.5
8
3
00
900
5785
445
3766975
Aug. 26
Random distribution.
AT
Sept. 6
5
0
2
20.8
0
0
180
335
515
258
144625
B,
Sept. 14
5
0
5
21.9
0
0
120
450
1160
232
336200
CT
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
E,
Oct. 9
11
0
8
17
0
1
110
900
4080
610
2914000
FT
Oct. 20
5
0
12
18.1
0
0
70
855
2760
230
1322900
GT
Oct. 28
4
0
14
15.5
0
0
50
260
2030
145
347900
HT
Nov. 5
5
0
18
15
3
0
70
710
4610
256
1623150
IT
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
KT
Dec. 2
4
0
17
14.5
0
0
50
760
4140
244
1597800
Lr
Dec. 9
5
0
12
15.2
1
0
90
390
2160
180
508800
MT
Dec. 16
5
0
9
18.1
1
1
60
900
1935
215
954725
NT
Dec. 23
4
0
15
12.8
2
0
100
470
3720
248
1068550
1916
O?
Jan. 1
Random distribution.
P7
Jan. 13
8
0
7
16
0
2
100
900
2875
411
1890875
Q7
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
TT
Feb. 16
5
0
20
17
3
2
90
900
6205
310
2922075
UT
Feb. 23
3
0
9
11.4
0
0
100
570
2095
233
667125
VT
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 light current interrupted
before experiment was completed
YT
Mar. 27
3
0
3
14.8
0
0
60
80
210
70
14900
ZT
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
D8
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
Gs
June 12
4
0
7
17.3
0
0
145
420
2185
312
735775
H8
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
Ks
July 18
4
0
20
21.3
2
7
150
900
L0600
530
7431800
Ls
July 24
3
0
6
21.4
0
1
100
900
1820
303
1053000
M8
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
Os
Aug. 14
4
1
14
18.7
2
1
90
900
3670
262
1816506
P8
Aug. 19
2
0
8
20.3
0
4
100
900
4110
514
3308900
Qs
Aug. 26
3
0
33
21
0
7
140
900
3320
404
8776400
R8
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 X«+ dark.
32 SELECTION IN CLADOCERA ON THE BASIS OF
TABLE 4. — Summary of selection data by broods for Line 695 minus.
a
3
S
3
i
2
.S
.S
.2
CD
h
•8
I
'i
M
1
I
I
1
|
I
.
J
1
I
l
3^
1
1
\
1
I'd
g
|
>> ta
.9
J
a
fl
1]
a m
1
2
S „
g
-.3
1
1
g
•sj
"o
°l
"8
a '
"8^
•g f
.11
el
"3 "a
a
°s
J
||
S
P
£*
ll
P
"3 £
1 *>
1
s.s
1912
I
April 6
10
2
10
14.5°C
2
2
200
900
5385
53.9
3491975
J
April 17
11
0
3
16.6
0
1
165
900
1537
512
1060009
K
May 6
11
0 or 1
6
16.3
0
4
215
900
4250
708
3475450
L
May 15
5
0
6
0
5
590
900
5090
848
4398100
L»
May 20
3
0
1
17
0
0
230
230
230
230
52900
M
May 25
4
0
5
19
0
0
185
645
1485
297
594875
N
June 1
10
0
12
18.5
0
1
80
900
2815
235
1243425
O
June 10
11
Oorl
5
17
0
3
520
900
4110
8SS
3492500
P
June 18
1
0
4
19
0
0
180
330
935
234
231625
Q
July 1
11
Oor 1
3
18
0
0
320
790
1495
498
874725
R
July 8
10
0 or 1
2
22
0
1
290
900
1190
595
894100
R»
July 9
11
0
2
22
0
0
230
345
595
288
171925
8
July 17
9
0
2
22
0
1
450
900
1350
675
1012500
B*
Aug. 1
3
0
4
21
1
2
600
900
3040
760
2389600
T
Aug. 16
2
0
1
20.5
0
0
235
235
235
S35
55225
U
Sept. 9
2
Oor 1
1
20
0
0
285
285
S85
81225
V
Sept. 16
2
0
1
19
0
0
25?
255
255
255
65025
V*
Sept. 20
5
0
1
19.8
0
0
460
460
460
460
211600
W
Oct. 2
1
0
6
14.1
2
0
95
665
1868
311
796624
X
Oct. 15
2
0
1
19.2
0
0
280
280
280
280
78400
X-
Oct. 18
3
0
8
18
1
0
70
615
2657
S3S
1150719
Y
Nov. 1
1
0
1
18
0
0
174
174
174
174
30276
Z
Nov. 16
1
0
2
15
0
0
235
445
680
340
253250
At
Nov. 25
2
0 or 1
4
17
0
0
310
595
1945
486
991725
Di
Dec. 7
3
0
5
16
0
0
170
230
980
196
194350
Cj
Dec. 17
2
0
12
19
0
0
105
605
4372
364
1987924
D»
Dec. 26
1
0
11
14
0
0
175
500
3058
278
790954
1913
Et
Jan. 6
11
Oorl
14
16.4
0
2
75
900
4967
set
2819819
F,
Jan. 13
1
0 or 1
4
14.4
0
0
90
190
555
139
84825
G,
Jan. 21
2
0
8
14.4
0
0
125
280
1625
203
356475
Ht
Jan. 28
3
0
8
13.4
0
0
95
365
1935
242
540675
It
Feb. 6
2
0
9
13
0
0
100
355
2075
231
553475
J,
Feb. 17
1
Oorl
9
13.4
0
0
140
510
2970
330
1066175
K.
Feb. 24
11
Oor 1
7
12.5
0
0
250
450
2470
353
908750
Li
Mar. 3
11
0 or 1
8
11.8
1
0
110
340
1675
209
416125
Mt
Mar. 20
1
0
1
17
0
0
315
315
315
315
99225
Nt
April 7
4
Oor 1
4
14.8
0
0
100
200
610
153
98850
0,
April 16
3
0
11
17.5
0
0
110
505
2600
236
777078
Pi
Q«
April 24
May 2
2
3
0
Oorl
14
6
17.5
15.8
3
0
0
0
80
95
395
250
3145
975
225
163
814325
173975
Ri
May 12
1
0 or 1
7
14
0
0
210
460
2140
306
699950
8,
May 20
2
0
5
14.5
0
0
330
660
2155
431
1002625
T,
May 29
3
0
6
16.5
0
0
70
135
640
107
71850
Ut
June 5
1
0
6
18
0
0
130
250
1105
184
212525
I*
June 12
4
0
9
17.2
0
1
95
900
2795
Sll
1364175
W,
June 18
2
0
5
19.3
0
0
130
610
1625
3S5
702425
x»
June 24
2
0
6
20.5
1
1
207
900
3067
511
1971599
Yt
July 1
2
0
1
21.7
0
0
138
138
138
138
19044
Zt
July 10
3
0
2
22
0
0
290
SOU
1090
645
724100
A,
July 19
2
0
4
22.3
1
125
900
1635
409
1011675
Bi
July 25
5
0
5
21.8
0
1
165
900
1945
389
1101375'
Ci
Aug. 1
2
0
6
22.8
0
2
430
900
3280
656
2356950
D!
Aug. 8
4
0
10
21.9
0
0
150
345
2180
SI 8
516350
EI
Aug. 14
2
0
11
0
0
80
635
2630
S39
889150
Fi
Aug. 20
3
0
2
20'.4'
0
1
850
900
1750
875
1532500
Gi
Aug. 28
4
Oor 1
11
21
0
0
100
320
2083
189
438627
Hi
Sept. 3
10
1
9
22.5
4
0
67
458
1466
163
360146
It
Sept. 9
2
0
13
20.6
0
1
122
900
5102
393
2701034
Ji
Sept. 17
10
0
8
18.4
0
0
146
460
1982
£48
564172
KI
Sept. 22
11
0
10
20
0
1
113
3011
301
1414841
I",
Sept. 29
10
1
6
16.5
0
0
113
640
1595
266
629195
Mi
Oct. 13
4
0
9
16.6
1
0
123
620
2565
285
961813
Nt
Oct. 20
11
0
6
17.8
0
0
110
244
1092
182
212072
8'
Oct. 28
11
0
8
19.3
0
0
94
370
1284
161
261488
p«
Nov. 14
11
0
7
16
3
0
74
440
1411
202
363415
Qi
Nov. 20
11
0
7
16.9
0
0
122
510
2069
S96
726711
'Selection was repeated from a later brood because of the loss of the earlier brood.
A PHYSIOLOGICAL CHARACTER.
33
TABLE 4. — Summary of selection data by broods for Line 695 minus — Continued.
1 '
8
1
J
3
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i
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1
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.3
1
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1
1
1
1
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1
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a
§
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1
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2
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$
*5! .
i
I «
5
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~1
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8 a
a «
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S
o
I5
B
fl
*
gi
Is3
0 0
a«
|I
f«
a
jp
1913
R.
Dec. 1
Rand
m distribution.
Si
Dec. 8
11
0
5
15.5°C
0
0
188
380
1468
294
454376
T,
Dec. 16
1
0
5
17.5
0
0
210
430
1615
323
553075
Ui
Dec. 26
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
A4
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
It
J4
April 15
May 2
5
3
0
0
8
19.7
19.7
0
1
0
0
120
490
820
490
3340
490
418
490
1937200
240100
K4
May 8
4
0
11
19
6
1
88
900
3603
328
1704519
Ll
May 14
2
0
8
21.2
0
0
100
360
1656
£07
410010
M4
May 22
2
0
9
18.7
4
1
97
900
4224
469
2663016
Ni
May 29
11
1
7
22
1
4
490
900
5608
801
4634574
o<
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
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
Z4
Sept. 7
5
0
16
21.8
2
0
100
800
6383
399
3386309
At
Sept. 17
11
0
6
16.3
0
0
85
395
1111
185
266303
B,
Sept. 23
3
0
11
19
8
100
900
3372
307
1670024
C,
Sept. 30
2
0
8
20.1
2
0
80
400
1701
213
468945
Di
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
F.
Oct. 20
2
0
14
19.9
0
2
131
900
5108
365
2750448
Gi
Oct. 26
11
0
13
17.3
0
0
90
550
3285
253
1063923
H,
Nov. 2
3
0
12
16.2
4
0
80
590
3020
252
1119750
h
Nov. 9
5
0
7
16.7
2
0
150
620
2125
304
886375
Jf
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
L.
Nov. 30
5
1
34
19
8
0
60
552
8346
246
2771592
Mi
Dec. 5
1
0
5
18.4
0
0
60
188
613
123
85569
N,
Dec. 11
4
0
I
16
1
2
145
900
2435
487
1766693
Oi
Dec. 19
2
0
12
16
4
1
100
900
4350
363
2196550
P,
Dec. 26
2
0
11
12.5
0
0
72
280
2307
210
522109
1915
Qi
Jan. 4
3
1
20
19.3
1
0
91
735
5364
268
1914546
R.
Jan. 11
3
0
14
20.7
3
1
205
900
6180
441
3273950
S.
Jan. 19
3
0
7
17.2
0
0
65
330
1140
163
252150
Ti
Feb. 1
3
1
11
17.5
3
0
100
630
3439
S+S
1296451
u,
Feb. 10
1
0
10
13.3
0
0
85
445
2340
2S4
678550
V,
Feb. 18
10
0
12
16.6
2
2
110
900
3977
331
2184369
w.
Feb. 24
3
0
10
20.2
1
0
105
338
1942
194
439790
X,
Mar. 5
3
0
13
16.3
0
0
55
395
2415
186
621425
Y,
Mar. 12
1
0
12
16
0
1
70
900
3052
254
1429104
Z.
Mar. 20
4
0
8
18.8
0
1
92
900
2078
260
1068210
Ai
Mar. 30
1
0
8
16.5
0
0
90
516
1962
245
717912
B,
April 6
7
0
9
15
0
0
112
323
2092
232
529828
Ci
April 13
11
0
10
17.5
2
0
150
515
3309
331
1258685
Di
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
F,
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.
.5
i
3
i
3
.2
.S
.S
1
A
1
"8
g
a
s
a
I
I
1
1
I
§
$
§
1-a
«
'•3
3
t
1
1
••a g
0
t
i
cd
|
|
1
•S
I
1 .
11
*^
M c
1!
1
c
1
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c
N
individu
an end o:
M
I*
•s
1
1
**•• .
1
M
o
W
ti
1*
•3
fc
h
a
0 ffi
.§ 0
§ S
£ a
I!
Ji
1
il
1915
G»
May 13
4
0
11
17.8°C.
3
0
70
436
2083
189
563453
H«
May 22
1
0
8
17.3
0
1
90
900
2060
258
1 1 14000
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
K,
June 15
2
0
11
20.6
7
0
171
395
3033
276
895395
L.
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
Oi
July 14
Random distribution.
Pe
July 20
Random distribution.
July 28
Random distribution.
Pe
Aug. 7
Random distribution.
Q<
Aug. 13
Random distribution.
PU
Aug. 19
Random distribution.
S<
Aug. 30
Random distribution.
Te
Sept. 7
5
0
7
22.5
1
3
150
900
4670
667
3614500
u,
Sept. 13
4
0
7
20.9
0
0
150
630
2000
286
740650
Ve
Sept. 20
4
1
20
20.2
0
6
130
900
12635
6S2
9572475
W,
Sept. 29
5
0
11
18.2
1
2
125
900
5660
515
3778100
X,
Oct. 9
12
1
11
16.5
3
0
70
450
2070
188
514750
Y,
Oct. 18
5
0
12
18.4
1
0
90
690
3445
287
1323225
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
126
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
H?
Dec. 25
5
1
9
11.9
5
1
120
900
2820
313
1456550
1916
Iy
Jan. 4
5
0
10
14.5
0
10
900
900
9000
900
8100000
J7
Jan. 13
8
1
8
18
0
80
900
2280
£85
1279400
K7
Jan. 23
3
1
9
19
1
0
105
600
2115
285
673725
Jan. 31
9
1
11
13.1
1
4
130
900
5870
534
4265500
M7
Feb. 7
4
17
15
0
1
50
900
3865
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
11
18
6
0
90
750
3845
350
1912925
Rv
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
VT
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
Zi
June 13
5
0
6
17.8
1
0
100
GOO
1390
232
493050
A,
June 20
4
0
13
17.8
1
0
75
420
2020
155
412700
Bi
June 30
4
0
12
18.8
0
0
80
360
2010
168
403300
C,
July 7
6
0
7
19.5
0
3
180
900
3970
567
2889700
D,
July 14
3
1
6
20.7
2
0
135
470
1655
276
521025
Ei
July 21
3
0
I
21.1
0
4
270
900
3870
774
3312900
F,
Aug. 3
Random distribution.
G,
Aug. 14
5
1
26
19
0
0
110
460
4100
168
869400
Hi
Aug. 19
12
0
7
19.8
0
0
120
360
1495
213
360025
It
Aug. 26
2
0
21
21
0
1
80
900
8540
407
4770700
Ji
Sept. 1
ai
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
oloeet 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.
1
*{
•3£
i
3
g
A
J
a*
Time period.
1
t
. of young tested
|
1
"o
6
li
"of
No. of individl
; brood divided
age of mothers.
gatively reacting
B.
idividuals failing
L end of the tank
minimum reacti
|
'3
g
'action-times.
1
I
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1
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c
I
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III
III
j|
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B
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1
1
O
2:
H
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'*'*
M
*
CQ
£
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
249.17
40.86
Oct.-Nov. 1912. . .
24-29
G
14
2.3
11.2
.21
0
0
167
316
3268
233
101.40
18.28
Dec. 1912-Jan. 1913
30-35
B
62
10.3
9.0
1.14
0
0
77
428
13761
222
Feb.-Mar. 1913...
36-42
7
56
8.0
8.3
.96
0
3
158
447
16930
302
Apr.-May 1913...
43-50
8
67
8.4
7.8
1.08
1
0
100
331
13345
199
June-July 1913...
51-58
9
35
3.9
7.2
.54
3
5
209
462
12355
353
Aug.-Sept. 1913...
59-67
9
74
7.2
6.7
.07
2
3
114
548
20737
280
Oct.-Nov. 1913. . .
68-75
8
66
8.4
7.5
.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
6
56
9.3
8.3
.12
4
10
158
634
24443
436
Apr.-May 1914...
90-98
9
75
8.2
7.1
.15
18
15
198
766
32718
436
June-July 1914. . .
99-105
7
40
6.1
6.3
.97
6
2
149
693
15033
376
224 64
23.96
Aug.-Sept. 1914...
106-114
9
97
8.2
6.6
.24
8
18
152
783
43400
447
Oct.-Nov. 1914. . .
115-122
8
106
12.5
7.5
.67
27
4
125
776
35712
337
Dec. 1914-Jan. 1915
123-130
8
66
8.4
7.1
.18
8
7
111
612
20726
314
Feb.-Mar. 1915... .
131-138
7
85
11.6
8.3
.40
5
4
114
664
26122
307
Apr.-May 1915...
139-145
8
81
10.0
8.6
.16
13
11
180
876
34741
429
279 ! 77
20 97
June-July 1915...
146-153
5
43
10.5
7.9
.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
7
107
17.7
8.4
.11
5
2
74
662
25300
236
Dec.1915-.Jan. 1916
167-174
7
80
12.4
8.5
.46
9
5
84
643
19710
246
206! 09
15, 54
Feb.-Mar. 1916. . .
175-181
6
70
10.3
7.9
.30
5
5
91
664
18570
265
Apr.-May 1916...
182-187
6
67
11.7
10.4
.13
6
10
88
787
22090
330
June-July 1916...
188-195
6
49
9.0
7.2
.25
6
12
163
612
21200
433
288 !si
27.78
Aug.-Sept. 1916...
196-200
0
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. .
bAH 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.
•S
o. of young tested.
•3
1
ii
No. of individuals
st brood divided by
3 age of mothers.
egatively reacting in- 1
ils. 1
individuals failing to
in end of the tank.
minimum reaction-
maximum reaction-
j
individual reaction-
i
"3
|
|
ce between means for
id minus strains.
§
•3
O
ce divided by proba- 1
)r. 1
O
1
Total N
| Average
1 brood.'
iii
Average
per fir
averagi
a g
P
6 8
£ *
I1
If
•s
1
J|
S
Probabl
1 Differen
1 plus ai
1
II
Apr.-May 1912...
9-13
6
31
5.2
11.8
0.44
2
12
264
746
17977
580
292.29
35.41
-166
47.13
3.52
June-July 1912...
14-19
7
30
4.7
8.3
.57
0
6
296
724
12470
416
302.16
37.21
+ 182
50.32
3.62
Aug.-Sept. 1912. . .
19-22
5
8
1.0
1
2
367
427
4275
534
254.67
60.73
-201
73.20
2.74
Oct.-Nov. 1912 ...
Dec. 1912-Jan. 1913
23-27
28-34
H
22
62
2.8
8.9
8l9
".'26'
1.00
3
0
0
2
194
119
462
439
7604
17492
346
282
174.94
25.16
-113
- 60
31.09
3.63
Feb.-Mar. 1913...
35-39
£
34
6.8
9.4
.72
1
0
183
394
9510
380
+ 22
Apr.-May 1913...
40-46
7
53
7.0
9.4
.74
3
0
142
372
12265
231
- 32
June-July 1913...
47-54
8
38
4.2
7.2
.58
2
4
160
675
13400
353
±000
Aug.-Sept. 1913. . .
Oct.-Nov. 1913. . .
55-64
65-70
10
5
85
37
8.5
5 5
6.4
8.0
1.33
.69
4
4
5
0
217
105
646
437
25079
8421
295
228
- 15
+ 15
Dec. 1913-Jan. 1914
71-77
7
45
6.4
9.2
.70
0
5
151
725
19682
437
+ 68
Feb -Mar 1914
78-85
g
60
7 5
7 9
95
4
g
195
728
26365
439
— 3
Apr-May 1914...
86-92
7
48
7.1
7.0
1.01
12
6
219
673
19836
413
+ 23
June-July 1914. . .
93-99
7
29
4.1
9.8
.42
7
6
374
670
13478
465
176; 94
21.41
- 89
32! is
2! 76
Aug.-Sept. 1914...
100-107
8
82
9.6
7.2
.33
22
11
176
762
3228S
394
+ 53
Oct.-Nov. 1914 . . .
108-116
9
123
12.4
7.0
.77
28
3
94
648
34167
278
+ 59
Dec. 1914-Jan 1915
117-124
7
74
8 7
8 1
07
g
4
105
605
22389
303
+ 11
Feb.-Mar. 1915...
125-131
8
84
11.0
8.1
.36
6
4
88
628
21205
252
+ 55
Apr.-May 1915...
132-139
8
76
9.9
7.6
.30
13
2
118
604
19978
263
197; 23
15l26
+ 166
25! 93
6146
June-July 1915...
140-145
5
36
8.7
7.6
.14
8
4
326
730
16613
461
233.14
26.21
- 99
38.01
2.60
Aug.-Sept. 1915...
146-153
4
45
8.8
8.4
.05
2
11
139
833
24965
555
292.86
29.45
-188
43.93
4.27
Oct.-Nov. 1915. . .
154-160
7
97
14.1
8.0
.76
9
1
78
522
20310
209
+ 27
Dec. 1915-Jan. 1916
Feb.-Mar. 1916...
161-168
169-175
8
7
91
100
12.1
11.3
8.4
7.6
.44
.49
15
9
17
3
194
83
752
704
(8860
J3C-SO
367
237
295195
20 .'93
-121
+ 28
26 1 06
Apr.-May 1916...
176-181
B
52
11.0
11.6
.95
3
6
95
748
15430
297
+ 33
June-July 1916...
182-188
7
54
6.5
8.6
.76
4
7
141
624
17065
316
264162
24 '.29
+ 117
36 190
3li7
Aug.-Sept. 1916...
189-195
4
65
8.8
7.3
1.21
2
1
118
620
18555
285
212.15
17.75
+ 192
30.02
6.39
• Including all the data available, some of which is additional to that for the broods tested.
b All available data is here used, including that for No. 2 mothers whose broods were not ordinarily tested for use in selection.
1, 1912, and subsequent to July 31, 1916. The data obtained previ-
ous to August 1, 1912, deserve less serious consideration than may
be given the data obtained later, since previous to that time improved
methods of handling the material and additional experimental pre-
cautions were occasionally being devised and utilized. From August
1, 1912, the methods of handling the Cladocera material were pretty
well standardized and there seems every reason to believe that the
data are such as would be obtained by repetition of the experiments.
A PHYSIOLOGICAL CHARACTER.
37
The mean reaction-times obtained in the tabulation by two-
month periods (tables 5 and 6) are used in the plotting of curves
(figure 2c), showing graphically the courses of the selection experi-
ment with Line 695.
Table 7 gives in abbreviated form selection summaries of the
data for 695 plus and 695 minus by the larger periods (mostly year-
periods) into which it was divided in tables 5 and 6; it also includes
summaries and results of the " test series" and summaries for selection
data for three-month periods during which the test series were con-
ducted. In table 7, in the vertical column to the left, are indicated
TABLE 7. — Selection summary for Line 695.
Time period.
Apr. 1, 1912-
July 31. 1912
Aug. 1, 1912-
July 31, 1913
Aug. 1, 1913-
July 31, 1914
Test series,
Aug., 1913...
July 1, 1913-
Sept.30, 1913
May, 1914 . .'
Apr. 1, 1914-
June 30, 1914
Aug. 1, 1914-
July 31, 1915
Test series,
May, 1915 . .
Apr. 1, 1915-
June 30, 1915
Test series,
June, 1915 . .
May 1, 1915-
July 31, 1915
Aug. 1. 1915-
July 31, 1916
Aug. 1, 1916-
Sept. 2, 1916
Strain.
Plus. ..
Minus
Plus. ..
Minus
Plus...
Minus
Plus...
Minus
Plus. ..
Minus
Plus. ..
Minus
Plus . . .
Minus
Plus . . .
Minus
Plus. ..
Minus
Plus...
Minus
Plus...
Minus
Plus. ..
Minus
Plus. ..
Minus
Plus...
Minus
8-19
8-19
20-58
19-64
59-10;
66-99
61
67
55-67
61-64
95
89
90-10(
86-95
106-153 45 10
100-147 46
1S2-14S IS
143
1431615
143-153
136-147
154-195
148-187
196-203
188-19S
732
497.3
499.1
260.2
£97.5
371.8
S71.S
522.2
296 'B
AA1 £
3Q8.7
SOS. 7
350.1
351. £
399.2
517.9
369
317.9
397.9
259.2
308.3
182.5
187.7
253.3
£46.6
284.4
£8£.6
225.4
£6£.S
222. 9
£16. £
279.4
244^3
242.5
£6 £.8
338.6
ese
6.19
21.80
18.67
8.19
24.49
8.99 + 0.5
+si:7
+ 2
- 1.0
+60.0
+61.5
-1-150.8
11. 69 3. S
13.100.04
'.96
10.686!62
6.
22.56
14.083.63
28! 63 2.
5.0
the periods covered by the various portions of the summary ; succes-
sive columns to the right of this indicate the strain (whether plus or
minus); the generations of descent during the period under con-
sideration; the number of broods used in selection and in obtaining
the data; the average number of individuals per brood of those
tested; the total number of individuals tested in making selections;
the number of individuals moving to the negative end of the tank;
the number failing to respond to the extent of reaching either end of
the tank; the average minimum reaction-time; the average maximum
reaction- time ; the mean individual reaction-time; standard deviation
209S52
38
SELECTION IN CLADOCERA ON THE BASIS OF
of this mean; its probable error; the difference between the mean for
the plus strain and that for the minus strain; the statistical probable
error of this difference; and the quotient of this difference divided by
its statistical probable error. For each period considered in this
table the corresponding data for the plus and minus strains are given
in successive horizontal columns, the plus first and the minus in
italics immediately below it. Thus the items for making the signifi-
cant comparisons between the plus and minus strains are found
together.
Table 8 gives a summary of the data for the " same-day" broods,
i. e., those broods of the mothers of the corresponding plus and minus
strains which reproduced on the same days and the young of which,
consequently, were tested under identical conditions. The arrange-
ment of the data is similar to that for table 7.
TABLE 8. — Same-day broods. Summary of data for Line 695.
I
i
3
i
A
A
d
1
S
J
|
%•%
'S
•|
*3
I
S .
S
2
a
53
0$
Cj
rt
S3
2 §
«
m
Time period.
Strain.
•s
1
"3
0
!2
1
viduals fa
nd of the
1
'3
S
3
1
1
•o
2
Between m
linus strai
"o
I
1
!i
•3
No. of negai
dividuals.
IS
si
1
||
Average mi
time.
1
fl O
Standard d(
Probable er:
1 Difference b
plus and n
i
§3
Apr. 6, 1912-
July 31. 1912
t Plus ....
( Minus . .
|
8.5
7.6
17
16
3
S
4
5
193
360
900
900
545.3
633.0
'-'88
Aug 1, 1912-
11
4 3
47
2
7
173
455
332 9
July 31, 1913
I Minus.'.'.
5.6
6S
S
4
7 SO
601
333.7
- ;
Aug. 1, 1913-
< Plus
0
7.1
64
1
3
138
527
290.6
July 31, 1914
\ Minus...
0
8.1
73
4
230
ffOS
SOS. 9
— 12
Aug. 1, 1914-
t Plus
C
10.5
63
6
2
125
662
314.2
+ 2
July 31, 1915
I Minus...
C,
11 .3
68
n
3
679
311.9
Aug. 1, 1915-
( Plus....
10
12.0
120
15
9
92
622
284.0
223.11
13.74
July 31, 1916
Aug. 1, 1916-
I Minus...
( Plus
in
:<
14.6
18.3
55
9
2
12
S7
110
677
900
«77./
383.6
250.74
324.97
/4-00
29.56
' 11 " 7
+ 122
';s!e/
35.10
o!s5
3.47
Sept. 2, 1916
I Minus...
:-;
;s.o
54
0
/
;os
573
206.33
18.94
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 anu 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 +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 strain 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 strain
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
I B
I 5
10
00
B
15
4-5 8-9 12-1 4-5 8-9 12-1 4-5 8-9 12-1 4-5 8-9 m 12-1 4-5 8-9
1912 1913 1914 1915 1916 -,2.0'
•tl M.rl
:
I 'I
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
r i - 3 A i
o
O O
0
7 ' .0 oo
-
—
032
0
|
O O |0
—
4 4
00 0
0
00 000
O O O -
2204
9
85^902
0 10 10
m
0
O
0 -
1
0
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
1912
1913
1914
1915
1916
.50
.25
00
.25
.50
600
450
300
150
4-5 8-9
1912
12-1
4-5 8-9
1913
4-5 8-9
1914
in TV
12-1 4-5 8-9
1915
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 two-month periods for Line 689 plus.
Time period.
Apr.-May 1912....
June-July 1912... .
Aug.-Sept. 1912. ..
Oct.-Nov. 1912. . .
Dec. 1912-Jan. 1913
Feb.-Mar. 1913..
Apr.-May 1913...
June-July 1913...
10-14
15-19
20-23
24-26
27-33
34-40
41-47
48-52
11.1
9.0
17.8
18.7
9.0
8.3
7.9
8.2
0.48
.58
.13
ll
r =
TABLE 10. — Selection data summarized by two-month periods for Line 689 minus.
1
*i
— j?
A
3
i
A
i
d
g
i
i
!
V
mothers
s were p
HI
I
2
a failing
the tank
g
P
|
1
c
o
n means
trains.
1
«a
•o
M
1
•5
«~i »O "
>>
2"s
3
3
•f
*
*s
«J n
V
•2
Time period.
Generations of
No. of broods.
Total No. of yc
0
1
Average No. <
per first broc
average age o:
No. of negative
dividuals.
No. of individi
reach an end
i
fi
Average maxin
time.
Sum of reactior
1
Standard devia
1
Difference betw
plus and minu
Probable error
Difference divic
ble error.
Apr-May 1912....
June-July 1912....
10-13
14-17
4
5
9
20
2.3
7.3
13.3
9.0
.17
.81
1
0
2
3
246
439
586
728
4048
9415
450
471
+ 53
+ 22
Aug.-Sept. 1912...
Oct.-Nov. 1912. . .
Dec. 1912-Jan. 1913
Feb.-Mar. 1913...
Apr.-May 1913....
June- July 1913....
18-19
20-23
24-30
31-36
37-44
45-50
2
(i
7
fi
8
(i
8
14
51
31
84
35
4.0
1.3
7.3
5.2
9.1
5.8
21.5
14.6
8.8
9.2
8.0
7.5
.10
.09
.83
.57
1.14
.77
0
0
2
0
2
0
0
2
0
0
0
138
155
114
173
113
138
268
306
383
321
492
468
1624
3375
13135
8175
-MS. -,11
8722
203
241
258
264
260
249
106.46
148.60
130 ! 89
25.39
26.79
li! 92
+ 131
+ 90
+ 59
+ 38
- 36
+ 79
56.70
44.57
27^43
2.31
2.01
2^88
A PHYSIOLOGICAL CHARACTER.
TABLE 11. — Selection summary for Line 689.
47
Time period.
Apr.- July 31,
1912
Aug. 1, 1912-
July 16, 1913
(Plus..
I M imu
Plus.
9-19
9-17
20-52
18-60
36
174
484**
295.1
255.1
260.7
£69.4
194.8
145. S
23.9
S3. 7
8.2
6.6
S §
+34.2
+40.0
10.5
0.83
3.71
TABLE 12. — Same-day broods. Summary of data for Line 689.
Time period.
Strain.
•3
rage No. of young
brood.
of individuals.
of negatively react-
individuals.
of individuals fail-
to reach an end
the tank.
rage minimum re-
ion-time.
rage maximum re-
ion-time.
n individual re-
ion-time.
erence between
an reaction-times.
i
{»
i
Z'2
°.s-s
l§
IS
£8
5§
Apr. 9, 1912-July 31,
JPlus..
l
5.0
5
0
3
315
900
712
+ 163
1912
1 Minus
i
4.0
4
0
0
440
690
543
Aug. 1, 1912-July 31,
JPlus..
10
8.9
89
4
2
125
464
295
+3
1913
1 Minus
10
7.2
72
i
*
131
#7
S9S
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.
1 For 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
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
:j
i
A
8?
1
i
r
.50
.Z5
00
,25
4-5
FJ
1912
8-9
T
12-1
B
T
i
4-5
1913
;
t
.\
i
o
A
4-50
300
150
00
4-5 8-9
1912
12-1 4-5
1913
FIGURE 3. — Line 689.
A. Reproductive indices, actual values.
B. Reproductive indices, superiority.
C. Reaction-time curves with superim-
there were no consistent differences poseK- cui'ves (? fa!?t lines) r?pre?enti,ng th*
combined reaction-time means for the plus and
m reaCtlVeneSS Lines 691, 695, minus strains of all Daphnia pulex lines in which
713. 714 and 751 CsPP fitnirp 3r^ significant reaction-time differences did not
uu 101 ^be< ngure 3C). arise— Lines 691, 695, 713, 714, and 751.
inspection ol 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
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.
Time period.
Ipr. 12, ]
July 31, 1912
Aug. 1, 1912-
July 31, 1913
Aug. 1, 1913-
June 12, 1914
Plus....
Minus . .
Plus
Minus . .
Plus....
Minut . .
10-19
11-18
20-55
19-57
56-93
68-95
480.9
4S4.9
248.8
£96.
391.0
416. S
299.0
500.5
164.5
g£0.6
265.8
£68. £
31.9
34-3
7.4
9.S
10.6
10.9
+46.0
-47.8
'-£S'.8
4.0M
50
SELECTION IN CLADOCERA ON THE BASIS OF
TABLE 14. — Same-day broods. Summary of data for Line 691.
Time period.
Apr. 8, 1912-
July 31. 1912
Aug. 1, 1912-
July 31. 1913
Aug. 1, 1913-
June 9. 1914
Strain.
Plus....
Minus . .
Plus
Minus..
Plus
Minus . .
634
303
192
343
413
370
.69
S7S.S6
12.58
28.34
-156
+43
mean was larger by 46 ±46. 8 seconds.1 For the year, August 1912-
July 1913, the two means were 248.8 and 296.6 seconds (225 and 257
individuals). The difference, —47.8 ±11.9 seconds, was 4.02 times
the probable error. The same-day broods for this year-period, of
which there were 8, gave a mean of 192 seconds for the plus and of
348 seconds for the minus strain and a difference of —156 ±31. 00
seconds — 5.03 times the probable error (table 14). In spite of the
irregular course of the curves, the data for this year might seem to
indicate a possible (though slight) effect of selection, except for the
fact that the further history of the line does not bear out this inter-
pretation.
For the following year (nearly), which concludes the data for
this line, the mean for the plus strain was 391.0 seconds (285 indi-
viduals); for the minus strain 416.2 seconds (273 individuals). The
difference was —25. 2 ±15. 2 seconds (table 13) and is not of statisti-
cal significance. There were 6 same-day broods for this period, for
which the plus mean was 413 seconds and the minus mean 370
seconds (table 14). This difference (+43 seconds), while not of
statistical significance, is in the reverse of an effect of selection. No
test series were obtained for Line 691.
Figure 4 shows the curves of the mean reaction-times for the
two strains for Line 691 by two-month periods. The curves are
quite irregular and indicate considerable fluctuation in mean re-
action-times, particularly for the minus strain, which, except for its
wide fluctuations, was persistently the less reactive of the two strains
after the first 4 months of selection. The curves for the two strains
follow each other in a general way, but are so irregular that they
cross 8 times. The rather persistently (though only slightly) higher
reaction-time of the minus strain is suggestive of a possible slight
'There were only 2 same-day broods, involving totals of only 7 individuals in the plus and
8 individuals in the minus strain. The means were 634 and 303 seconds, the difference being
+331 seconds. These broods came in July, when the plus strain was relatively slightly reactive
A PHYSIOLOGICAL CHARACTER.
51
effect of selection within this line. However, the actually smaller
difference in mean reaction-times during the last year as compared
with the preceding year does not lend support to this supposition,
and the same-day-brood data for the last longer period are in oppo-
sition to an effect of selection, so that a result of selection within
Line 691 must be considered very questionable.
Obtaining the data for reproductive vigor (from the daily
laboratory notes) is laborious and was not done for Line 691 and a
number of the other lines in which there was obviously no effect of
selection or in which the results were inconclusive.
The parallel effect of broad environmental influences upon the
reaction-time means for the two strains is shown in the curves
(figure 4) for the two strains, they in a general way following the
same course.
600
300 -
1912
FIGURE 4. — Line 691. Reaction-time curves.
LINE 711.
Tables 15, 16, 17, and 18 and figure 5 present the data for Line 711,
TABLE 15. — Selection data summarized by two-month periods for Line 711 plus.
2 .
£
i
£
a
i
•
I
I
|
1
3
a
I
1
1
i «
£
2
i
2
•3
i
1
l
1
i!
|
S
•
I
1
|
1
Time period.
*o
•j
R
.£
•i
•-
•j3
~Q
*£
1
§
a
1
"3
X
1»
G fl
1
I
|
i
•S
s
I
0
fc
Total N<
§§
p
•" a
"o -g
|2
fi
Average
time.
•3
li
1
1
1
Apr -May 1912....
7-12
6
30
3
8
299
783
13605
454
308.91
38.040
June-July 1912....
13-17
6
22
0
10
345
668
11805
537
Aug.-Sept. 1912... .
18-19
2
6
0
0
144
220
1043
174
41.77
11.5.
Oct.-Nov. 1912
20-23
4
14
2
0
124
449
3662
262
Dec.1912-Jan.1913..
24-31
8
83
1
4
114
506
22875
276
Feb.-Mar. 1913
32-36
5
34
0
0
141
397
9225
271
Apr .-May 1913
37-43
7
62
1
0
119
349
13095
211
June-July 1913
44-50
7
36
1
0
148
343
6792
189
96.75
' ' 10.88'
52 SELECTION IN CLADOCERA ON THE BASIS OF
TABLE 16. — Selection data summarized by two-month periods for Line 711 minus.
i
5
J
*
i
i
I
.
i
I
i
i
1
11
•g
1
j
1
s
i
!i
1
I
8
c
1*0
§
'•?
1
.2
•o
I;
•o
"S
Time period.
"5
1
•s
"S
'fl
S
1
•S
1
ii
1
I
§
I
O
•8
c
Total No
|J
f §
o £
i'
Jj
Sum of re
||
Standard
1
rDifferenc
plus and
Probable
Differenc(
ble error
Apr.-May 1912...
7-12
1
25
i
14
458
900
17440
698
265.71
35.84
-244
52.27
4.66
June-July 1912...
13-18
6
24
i
8
308
643
11381
474
+63
Aug.-Sept. 1912...
Oct.-Nov. 1912. . .
18-19
20-25
3
7
5
17
2
I
0
0
348
229
572
335
2385
5019
477
295
171.16
51.63
-303
-33
52.89
5.33
Dec. 1912-Jan. 1913
26-31
6
41
1
2
113
380
12484
304
-28
Feb -Mar. 1913 . .
32-38
7
4'-!
2
0
123
418
12550
292
—21
Apr-May 1913...
June-July 1913...
39-45
46-51
7
6
40
44
1
10
0
2
131
132
341
573
9515
12275-
238
279
20i!87
'26 '.53
-27
-90
2s:23
'sis?'
TABLE 17. — Selection summary for Line 711.
Time period.
Apr. 1912-
July 31, 1912
Aug. 1, 1912-
July 31, 1
Strain.
Plus
Minus . .
jPlUB....
1 Minus . .
6-17
6-18
18-50
18-61
4-1
488.7
688. £
241.2
286.9
327.7
SIB. 1
163.3
193.8
30.7
30.4
7.2
9.6
-45.7
43.1
3.84
TABLE 18. — Same-day broods. Summary of data for Line 711.
Time period.
Mar. 26, 1912-July 31
1912...
UU»13.'.
1, 1912-July 31
Strain.
Plus..
Minus
Plus..
Minus
4.0
f.7
8.3
Hi
477
SS3
128
900
8£0
398
348
680
638
224
l|
ii
A PHYSIOLOGICAL CHARACTER.
53
750
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
00
190
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.
£
I.
A
2
J
1
|
i
i
0
|
1
-a 15
i!
1
11
1
1
1
i
d
03
i
fa
s.
•o
1
!i
H
1
is
|
g
S
3
J
1
I
I
•o
"2
12
Time period.
Strain.
•3
^
00
.S g>
>
*> c
•|
"P
'«
0
Is
0
*>
1
|
fc-a
P
f-3
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!
1
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I
|s
O
6
!*
1*
11
Jl
fl
w
||
00
3
1
|1
1
•
Apr. 1912-
July 31. 1912
Plus....
Mint**..
9-18
10-19
10
14
5.4
5.7
54
67
3
2
15
232
256
644
476.7
459.7
314.5
277.4
28.9
24.0
+ 17.0
37.6
.45
Aug. 1, 1912-
Plus ....
19-53
37
6.4
236
3
5
139
401
244.6
163.0
7.2
July 31. 1913
Minus . .
20-54
36
5.6
203
7
77
793
437
255.4
755.5
5.9
-40 '.5
77^5
Si 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
Minus . .
55-93
40
5.2
327
20
42
174
732
404 3
267.0
9.7
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
15
75.2
273
32
6
720
629
247.3
757.6
7.4
Feb. 1914-
Plus .
76-84
9
8.3
75
4
15
149
758
425.3
Apr. 31. 1914
Aug. 1, 1914-
Minus . .
Plus. ...
73-53
97-136
70
40
7.7
11.9
77
477
d
9
39
245
145
722
633
457.6
306.0
'241.3
"i'.s
-32.3
July 7. 1915
Test series,
Minus . .
Plus....
94-139
125
1
72.0
20.0
550
180
85
31
fi
141
91
695
778
349.6
342.9
236.9
247.2
6.8
12.4
-43.6
io'.i'
4 '.32
Mar. 1915...
Minus . .
725
9
79.5
775
22
77
704
796
354.4
267.7
73.2
-47^5
"is'.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.
Minus
726
SI
39.6
7265
247
774
95
567
377.7
245 4
4 7
Feb 1, 1915-
Plus. ..'
120-130
11
14.3
157
19
17
104
678
335.8
+ i9.3
Apr. 30, 1915
Minus . .
779-730
72
77.5
735
70
702
602
376.5
....
TABLE 20. — Same-day broods. Summary of data for Line 713.
Time period.
Apr. 13, 1912-
July 31, 1912
Aug. 1, 1912-
July 31. 1913
Aug. 1. 1913-
July 31, 1914
Aug. 1, 1914-
July 7, 1915
Strain.
Plus....
Minus . .
Plus....
Minus . .
Plus
Minus . .
Plus....
Minus . .
2.7
3.0
14 7.1
i-i
5.4
14 13.8
1473.7
100
KM
61
M
19:i
fftf
Il
400
tf7
fe4
9tM
f,i>o
890
3D.-)
SM
154.42
208.08
290.23
277.45
+53
10.42
73.53
iM.Sf, fL'i'J
23.34
77.37 5.
34.446.96
difficult to ignore the very consistent differences between the two
strains after the first 4 months of the experiment. It is to be re-
gretted that no test series was conducted with this line.
This case is very similar to that for Line 689, with which there
was a similar difference, but in the reverse direction.
A PHYSIOLOGICAL CHARACTER.
55
With Line 711, as well as with most of the other lines, the courses
of the curves for the two strains illustrate the marked effect of
environmental influences upon reaction-time (figure 5).
LINE 713.
The data for Line 713 are shown in tables 19 and 20 and in
figure 6. Scarcely more than a glance at figure 6 is necessary to
convince one that with Line 713 there was no effect of selection,
though the selection data for the last year period are suggestive of a
possible selective effect. Environmental influences were obviously
at play in directing the general course of the curves, which follow
each other in an interesting way. The curve for the plus strain is
quite irregular, while that for the minus strain is surprisingly free
from large minor fluctuations.
600
450
300
4-5 8-9
1912
4-5 8-9 12-1 4-5 8-9 12-1 4-5
1913 1914- 1915
FIGURE 6. — Line 713. Reaction-time curves.
For the first 4 months the means for the two strains were
476.7 and 459.7 seconds (table 19). The difference is +17.0 ±37.5
seconds. There were 3 very small same-day broods for each strain,
with the average for the plus strain 53 seconds the larger (table 18).
The large upshoot in the curve for the plus strain for June- July is
from an average obtained from a very small number of individuals.
The data for the year, August 1912-July 1913, show these
means for the plus and minus strains: 244.6 and 285.4 seconds. The
difference is —40. 8 ±11. 5 seconds, 3.55 times the probable error.
The 14 same-day broods for this period have as averages 233.7 and
321.9 seconds, the minus-strain average being larger by 88 ±17.31
seconds. The extremely fluctuating course of the curves for this
year-period indicates that the differences obtained are not due to
an effect of selection.
During the next year, August 1913-July 1914, the plus mean
is higher by 27.0 ±13. 89 seconds. (The plus mean is 431.3 seconds
and the minus mean 404.3 seconds.) The 7 same-day broods averaged
56 SELECTION IN CLADOCERA ON THE BASIS OF
620 and 380 seconds, the plus mean being 240 ±34. 44 seconds the
larger.1
A small test series (268 and 273 individuals in the two strains)
conducted during March 1914, gave 265.9 and 247.3 seconds as the
plus and minus mean reaction-times (table 19). The difference
( + 18.6 ±11.62 seconds) was 1.58 times the probable error.
For the final year of the experiment with Line 713 (August
1914- July 7, 1915) the means were 306.0 and 349.6 seconds. The
difference was —43. 6 ±10.1 seconds, or 4.32 times the probable
error. For the 14 same-day broods the means were 305 and 333
seconds, the difference being only —28 seconds. Two test series were
conducted during March of this year. The first consisted of a com-
paratively small number of individuals (180 and 178). The means
were '342.9 and 384.4 seconds. The difference (-41.5 ±18.15
seconds) is 2.28 times the probable error and is not a significant
difference. A much larger and more adequate test series was con-
ducted after that just referred to. The numbers were 1,266 and
1,268 for the plus and minus strains, respectively. The means were
372.7 and 371.7 seconds. The difference was only +1.0 ±6.88
seconds. This test series was conducted with every care and it is
believed may safely be regarded as a crucial test. It brings out
clearly enough that, in spite of the small but rather consistent dif-
ference in mean reaction-times during the last year of the experi-
ment, there is not an effect of selection in Line 713.
LINE 714.
Line 714 was one of the original lines of D. pulex. Selection
was begun in the eighth generation and continued for more than 53
months, 184 generations in the plus strain and 196 generations in
the minus strain. The data for Line 714 are given in tables 21 and
22 and in figure 7.
For the first 4 months of the experiment the mean reaction-time
for the plus strain was 495.4 and for the minus strain 618.9 seconds
(table 21). The difference was rather large (-123. 5 ±32.0 seconds,
3.86 times the probable error). There were only two small same-
day broods for this period (table 22), for which the mean for the
minus strain was 231 seconds the larger.
For the year-period (August 1, 1912- July 31, 1913) the mean
reaction-time for the plus strain was 309.2 seconds and that for
the minus strain 278.5 seconds. The difference was +30.7 ±12.4
seconds (2.48 times the probable error). This difference is in the
reverse direction to that for the earlier (four-month) period. The 3
same-day broods had a mean for the plus strain 226 ±27.62 seconds
the larger.
1 Of these 7 same-day broods, 4 occurred during May, when the plus strain for some un-
accounted-for reason was rather non-reactive for D. pulex.
A PHYSIOLOGICAL CHARACTER.
57
For the next year-period (August 1, 1913-July 31, 1914) the
means were 406.4 and 439.2 seconds. The difference (-32.8 ±14.5
TABLE 21. — Selection summary for Line 714-
Time period.
Apr. 1, 1912-
July 31, 1912
Aug. 1, 1912-
July 31, 1913
Aug. 1, 1913-
July 31, 1914
Aug. 1, 1914-
July 31, 1915
Oct., 1914...'
Sept. 1, 1914-
Nov.31,1914
Test series,
Nov. 1914 ...
Oct. 1, 1914-
Dec. 31, 1914
Test series,
Mar., 1915..
Feb. 1, 1915-
Apr. 30, 1915
Aug. 1, 1915-
July 31, 1916
Aug. 1, 1916-
Sept.16, 1916
Strain.
Plus . . .
Minus
Plus. . .
Minus
Plus...
Minus
Plus
Minus
Plus. ..
Mi us
Plu
Mi us
Plus. ..
Mi us
Plu ...
Mi us
Plu
Minus
Plus. ..
Minus
Plus...
Minus
Plus. ..
Minus .
8-21
8-21
22-55
£2-54
56-100
55-101
101-144
102-147
112
114
105-116
107-118
113
115
109-121
111-123
129
131
125-135
127-138
145-185
148-195
186-194
196-202
11
36
M
44
ta
41
45 10.3
16 15.6
16 18.1
12 10.4
3530.1
IS 11
3631.2 1123
36 31.5 1133
39 11.1
7
841
331.8
397.1
383.1
380.2
421.
S9S.
317.5
415. 8
301.1
256.9
272.6
209.8
175.0
268.4
284.2
268.4
259.4
222.9
£18. 1
258.9
185.2
240.1
243.5
287.3
276.3
250.6
222.1
227.1
284.4
8.2
7.2
18.9
26.4
-123.5
+ 30.7
- 32.814.6
+ 15.812.3
23.4
21.8
9.5
7.9
10.3
10.3
9.2
8.1
9.5
8.7
15.6
10.6
4.9J
4.9t
5.78 + 25.3
6.52
+ 13.0 12.9
63^8
8.0
.
2.48
g.ea
1.28
1.01
5!03
7. 72 2. 46
3.16
TABLE 22. — Same-day broods. Summary of data for Line 714-
Time period.
Apr. 13, 1912-
July 31, 1912
Aug. 1, 1912-
July 31, 1913
Aug. 1, 1913-
July 31, 1914
Aug. 1, 1914-
July 31, 1915
Aug. 1, 1915-
July 31, 1916
Aug. 1, 1916-
Sept.16, 1916
Plus
Minus..
Plus
Minus . .
Plus....
Minus . .
Plus
Minus . .
Plus
M inus . .
Plus....
Minus . .
669
429
SOS
406
342
331
403
351
261.04
27 4. *4
.12
250.26
269.48
289.43
78.74
61. 30
17.05
21.73
29.84
30.72
12.12
15.20
-231
+226
-106
'-114
27.628.
42.82 8.46
19.445.86
'•45
;.18
seconds) is 2.26 times the probable error. There were 6 rather small
same-day broods, for which the mean for the minus strain was
105 ±42.82 seconds the larger. Again, as compared with the pre-
58
SELECTION IN CLADOCERA ON THE BASIS OF
2.00
I.5O
I.OO
.50
00
ceding long period, there is a reversal in this longer period of the
direction of the difference in the mean reaction-times for the entire
data and in that for the same-day broods.
Tor the next year-period (August 1, 1914-July 31, 1915), the
means were 378.1 and 362.3 seconds. The difference, which was
+ 15. 8 ±12. 3 seconds, is not statistically significant and is again in
the reverse direction from that for the preceding period. During
this year-period the same-day-brood data consisted of 13 broods and
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
1912
1913
A
1914-
1915
-I
. --i*:'
1916
T M B t ,
T ill
il Mil
60O
4-5O
300
150|- 1912 1913
4-5 8r9 12-1 4-5 8-9
.75
.50
.25
00
.25
.50
.75
12-1
1914- 1915
4-5 8-9 12-1 4-5 8-9
12-1
1916 .
4-5 8-9
FIQUBE 7. — Line 714.
A. Reproductive indices, actual values.
B. Reproductive indices, superiority.
C. Reaction-time curves.
the mean for the minus strain was 114 ±19. 44 seconds higher than
that for the plus strain. This is a difference almost 6 times the
statistical probable error.1
A test series conducted during October 1914, containing 249
individuals in the plus strain and 289 in the minus strain, gave
1 These same-day broods are pretty well scattered through the year period, except that none
occurred during the last 4 months. The averages obtained for the same-day broods are more
dependable than the averages for the data as a whole when there are sufficient numbers, and 129
and 165 individuals constitute fairly good numbers. It is unusual that the reaction-time means
for the complete data and for the same-day-brood data for the same period should differ so widely
as in the present case.
A PHYSIOLOGICAL CHARACTER. 59
means of 344.8 seconds for the plus strain and 331.8 seconds for the
minus strain. The difference, +13.0 ±12.9 seconds, is not of statis-
tical significance. A second test series conducted in November 1914
contained 1,055 individuals in the plus strain and 1,089 in the minus
strain. The difference in the mean reaction-time was +18. 9 ±7. 72
seconds, 2.46 times the probable error. A third test series was con-
ducted during March 1915. The numbers were large, 1,123 and 1,133
individuals in the two strains. The mean reaction- times were 421.9
and 396.6 seconds. The difference was +25. 3 ±8.0 seconds and is
3.16 times the probable error.
It will be seen that there is a rather striking similarity in the
differences obtained for the 3 test series and for the entire selection
data for this year-period, the differences being +13.0, +18.9, and
+25.3 for the 3 test series and +15.8 seconds for the selection data
as a whole. Such harmonious results seem to indicate that the plus
strain of Line 714, during this year-period, was actually less reactive
in spite of selection, although the same-day-brood data oppose this
conclusion.
For the next year-period of selection with Line 714 (August 1,
1915-July 31, 1916) the means were 301.1 and 303.5 seconds. The
difference was —2.4 ±10. 9 seconds. There remains a short period
of \}/2 months during which selection was continued. There were
only 66 individuals in the plus and 53 individuals in the minus strain.
The difference (+63. 8 ±32. 4) is not statistically significant.
Considering the data for Line 714 as a whole, it is clear that
there is not an effect of selection.
Figure 7c, showing the reaction-time curves for the two strains
of Line 714, presents curves with large minor irregularities. It will
be noted that throughout the first 6 months of selection with Line
714 there is a considerable divergence, the minus strain having much
the higher reaction-timei Such consistencies in reaction-time differ-
ences for limited periods also occur in Line 695 for a period of 8
months and in Line 740 for a period of 10 months. It would seem
that these are more than mere chance results, but in each of these 3
cases further selection not only did not result in increasing the
divergence, but in all 3 of these cases the divergence disappeared.
Assuming for the moment that an effect of selection was present in
these cases, it was later lost; but in what manner this loss could
have occurred is not clear. Selections were not relaxed, and there
seems to be no reason to suppose that a mutation had affected one
of the strains in each of these 3 lines.
However suggestive of an effect of selection the early portion of
the curves for Line 714 are, it is "clear from the remainder of the
curves that a selective. effect is not present (figure 7c). The detailed
analysis of the data has already shown this.
60 SELECTION IN CLADOCERA ON THE BASIS OF
The minus strain of Line 714 was on the whole slightly more
vigorous than the plus strain.1 The plus strain was superior in vigor
during 13 two-month periods; the minus strain more vigorous during
14 two-month periods. The differences in favor of the minus strain
were somewhat greater than those in favor of the plus strain.
There is no apparent relation between vigor and reaction-time.
Casual inspection of the reaction curves and the reproductive index
figure (figure 7) gives this impression, and the following facts bear
it out: During 11 periods the temporarily more vigorous strain was
the more reactive, during 15 periods the more vigorous strain was
the less reactive, and during 1 period the reaction-times were the
same. Selecting the 9 periods during which the mean reproductive
indices differed by more than 0.30, it is found that in 5 periods the
more vigorous strain is the less reactive, in 3 periods the more
vigorous strain is the more reactive, and for 1 period the mean re-
action-times were the same. Selecting the 9 two-month periods
during which the reaction- time means differed by more than 100
seconds, it is found that in 2 cases these large differences in the mean
reaction-time correspond with periods in which the more reactive
strain was also the more vigorous, but that in 7 cases the more re-
active strain was the less vigorous of the two.
There is an interesting feature of the curves (figure 7c) in the
marked influence of environment upon the mean reaction-time of the
two strains. In spite of considerable irregularities of the curves for
the two strains, they follow each other to a remarkable extent.
Nothing other than environmental influences would seem to account
for such a coincidence.
LINE 719.
The data for Line 719 will be found in tables 23, 24, 25, and 26
and in figure 8. This is one of the original lines of Daphnia pulex
taken into the laboratory in November 1911. Selection was begun
(March 28, 1912) in the eighth laboratory generation and continued
for 130 and 138 generations, until both strains were lost in June 1915.
Consultation of the curves (figure 8c) shows that there is no
effect of selection; some interesting features of this data, however,
will be brought out by a somewhat detailed analysis.
For the first 4 months the mean reaction-times for the plus and
minus strains were 450.1 and 493.1 seconds. The difference is
- 43 ± 31.8 seconds (table 25). For the same-day broods (only 3) the
corresponding data are 718 and 539 seconds, the difference being
+ 179 ±76.00 seconds.
' The average reproductive index (obtained by dividing the sum of the reproductive indices
by two-month periods by the number of two-month periods) for the plus strain was 1.006, for
the minus strain 1.034.
A PHYSIOLOGICAL CHARACTER. 61
TABLE 23. — Selection data summarized by two-month periods for Line 719 plus.
ft
«2
•3£
h
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i
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f,l
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Time period.
•3
rf
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1*1
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1
1
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1
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il
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z'0
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^
£
s~
CD
£
Apr.-May 1912...
9-13
8
27
6.8
9.6
0.71
0
16
214
534
15738
583
June-July 1912. . .
14-19
6
61
6.8
11.4
.60
4
3
240
635
19370
380
Aug.-Sept. 1912...
20-21
2
3
1.5
11.5
.13
0
0
288
295
825
275
*
Oct.-Nov. 1912. . .
21-24
4
16
2.3
12.8
.18
0
0
220
448
4138
259
Dec. 1912-Jan. 1913
25-31
7
67
10.2
9.1
1.12
2
6
115
476
18737
280
Feb.-Mar. 1913...
32-38
7
51
7.3
8.5
.86
1
5
147
437
16685
327
224: 17
21 17
Apr.-May 1913...
39-45
7
47
6.7
8.3
.81
1
0
111
316
9910
211
June-July 1913. . .
46-54
9
57
6.0
7.2
.83
2
12
173
595
22603
397
Aug.-Sept. 1913...
Oct.-Nov. 1913. . .
55-62
63-68
8
5
86
37
9.2
6.3
6.3
7.5
1.46
.84
9
2
7
0
105
141
539
394
31176
9044
363
244
251.87
18.32
Dec.1913- Jan.1914
69-75
7
48
6.2
8.4
.74
1
9
319
725
22299
465
258:36
25 15
Feb.-Mar. 1914...
76-81
6
32
5.6
8.8
.64
0
3
233
626
14002
438
Apr.-May 1914...
82-90
9
81
8.5
7.1
1.20
13
11
186
801
34353
424
June-July 1914. . .
91-96
6
31
4.6
7.2
.64
6
9
326
814
18139
585
24§: 35
'36:69
Aug.-Sept. 1914. ..
97-104
8
73
9.6
7 1
1 35
11
13
165
834
33103
453
Oct.-Nov. 1914 . . .
105-113
9
108
12.2
7.4
1.65
24
2
87
517
28021
259
Dec. 1914- Jan. 1915
114-121
8
65
8.1
7.9
1.03
3
9
119
698
24354
375
Feb.-Mar. 1915...
122-127
5
59
12 5
9 1
1 37
9
7
137
623
20971
355
Apr.-May-June '15
128-137
10
79
8.4
8.2
1.02
6
18
315
768
33398
423
297.34
22.57
TABLE 24. — Selection data summarized by two-month periods for Line 719 minus.
J
11
1!
-
3
B
i
i
d
.2
0
i
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1
1
1
si
I
11
1
1
1
*S5
d
p
1
1
•S
|
1
fl
I!
1
:»j
3-s
a
|
J*
1
O
a
!
1
li
•3
1
Time period.
•3
1
1
0
1
1
H«
lj
ii
'a
1
o
*?
T) -
i
£ a
!«
1
=5
i>i
"S
•8
0
I
!
j|
fi!
°|
d 1
fj
•Si*
•3
i
II
i
1
C, 03
II
I
6
Apr.-May 1912...
9-14
7
37
6.2
9.1
0.68
0
14
292
896
21155
572
+ 11
June-July 1912. ..
15-21
8
39
5.6
8.5
.66
o
7
210
719
16320
418
-38
Aug.-Sept. 1912. . .
22-24
3
4
.3
17.5
.07
0
0
200
251
854
214
+61
Oct.-Nov. 1912 .
Dec. 1912-Jan. 1913
25-28
29-36
6
8
15
V,
.5
.9
19.5
8.4
.08
.82
0
0
0
0
184
93
239
333
3809
12566
254
228
+5
+52
Feb.-Mar. 1913. . .
37-42
6
us
.8
9.0
.64
0
0
126
353
7938
227
113:26
i2:6i
+ 100
"24:79
4.03
Apr.-May 1913...
43-50
8
50
.5
7.4
1.00
6
0
124
423
15220
254
-43
June-July 1913...
51-58
10
41
.6
7.2
.64
4
1
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. ..
68-74
7
8.0
8.1
.99
0
3
149
403
14036
251
—7
Dec. 1913-Jan. 1914
75-81
7
S8
8.3
8.3
1.00
4
5
179
701
21682
374
207:78
j.8 40
+91
"si:i7
2:91
Feb.-Mar. 1914 . . .
82-89
7
5.7
9.0
.63
2
2
255
722
16870
422
+ 16
....
Apr.-May 1914...
90-97
8
84
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
24i:is
26:56
+ 118
"36:46
3:24
Aug.-Sept. 1914...
Oct.— Nov 1914
106-112
113—120
7
0
50
6.6
9 5
7.3
7 4
.90
1 28
15
20
6
206
183
769
646
20071
22045
401
266
+52
7
Dec. 1914^-Jan. 1915
121-128
8
S7
7'.8
9
5
168
697
31633
364
+ 11
Feb.-Mar. 1915 . . .
129-135
7
70
9.5
8.5
1:12
8
7
116
639
26135
373
-18
Apr.,May,Juce,1915
136-145
9
65
8.1
7.8
1.04
19
6
147
606
22580
347
257:35
2i:53
+76
"si." is
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.
Time period.'
Apr. 1912-
July 31, 1912
Aug. 1, 1912-
July 31, 1913
Aug. 1, 1913-
July 31, 1914
Test series,
May 1914 . .
Apr. 1, 1914-
June31, 1914
Aug. 1, 1914-
June 25, 1915
Test series,
Nov. 1914..
Oct. 1. 1914-
Dec. 31, 1914
Strain
Plus . . .
Minus
Plus. ..
Minus
Plus. ..
Minus
Plus . . .
Minus
Plus. ..
Minus
Plus. ..
Minus
Plus. ..
Minus
9-19
9-21
20-54
22-58
55-96
69-105
82-93
90-101
97-137
106-145
110
118
105-117
11 3-1 £4
5
6
5
7
8
13
23/4
12
12
40
39
4322.
3
1211.
794
450.1
493. 1
302.5
256. 3
409.6
366.3
520.8
463.6
453.7
434.5
364.2
345.0
310.9
290.6
261.1
288.1
275. S
sos. 4
232.3
142.9
257.6
966. S
289.2
271.
264.9
239.8
217.2
197.0
21.0
23.8
10.1
8.9
10.9
10.1
9.1
8.6
-43.0
+46.2
+43:3
+'57:2
+ 1JK2
+ 19'. 2
+'26:3
31.8
12.1
TABLE 26. — Same-day broods. Summary of data for Line 719.
Time period.
Apr. 8, 1912-
July 31, 1912
Aug. 1, 1912-
July 31, 1913
Aug. 1, 1913-
July 31, 1914
Aug. 1, 1914-
June25, 1915
Strain.
( Plus....
I Minus . .
( Plus
» Minus . .
( Plus. ...
I Minus . .
I Plus
i Minus . .
-ss
7is
580
314
3-1 •)
884
351
978
257.80
320.75
262.11
44-66
207.54
249. 6 S
7
18.85
10.71
15.01
/7.95
II
if
54.99 +179
+60
-64
-27
76.002.35
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 curved (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
I,;,1,,../".:
8 I
4-5 8-9 12-1 4-5 8-9 12-1 4-5 8-9 12-1 4-6
1912 1913 1914 1915
'. . t 1 T T I
.1 11 r i
.50
.25
00
.25
.50
. A i
i J 1 ']
600 h
450
300
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.
Time period.
Nov. 13, 1912-
July 31, 1913
Aug. 1, 1913-
Junel 1,1914
Test series
Jan. 1914
Dec. 1, 1913-
Feb. 28, 1914
Strain.
Plus.
Minus . .
Plus
Minus . .
Plus....
Minus . .
Plus. ...
Mimu..
1-31
1-31
32-71
32-70
54
63
48-58
47-67
El
282.2
£70.8
375.4
400.7
416.1
433.9
414.0
179.6
164- 1
252.7
£56.7
260.0
£67. 7
233.0
S5K.6
fe
i
Is
7.73 +11.4
6.78
9.51
9.62
9.7
10.0
-74.7
10.28 1.10
IS. 46 1.88
IS. 9
i.es
TABLE 28. — Same-day broods. Summary of data for Line 751 .
Time period.
Nov. 13, 1912-
July 31, 1913
Aug. 1, 1913-
June 11, 1914
Strain.
Minus..
325
170.04
II
+37
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
"All the lines of D. pulex except 689 and 751 came from Pond II, the spring-fed pond in
the woods.
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, 10)^ 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,
FIGUBE 9.-Line 751. e t fm ^ two_month period.
Reaction-time curves. _., . . , , ,, .
But in view of 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
8-9 12-1 4-6
1913 I9M-
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.
Time period.
Nov. 1912-
July 31, 1913
Aug. 1, 1913-
Sept. 8. 1913
Plus
M inut . .
Plus....
Minus . .
1-30
1-28
31-36
29-35
6.4
9.8
11 .3
366.2
377. 3
269.3
4SS.S
229.2
£57.1
224.1
305.6
10.5
12.3
19.7
23. 2
SO. 4£ 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 IOA)
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 ifl 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
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 4-5
1913
FIGURE 10.
A, B, C. Reaction-time curves for Daphnia longiapina — 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 environmental 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.
Time period.
Oct. 1, 1913-
Apr. 1914
Strain.
Plus . . .
Minus .
39_61
40-66
gl.64S.7f
periment. The mean reaction-times for the two strains for the
entire period of the experiment were 327 and 385.8 seconds (table
30). While the difference is -58.8 seconds and is 2.72 times the
probable error, it is largely due to the difference in mean-reaction
time obtaining for the first two months. There were two same-day
broods containing totals of only 1 1 and 9 individuals in the plus and
minus strains, respectively. The averages were 222 and 250 seconds
and the difference was —28 seconds, a difference which, with so few
individuals, is not significant. There is no effect of selection.
The effect of environmental influences upon reaction-time is
indicated by the generally coincident up-and-down courses of the
curves of the two strains of Line 762 and Line 766 (see figure 10,
A and B).
LINE 768.
Line 768 was the second of the two genetically distinct lines of
D. longispina. It was taken into the laboratory from Pond IV in
November 1913, but selection was not Begun until December 1914
A PHYSIOLOGICAL CHARACTER.
71
in the fifty-fourth laboratory generation. Selections were continued
for 21 months, 75 and 74 generations in the plus and minus strains,
respectively. The data will be found in tables 31 and 32 and in
figure lOc.
The data for the first longer period (December 1914-July 1915)
consists of 29 and 28 broods from the two strains, there being 238
and 239 individuals. The mean reaction-times were 426 and 443
seconds. The difference is —17 ±19. 5 seconds (table 31). There
TABLE 31. — Selection summary for Line 768.
Time period.
Dec. 21, 1914-
July 31. 1915
Aug. 1. 1915-
July 31, 1916
Aug. 1, 1916-
Aug. 22, 1916
Plus
Minus . .
Plus
Minus . .
Plus..
Minus . .
54-83
64-82
84-12J
83-124
126-128
1 26-1 2
ve
per
9.7
i
i
426.0
315!?
325.0
350.0
331.2
293.7
242.1
230.*
268.5
14.5
1S.1
8.4
9.1
33.6
316. 8
-1S.S
+151.6
19.60.87
TABLE 32. — Same-day broods. Summary of data for Line 768.
%
£
Till
£
g
£
|
I
"o
S
3
f!
S
a
'H .
1
•|
Time period.
Strain.
1
!
|1
11-
ll
1
•3
I-S
"3
•sg
•s5!
1.1
H
§
6
"Z,
3*
1
1*
1-9 "8
> 2
•<
l§
5
Dec. 21, 1914-July 31,
f Plus
17
10.6
180
2
52
212
808
454
+ 16
1915
1 *f
438
Aug. 1, 1915-July 31,
jPlus....
4
12.5
50
1
3
106
603
374
+38
4
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.
Time period.
Dec. 23, 1914-
July 31, 1915
Aug. 1, 1915-
July 31, 1916
Test series,
July 1916...
June 1. 1916-
Aug. 31, 1916
Aug. 1, 1916-
May 1, 1917
Plus...
Minus
Plus...
Minus .
Plua...
Minus.
Plus. ..
Minus .
Plus...
Minus .
1-27
1-28
28-63
27-61
62
61
59-67
69-64
64-91
715.6
9.7
[5.1
529.4
451.8
483.8
689.1
395.9
395. S
314.4
409.6
309.9
373.*
334. S
S24.9
266.4
10.7
7.1
17.2
£ §
II
+77.6
—105.S
+ .7
15.64.97
33. 9 S. 80
ie'.S4.18
TABLE 34. — Same-day broods. Summary of data for Line 794.
Time period.
Deo. 23. 1914-
July 31, 1915
Aug. 1, 1915-
July 31, 1916
Aug. 1, 1916-
May 1, 1917
Plus
Minus..
Plus. ...
Plus"! .' .'
Minus . .
7 13.1
7 11.6
5 16.2
13.8
2 12.0
21S.O
475
S59
463
4!>0
823
S48.69
ii
H
1
48 +116
49.66 2.69
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 db 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 12s) 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
-
-
2.00
1.50
VtVi'j.
lir-
1.00
1
.50
_ _
12-1 4-5 8-9 12-1 4-5 8-9 12-1 A
00
1915 1916 1917
-11,1,
T T
300
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.
Time period.
Dec. 24, 1914-
July 31, 1915
Aug. 1, 1915-
July 31, 1916
Test series,
June 1916
May 1, 1916-
July 31, 1916
Aug. 1, 1916-
May 1, 191
Strain.
Plus..
Jtftnus
Plus. ...
Minus
Plus..
Minus
Plus. .
Minus
Plus. .
Minus
1-27
1-27
28-65
28-64
60
60
57-65
66-64
66-94
65-96
6-S
in. 4
15.7
II
1
703.3
57/. 4
457.2
502.0
254.4
299. 1
341.7
326.4
330.1
10.0
12.1
8.6
11. S
6.0
0.0
18.8
25.5
11.7
13.1
+ 131.9
'+94'.3
'+'23^8
!
15.78.40
8.92.67
3i'.64
TABLE 36. — Same-day broods. Summary of data for Line 795.
Time period.
Dec. 24, 1914-
July 31, 1915
Aug. 1. 1915-
July 31. 1916
Aug. 1, 1916-
May 1, 1917
Strain.
Plus....
Minus . .
Plus
Minus..
Plus ....
Minus . .
12 11.6
IS 10. 7
12 13.5
ISIO.S
512.6
6 14.6
458
277.77
17
.21
313.97
+63
17.16+138
SO. 61
22.37
.63
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-
700
;: ; 111]
,1
1L1-
1.50
1.00
<!
1
i
.50
_
_
12-1 4-5 8-9 12-1 4-5 8-9
12-1 «
GO
1915 1916
1917
FIGUKE 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.
Time period.
Dec. 25, 1914-
July 31, 1915
Aug. 1, 1915-
July 31, 1916
Aug. 1, 1916-
May 1, 1917
Test series,
Aug. 1916. . .
July 1, 1916-
Sept. 30, 1916
Strair
Plus....
Minus . .
Plus....
Minus . .
Plus
Minus . .
Plus
Minus . .
Plus
Minus . .
1-28
1-27
28-61
67-95
65-71
12.3
27 13.0
38 15.9
33 8.0
22 13
SO
2033.1
619.1
697.6
449.4
393.7
675.6
653.7
662.4
453.8
289.5
290.9
291.9
297.2
331.1
311.9
286.6
302.8
201.3
319.9
10.5
10.5
8.0
12. 3
12.8
8.1
12.6
21.7
-43.3
-78.4
+55.7
+21.
§ fi
14.93.31
14.76'.3S
TABLE 38. — Same-day broods. Summary of data for Line 796.
Time period.
Dec. 25, 1914-
July 31, 1915
Aug. 1, 1915-
July 31, 1916
Aug. 1, 1916-
May 1, 1917
Strain.
Plus...
Minus .
Plus. ..
Minus .
Plus. ..
Minus .
I
634
523
674
680
723
503
283.10
281.80
176.68
898.67
291.84
303.64
24.86 +111
10.11
21.84
23.87
SB. 7 9
is
n
+220
The mean reaction-times for the first longer period (8 months)
of the experiment were 555.5 and 604.8 seconds, the plus strain
being the more reactive by a margin of 49.3 ±14.9 seconds, a differ-
ence 3.31 times the probable error (table 37). The 7 same-day
broods for this period gave averages indicating greater reactiveness
for the minus strain by a difference of +111 ±36. 23 seconds, more
than double the difference obtained from the entire data for this
period and in the reverse direction (table 38).
82
SELECTION IN CLADOCERA ON THE BASIS OF
For the second longer period the reaction-time means were
619.1 and 697.5 seconds. The difference was -78.4 ±14.7 seconds,
5.33 times the probable error. The 10 same-day broods gave averages
differing by only -6 ±24.07 seconds.
The data for the last longer period (August 1916 to the end of
the selection experiment in April 1917) gave as means 449.4 and
A 1
2.00
-J
j
> _
1.50
- r
I 1
T-
. I
I ^
1.00
~~ 1
1
o -
1
)
.50
(
6
i
IM V5 8-9 IZ-1 VS 8-9 IM 4
1915 1916 1917
300
B
iL
1915
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 in 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 13s), 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.
Time period.
Aug. 27, 1912-
July 31, 1913
Aug. I, 1913-
July 31. 1914
Test series,
Apr. 1914. ..
Mar. 1914-
May 1914...
Aug. 1, 1914-
July 31. 1915
Aug. 1. 1915-
July 31, 1916
Test series,
Mar. 1916..
Feb. 1916-
Apr. 1916...
Test series,
June 1916...
Mav 1916-
Jufy 31, 1916
Aug. 1, 1916-
May 1, 1917
Test series,
Apr. 1917...
Mar. 1917-
May 1, 1917
Strain.
Plus...
Minus
Plus. ..
Minus
Plus...
Minus
Plus. ..
Minus.
Plus...
Minus
Plus. ..
Minus
Plus. ..
Minus
Plus...
JMrnua
Plus...
Minus
Plus. ..
jtftnus
Plus...
Minus .
Plus...
Minus .
Plus. ..
Minus .
139-146
140-147
147-153
148-154
154-181
55-184
179
181
175-181
178-184
1-34
1-S4
35-76
35-79
65
66
61-68
62-71
77-1 U
79-119
119-153
180-16635 15.3
1423431.7 1078
143 34 32.0 1089
8 17.9
8 17.6
151 32 38.5 1231
40.1
14.3
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0
18.6
29.1
8619.1
6 12.7
3.6
8*8
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848
884
884
735.2
757.0
597.1
656.9
666.4
711.9
569.2
696. S
642.5
607.7
574.6
600.0
641.6
577.9
653.8
641. 8
420.2
525.7
574.7
710.4
311.1
395. 8
314.5
329. 8
262.0
165.7
£44-7
298.7
302.4
294.6
S78.3
307.6
300.0
335.6
326. 9
324.6
331.7
468.6
243.0
334.0
256.3
267.0
248.8
184.6
8.7
8.7
7.8
7.6
9.8
9.7
10.4
9.7
6.7
6.3
6.5
19.2
-46.
-£7.1
+34.8
-25.4
+63.7
-135. 7
-84.7
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+96.3
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9.5
21.1
4-4S
4.17
1.79
6.71
10.3 10. £4
TABLE 40. — Same-day broods. Summary of data for Line 740.
Time period.
Aug. 1,
July 31
Aug. 1.
July 31
Aug. 1.
July 31
Aug. 1,
July 31
Aug. 1,
Slay 1
1912-
, 1913
1913-
, 1914
1914-
, 1915
1915-
, 1916
1916-
, 1917
Plus. ...
JWtnu*..
Plus....
Minus . .
Plus
Minus..
Plus
Minus . .
Plus....
Minus . .
6 10.0
312
14 16
410.6
M
£
us
37
SO
216
22>i
42
41
fi.32
834
815
+31
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+ 19
+ 137
SI .63 3.80
34.034.02
A PHYSIOLOGICAL CHARACTER. 85
until the experiment was completed. Both strains are still in our
laboratory.1 The data are summarized in tables 39 and 40 and figures
14 and 15.
The first longer period of the experiment, approximately 11
months, contains the data for 37 and 36 broods of the two strains.
The mean reaction-times (427 and 357 individuals) are 735.2 and
757 seconds (table 39). The difference (-21.8 ±12.3 seconds) was
1.77 times the probable error. There were 6 same-day broods for
this period (table 40) for which the mean reaction-times were 713
and 682 seconds, the difference being +31 seconds.
For the second year-period of selection with Line 740, the mean
reaction-times (424 and 482 individuals) were 597.1 and 656.9
seconds. The difference was -59.8 ±13. 5 seconds. This difference
was 4.43 times the statistical probable error. The 9 same-day
broods for this period gave as mean reaction-times 607 and 632
seconds, the difference being —25 ±31. 18 seconds.
A test series was conducted in April 1914. There were 653
individuals of the plus strain and 614 of the minus strain. The mean
reaction-times were 666.4 and 711.9 seconds. The difference was
—45. 5 ±10. 9 seconds, or 4.17 times the probable error. There is
a suggestion of an effect of selection for this year-period in that the
data as a whole, the data for the same-day broods, and the test-
series data all show higher reaction-times for the minus strain. But
in view of the later data this evidence is overruled.
For the next year-period (August 1914-July 1915) the mean
reaction-times (452 and 454 individuals) were 642.5 and 607.7. The
difference (+34.8 ±13. 8 seconds) is 2.52 times the probable error.
Hence the difference is of statistical interest, but is opposed to an
effect of selection. There were only 3 same-day broods for this year-
period for which the mean reaction-times were 834 and 815 seconds.
The difference was +19 seconds.
For the year-period (August 1915-July 1916) the mean reaction-
times (471 and 519 individuals) were 574.6 and 600.0 seconds. The
difference (-25.4 ±14.2 seconds) is only 1.79 times the probable
error. There were 14 same-day broods for this period, for which the
mean reaction-times were 558 and 640 seconds. The difference was
-82 ±21.53 seconds.
A test series was conducted during March 1916 and contained
1.078 individuals in the plus strain and 1,089 in the minus strain.
The mean reaction-times were 641.6 and 577.9 seconds. The differ-
ence (+63. 7 ±9.5 seconds) is 6.71 times the probable error. This
test series was conducted with great care, and the fact that the plus
strain was the less reactive of the two by a margin of statistical
significance largely counterbalances the small difference in the
1 It was in one of the substrains of this line (740 - Special B) that the sex-intergrades were
first discovered.
86 SELECTION IN CLADOCERA ON THE BASIS OF
opposite direction for the selection data as a whole and the larger
difference in the opposite direction for the same-day-brood data for
this year. Another test series was conducted during this period,
during June 1916; it contained 1,231 individuals in the plus strain
and 1,282 in the minus strain. The mean reaction-times were 420.2
and 525.7 seconds. The difference (-105. 5 ±10.3 seconds) is 10.24
times the probable error. The result of this test series, conducted
only 3 months later than the earlier one, is quite contradictory to the
result of the former.
The results of these test series are somewhat puzzling, but table
39, giving the selection summary, shows that for the three-month
period February- April 1916 (including the month in which the
earlier test series was made and that just preceding and just succeed-
ing) the mean reaction-time was much higher for the plus strain. The
similar period of 3 months, May-July 1916, during which the June
test series was conducted, shows a mean reaction-time in the minus
strain 135.7 seconds greater than that for the plus strain. One sees,
in these test series and the selection data for the contingent periods,
that the relative reactiveness had changed here in a comparatively
short time. Reference to the curve, figure 15, shows that for the
two-month period, February-March 1916, the two curves met, but
for the later two-month periods the minus strain for a considerable
time was much less reactive than the plus strain.
For the final longer period of the experiment with Line 740
(August 1916 to the close of the experiment, May 1, 1917) the mean
reaction-times were 311.1 seconds and 395.8 seconds for the two
strains. The difference (-84.7 ±16.2 seconds) was 5.23 times the
probable error. There were only 4 same-day broods, for which the
mean reaction-times were 358 and 221 seconds, the difference being
+ 137 ±34. 03 seconds. Here the number of same-day broods was
too small to be given serious consideration, but it is interesting to
note that with these same-day broods the plus strain was so much
less reactive than the minus, in opposition to the difference shown
by the data as a whole for this period. Two of these 4 same-day
broods, however, occurred during the final 3 months of the experi-
ment, when the data as a whole showed that the minus strain was
temporarily the more reactive.
Reference to the curves of the mean reaction- times (figure 15)
shows that for 10 months (from April 1916 to January 1917, inclusive)
the minus strain was the less reactive and that the margin of differ-
ence increased until in the December 1916 and January 1917 periods
the reaction-time for the minus strain was more than double that
for the plus strain. The data for a portion of this ten-month period
was not very satisfactory. We were troubled with poor food con-
ditions, and, at times, when the stock was not well nourished, the
selections were not made by tests, but individuals were chosen at
A PHYSIOLOGICAL CHARACTER.
87
random. Because of this fact, the data for 4 months (October 1916-
January 1917) are not very complete and the differences for those
2 two-month periods are not to be credited to the same extent as
the differences for the remainder of the experiment.
Reference to tables 48 and 49 (columns 5 and 6) and figure 15
and 18s shows that this consistent divergence in reactiveness be-
2.00
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4-5 8-9
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4
00
1912 1913
1914
1915
1916
1917
900
_
, ll .
6 tr
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' 11 ' 1 '
T:
FIGURE 14. — Line 740.
A. Reproductive indices, actual values. B. Reproductive indices, superiority.
FIGURE 15.— Line 740.
Reaction-time curves with reaction-time curves for Line 757 superimposed.
88 SELECTION IN CLADOCERA ON THE BASIS OF
tween the two strains of Line 740 for the period April 1916- January
1917 is due to exceptionally high means for the minus strain during
the period April-September and to exceptionally low means in the
plus strain from October to the following January, both relative to
the means for corresponding two-month periods for the other S.
exspinosus strains (Lines 794, 795, and 796). The following of one
of these two exceptional reaction- time periods just upon the other
produces the long period of divergence which at first sight seems very
suggestive of an effect of selection. Since it is seen in this case that
neither the plus nor the minus strain of Line 740 is consistently
different from the composite curve for the other plus and minus
strain of lines of S. exspinosus throughout this ten-month period, it
seems pretty evident that genetic change is not responsible for this
divergence. It is believed that the explanation lies in the unavoid-
able differential treatment of the two strains of the same line (see
pages 140-142).
A final test series was conducted in April 1917. There were 756
individuals of each strain. The mean reaction-times were 314.5
seconds and 329.8 seconds. The difference ( — 15. 3 ±9.1 seconds)
was only 1.68 times the probable error. For the contingent two-
month period the plus strain was the less reactive by 96.3 seconds.
This test series, fortunately conducted at the close of selection,
showed conclusively that there was no effect of selection in this
line; or, if there had actually been an effect of selection, that it
had been lost. The selections were somewhat relaxed during the
October- January period, but it does not seem probable that a real
selective difference would have been lost so quickly. Hence, every-
thing considered, the writer does not believe that the data for the
ten-month period in which there was a consistent reaction-time
difference between the two strains indicates an effect of selection.
Line 740 is somewhat unique among the lines of Cladocera for
which the data for vigor has been worked out, in that the minus
strain was on the whole more vigorous. Of the 29 two-month periods,
the minus strain was the more vigorous as measured by the repro-
ductive index for 18 periods and the plus strain was the more vigorous
of the two for only 11 periods. The differences in favor of the plus
strain average somewhat larger than those favoring the minus strain,
however. Examining these data with reference to periods of superior-
ity in the reproductive index on the part of the two strains, one fails
to find any correlation between relatively greater reactiveness and
greater vigor. For the 5 two-month periods during which the re-
productive index for the plus strain was superior to that for the
minus strain by 0.25 or more, the plus strain had a higher reaction-
time twice and a lower reaction-time three times. For the 6 two-
month periods for which the reproductive index for the minus strain
was superior to that for the plus strain by 0.25 or more, the reaction-
A PHYSIOLOGICAL CHARACTER. 89
time mean for the minus strain was higher four times and lower
twice. Further detailed analysis fails to show any relation between
mean reproductive index and mean reaction-time.
The curves for Line 740 indicate clearly a marked general effect
of environmental conditions upon the reactiveness of these animals.
There are irregularities in the curve, but in a general way the reaction-
time curves for the two strains follow each other remarkably well.
It is possible that a mutation occurred in the minus strain of
Line 740 in the forty-third generation of selection. A brood of 8
young on September 28, 1913, had the remarkably low reaction-
time mean of 159 seconds. These were day-old young, however, and
day-old young are sometimes abnormally reactive. Further, 4 of
these 8 young were negative in their reactions, an unusual result,
indicating unusual experimental conditions (see page 15). However,
on the following day a brood of 8 from another mother of this strain
had a mean of only 280 seconds.
Examination of the other selection data for the same days
showed that the broods of the strains of D. pulex from which selections
were made were unusually reactive. Fairly extensive tests of addi-
tional broods of 740 minus and other strains of S. exspinosus extend-
ing over several generations failed to show any consistently greater
reactiveness in 740 minus, though certain descendants of the very
reactive individuals of the minus strain of Line 740 produced broods
some, but a minority, of which were very reactive.
The results of these tests and the fact of the unusual reactive-
ness of many of the broods of other strains tested during this period
led to the conclusion that a mutation had probably not occurred and
that the unusual reactiveness of the minus strain of Line 740 at that
time had probably been caused by unusual experimental conditions.
If a mutation did occur it should have remained apparent (in
the absence of segregation) at least for a considerable period, in
spite of any effect of selection. But after the second generation the
reaction-means for 740 minus were normal, running usually above
700 seconds.
LINE 757.
PRESENTATION OP DATA.
The data for Line 757 will be examined with great care, for with
this line the result is different from that with most of the other lines
used in these selection experiments, and it is desired to be conserva-
tive regarding any conclusions that may be reached regarding the
effect of selection within this line.
The original mother of Line 757 was obtained October 19, 1912,
from Pond I, the temporary surface-water pond. Selection was
begun with her first laboratory brood of young and continued for 181
generations, covering a period of 4^ years.
90 SELECTION IN CLADOCERA ON THE BASIS OF
The tabulations of the data and the diagrams employed are
practically identical with those used in presenting the data for Line
695 and the other lines to which rather careful treatment has been
accorded. They are: tabulations of the data by broods for the 757
plus strain and the 757 minus strain (tables 41 and 42) ; tabulation
by two-month periods (tables 43 and 44) ; a summary of the selection
data, including the results of the test series (table 45) ; a tabulation
of the data for the same-day broods (table 46) ; summary of data for
more reactive individuals (table 47); figure 18s, showing graphically
the mean reaction-times of the two strains by two-month periods;
figure 19, showing the reaction-time curves by six-month periods;
figure 16, showing the relative rates of descent of the two strains;
figure 17 A, representing the average brood-size for the two strains;
figure 17s, showing average age of mothers at the time the first
broods were produced; figure 17c, giving the reproductive indices
for the two strains; and figure ISA, showing graphically the differ-
ences in vigor of the two strains as indicated by the reproductive
indices. Tables 48 and 49, comparing the data'for Line 757 with that
for the other S. exspinosus lines, are also presented, and the data in
them are graphically presented in figures 15 and 18s.
Examination of tables 41 and 42 shows that in Line 757, as with
all the lines subjected to selection, there are considerable fluctuations
in the reactiveness of individual broods. During the first 5 months
of selection the range in mean reaction-time by broods for the plus
strain was from 472 to 900 seconds and for the minus strain from
586 to 900 seconds. The fluctuations in reaction-time means by
broods were still large after the two strains became widely different
in their reactiveness to light. For the last 5 months of selection with
Line 757 the plus strain means ranged from 93 to 651 seconds and
the minus strain means from 133 to 814 seconds.
Although brood-by-brood comparison (tables 41 and 42) shows
clearly that in general, after the early part of the selection experi-
ment with Line 757, the minus broods were very much the less re-
active, the rather wide fluctuations by brood averages made it desir-
able to treat the data by larger units.
Tables of the data by one-month periods were made.1 From
the averages in these tables it was found that for the first 9 months
of selection the plus strain of Line 757 was the more reactive for 7
months; for, the second longer period (the year August 1913-July
1914) the plus was the more reactive during 10 months; for the third
longer period the plus was the more reactive during 10 months; for
the remaining 21 months of the experiment the plus was the more
reactive during each month. After the first 5 months of selection
1 These are omitted from the paper for the sake of reducing the cost of publication and in
favor of retaining in the paper the tables of data by broods in which the original data are given
in sufficient detail to make possible their utilization by anyone who might wish to examine the
data statistically.
A PHYSIOLOGICAL CHARACTER.
91
TABLE 41. — Summary of selection data by broods for Line 757 plus.
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6211
565
4363339
0,
June 10
1
0
9
19.8
0
6
307
900
6544
727
5315338
92 SELECTION IN CLADOCERA ON THE BASIS OF
TABLE 41. — Summary of selection data by broods for Line 757 plus — Continued.
a
3
2
a
q
.2
•S
J
1
0
•5
1
W
1
I!
1
1
1
§
1
8 .
1
ii
3
i
.2
1
atively reai
lividuals it
end of the
43
reaction-
1
tion-timeiE
1
o
If
4)
I*
1
I
«-i
- 05
S «;
1 «
— o
I
I
1
8
|
W
it
"5
0
JJ
o|
6 §
1|
J!
if
OQ
B
i
I'l
Pi
1914
June 18
5
o
3
19.0°C.
0
2
305
900
2105
702
1713025
June 26
11
o
5
22
0
3
365
900
3883
777
3232349
T*
July 4
11
o
11
19.7
0
11
900
900
9900
900
8910000
Si
July 13
4
1
24
21.1
0
24
900
900
21600
900
19440000
Ti
July 18
12
o
8
24
0
7
110
900
6410
801
5682100
July 25
1
0
5
22
0
4
120
900
3720
744
3254400
Vi
Aug. 4
2
o
12
21.8
1
5
200
900
6545
545
4688525
W
Aug. 10
3
o
11
23.2
1
1
140
900
3753
341
1790699
V
Aug. 17
2
1
10
21.4
0
0
162
775
4257
426
2229417
Yi
Aug. 22
5
o
10
22.9
0
9
402
900
8502
850
7451604
n
Aug 28
4
o
10
19.7
0
6
480
900
7950
795
6533700
A
Sept 7
4
1
•40
21.6
0
19
128
900
24689
617
18907437
B<
Sept 17
11
o
6
16.3
0
0
170
261
1291
215
283171
C4
D4
Sept. 24
Oct. 5
3
3
0
1
21
16
20
19.6
0
0
8
0
136
70
900
570
12592
3352
600
210
9322494
914534
Oct. 12
4
o
12
19.1
1
2
145
900
5146
429
2869308
F*
Oct. 20
3
0
16
21
0
11
210
900
11402
713
9395010
Gt
Oct 28
5
0
16
18
1
13
305
900
12890
806
11021250
H4
Nov. 6
3
o
18
18.1
0
11
90
900
12694
705
10183674
II
Nov. 13
2
0
15
18
0
13
310
900
12350
10741700
J4
Nov. 20
4
0
14
21.1
0
12
543
900
12131
867
10635793
K4
Nov 28
11
o
12
18
0
7
270
900
8275
690
6602575
L,
Dec.' 4
3
0
10
18.4
0
10
900
900
9000
900
8100000
M.
Dec. 11
6
0
7
18
0
7
900
900
6300
900
5670000
N4
Dec 22
4
o
12
15.7
1
2
168
900
5319
443
3079429
Ot
Dec. 31
10
1
12
15.2
0
6
133
900
7831
653
6116351
1915
p
Jan. 8
3
0
8
19.6
0
4
230
900
5656
707
4498818
Q4
Jan. 15
2
0
8
19.2
0
8
900
900
7200
900
6480000
R4
Jan. 25
3
0
12
17
0
7
220
900
7982
666
6282834
s;
Feb. 4
4
0
14
13.5
0
9
65
900
9133
652
7584279
T4
Feb. 16
1
0
12
17.3
0
2
185
900
•6308
442
3055384
Feb. 24
4
0
10
20
0
0
75
465
1910
191
505550
V4*
Mar. 6
5
0
7
19.2
0
0
80
388
1022
146
315738
Mar. 15
3
0
13
17.9
0
1
136
900
4182
322
1879940
X4
Mar. 24
2
0
10
17
0
4
185
900
5627
563
4022017
Y,
April 2
10
0
15
16.6
0
6
120
900
9619
641
7424255
Z4
April 12
5
0
14
20.5
0
13
500
900
12200
459
10780000
B;
April 21
May 5
8
8
Random
18.9
distr
1
butic
1
>n.
183
900
4146
518
2654998
2
0
c,
May 15
3
0
10
16
0
4
340
900
6782
678
5107074
D(
May 28
3
0
6
16
0
0
160
715
1816
303
767146
E
June 8
0
16
18.9
0
6
210
900
8546
534
5928436
F,
June 14
0
17
20.5
0
3
183
900
7695
453
4353433
Oi
June 22
0
9
20
0
5
400
900
6731
748
5390501
Hi
July 1
0
10
20.4
0
3
372
900
6375
638
4499029
I,
July 8
0
11
21
0
11
900
900
9900
900
8910000
J<
July 15
0
11
21.9
0
6
120
900
7571
688
6273601
K,
July 22
0
7
21.1
0
0
115
727
3522
503
2063154
July 30
0
4
23
0
0
130
390
976
244
275146
M«
Aug. 13
0
6
23
0
5
410
900
4910
818
4218100
Ni
Aug. 19
0
5
19.3
0
3
65
900
3225
645
2645825
Oi
Aug. 26
0
7
20.2
0
5
210
900
4990
713
4172500
P,
Sept. 6
0
8
20.3
0
2
200
900
4195
524
2842925
Q»
Sept. 12
0
12
21.2
6
3
120
900
7065
589
5263925
K
Sept. 17
0
10
23.9
1
6
180
900
7180
718
5814SOO
Sept. 24
0
12
17.3
0
2
120
900
3670
306
2005500
Ta
Oct. 4
15
17.6
0
7
150
900
8795
686
6556825
U»
Oct. 13
0
13
17.5
0
9
185
900
9810
765
8112150
V,
Oct. 21
0
20
18.9
0
12
70
900
12760
6S8
10517450
w,
Nov. 1
5
0
22
17.1
0
1
130
900
5995
273
2162325
X.
Nov. 10
4
0
18
14.1
0
0
100
390
4455
248
1264675
Y,
Nov. 19
2
0
23
16.3
0
0
145
450
7550
SS8
2092500
•Probably two broods, the first brood having been overlooked until the second appeared. Inasmuch
as the second brood (if there were two) was a day old before this test was made, the first brood must then have
been near 72 hours old. Obviously data for three^day-old individuals is not strictly comparable to the other
data, nearly all of which was obtained from individuals less than 24 hours old. Hence, much as the writer
dislikes to cull his data in any manner, the data for this day's test is omitted. However, it may be added that
thia is the only instance in the data of this publication in which any data has been rejected.
A PHYSIOLOGICAL CHARACTER. 93
TABLE 41. — Summary of selection data by broods for Line 757 plus — Continued.
«
J
3
.2
3
fl
.2
.2
i
A
a
•3
a
•g
1
I
H
11
1
1
!
1
1
2
1
I
a
1
a
i
.2
£
1
.3 **
3-S
3-0
> A
J
1
i
j
*o
1
*i"§
1
0«J
>> a
1
1
B-3
3§
1«
a £
*. •
§
I
1
•oj
•3
|
oj
*o
N
*3
3 13
_e g
'3 o
||
"o •§
a °
i
*0 a
a a
o
03 -£
Q *"
•
5s-
o
|j
1*
6 g
§g
1 *
Is
i
1*
1915
z,
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
C,
Dec. 27
5
1
21
14.9
5
95
900
5400
257
2031375
1916
D,
Jan. 7
8
0
5
16.5
0
5
900
900
4500
900
4050000
E>
Jan. 17
9
0
6
16.8
0
0
140
240
1105
184
210725
F,
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
H«
Feb. 15
8
0
15
16.6
0
6
200
900
7750
517
5206900
Ii
Mar. !•
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
K,
Mar. 22
4
0
19
15.9
0
3
90
900
5690
379
3423300
Li
Mar. 31
9
0
5
15.1
1
0
135
580
1770
354
756750
May 4
7
0
4
14
0
3
190
900
2890
723
2466100
N.'
May 15
4
0
24
15.8
0
3
170
900
8625
359
2625625
0,
May 25
3
0
17
18.5
0
16
210
900
14610
859
13004100
P«
June 8
3
7
19.6
0
4
175
900
4370
624
3455450
Q«
June 19
4
15
20.2
0
5
270
900
8805
587
6066825
R»
June 28
5
13
20.6
0
0
75
805
4483
345
2002129
July 5
4
11
18.9
0
1
110
900
3560
324
1777550
T«
July 12
5
11
23.7
0
1
150
900
3520
320
1565800
U,
July 18
3
9
21.5
0
0
130
330
2060
229
506400
V,
July 24
4
8
22
0
4
90
900
4390
549
3418900
W,
Aug. 1
3
0
19
20.6
0
6
85
900
10240
539
6987650
X«
Aug. 9
Selection made
>ut record incomplete.
Y,
Aug. 16
11
0
14
19.9 j
0
1
120
900
4110
294
1737800
Z.
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
BT
Sept. 5
2
1
13
20.5
0
0
60
390
2360
182
559150
CT
Sept. 11
3
0
14
16.5 |
0
7
210
900
8780
627
6614000
DT
Sept. 30
Random distribution.
ET
Oct. 21
'"3"
Random distribution.
FT
Oct. 31
22
Random distribution.
GT
Nov. 10
4
0
11
17 || 0 0 1 70
390
2180
198
500200
HT
Nov. 20
14
Random distribution.
IT
Nov. 29
9
Random distribution.
JT
Dec. 11
'"2"
"i"'
12
11.5 || 0
1 1 130
900
4480
337
2272600
KT
Dec. 20
3
0
6
14 II 0
1 1 190
900
2220
370
1173600
Lr
Dec. 30
19
Random distribution.
1917
|
MT
Jan. 10
2
0
12
18 [I 0
130
900
7810
651
6008700
NT
Jan. 19
........
7
Random distribution.
Or
Jan. 30
'"4"
6
15 II 0 I 0 I 40
180
560
93
67400
PT
Feb. 7
Random distribution.
§:
Feb. 16
Feb. 24
'"2"
3
0
0
9
14
9.8
12
0
0
0
2
120
90
460
900
1895
3730
211
266
509025
2003100
ST
Mar. 4
2
0
15
11
0
3
100
900
5020
335
2947000
TT
Mar. 13
3
0
12
13
0
2
70
900
3946
329
2332716
UT
Mar. 21
3
0
16
14
0
0
60
270
2205
138
352225
VT
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
XT
April 18
2
1
10
15
0
0
40
525
1671
167
466759
Y,
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 L« generation which was used in con-
;ing a test series.
ducting a test series
94
SELECTION IN CLADOCERA ON THE BASIS OF
TABLE 42. — Summary of selection data by broods for Line 757 minus.
|
g
1
a
5
.2
.2
.2
•S
A
g
1
1
•si
1
1
I
i
1
j
it
g
.2
j
viduals ft
ad of the
i
|
reaction-t
0
i
1
a
"3*8
&
s'".
§
J3
S «g
TJ *
a
'•§
1
!
•sj
M
1
it
V
0
|!
a~*
•M o!
o i
& x
•3 1
§ E
*S|
03 "
J
•8 j-
1912
j^
Nov. 15
i
0
5
18
0
5
900
900
4500
900
4050000
jj
Nov. 27
2
0
3
16
0
3
900
900
2700
900
2430000
Q
Dec. 9
2
Oor 1
6
13.5
0
4
245
900
4130
688
3381250
D
Dec. 20
2
0
13
19
0
11
190
900
10615
817
9221725
E
Dec. 30
2
Oorl
14
16
0
8
260
900
10090
7H
8078950
1913
F
Jan. 8
2
0
9
17.5
0
5
65
900
5270
586
4255950
G
Jan. 16
0
2
16
0
2
900
900
1800
900
1620000
H
Jan. 24
2
0
6
15.5
0
2
184
900
3975
663
2959925
I
Feb. 3
3
Oor 1
2
14.2
0
2
900
900
1800
900
1620000
J
Feb. 12
2
0
12
13
0
11
320
900
10220
852
9012400
K
Feb. 21
2
0
18
17
0
16
290
900
15120
840
13229000
L
Mar. 3
1
Oor 1
9
11.8
0
7
250
900
7105
789
6040525
M
Mar. 12
2
0
9
13.5
0
6
250
900
6570
730
5365700
N
Mar. 21
5
0
8
0
8
900
900
7200
900
6480000
O
April 5
2
0
16
19.9
0
14
555
900
13825
864
12096925
P
April 14
4
Oor 1
7
18.5
0
4
372
900
5082
723
4005684
Q
April 21
2
Oor 1
10
13.4
0
10
900
900
9000
900
8100000
R
8
April 28
May 7
3
2
Oor 1
0
9
7
17.6
17
0
0
0
7
900
900
900
900
8100
6300
900
900
7290000
5670000
T
May 17
4
0
14
16.2
0
14
900
900
12600
900
11340000
U
June 4
5
0
19
19.2
1
17
540
900
16550
876
14565700
V
June 13
3
0
6
17.5
0
6
900
900
5400
900
4860000
w
June 20
2
0
9
19.5
0
9
900
900
8100
900
7290000
X
June 26
3
0
16
20.6
0
16
900
900
14400
900
12960000
Y
July 3
3
0
7
22.3
0
7
900
900
6300
900
5670000
Z
July 10
July 14
4
3
0
3
11
5
22
22
0
0
11
5
900
900
900
900
9900
4500
900
900
8910000
4050000
At
July 23
4
0
3
22
0
3
900
900
2700
900
2430000
2«
July 29
2
0
6
22.5
0
5
600
900
5100
850
4410000
c,
Aug. 5
4
0
6
21.5
0
6
900
900
5400
900
4860000
Dl
Aug. 13
5
0
4
20.7
0
4
900
900
3600
900
3240000
Ki
Aug. 19
7
0
12
23
0
12
900
900
10800
900
9720000
F,
Aug. 26
4
0
8
20
1
7
840
900
7140
893
6375600
Gi
Sept. 1
10
0
17
21.6
0
17
900
900
15300
900
13770000
Hi
Sept. 8
1
2
13
22
0
13
900
900
11700
900
10530000
It
Sept. 16
11
0
11
13.9
0
11
900
900
9900
900
8910000
Ji
Sept. 22
10
0
15
20
0
6
167
900
8720
681
6326504
Kj
Sept. 29
11
0
17
17.1
0
17
900
900
15300
900
13770000
Li
Oct. 7
4
0
6
19.8
0
6
900
900
5400
900
4860000
Mi
Oct. 14
10
0
6
15.1
1
5
410
900
4910
818
4218100
Nt
Oct. 21
3
0
12
18.4
0
9
168
900
9843
820
8567649
Of
Oct. 29
4
0
8
19.1
0
8
900
900
7200
900
6480000
Pi
Nov. 8
11
0
6
17.7
0
3
242
900
3830
638
2942004
81
Nov. 17
11
0
13
13.6
0
3
255
900
6454
497
3915506
R'
Nov. 28
11
0
15
15.5
0
13
305
900
12347
823
10739989
1?
Dec. 6
11
0
11
14.2
0
11
900
900
9900
900
8910000
Tt
Dec. 15
12
0
12
12.2
0
12
900
900
10800
900
9720000
Ut
Dec. 24
1914
10
0
13
10.7
0
10
424
900
10534
810
8917876
V,
Jan. 2
3
0
10
9.1
0
10
900
900
9000
900
8100000
Wj
Jan. 10
0
11
13.9
0
10
62S
900
9628
875
8494384
Xt
Jan. 21
1
11
14.3
0
10
378
900
9378
853
8242884
Yl
Jan. 27
0
7
14
0
7
900
900
6300
900
5670000
Ai
Feb. 9
Feb. 21
Rand
0
om distrib
1
ution.
9.8
0
1
900
900
900
900
810000
H|
Mar. 2
1
13
16.2
1
1
146
900
4907
378
2538887
Ci
Mar. 11
0
10
22
0
10
900
900
9000
900
8100000
Dl
Mar. 20
0
9
18.3
0
9
900
900
8100
900
7290000
E*
Mar. 28
0
17
19
0
12
80
900
12530
737
10623900
FI
April 6
0
9
17.2
0
7
300
900
6930
770
580S900
Gi
April 15
0
18
19.7
.0
13
80
900
13490
749
11352300
Hi**
May 7
0
4
20.7
0
3
735
900
3435
859
2970225
9
May 18
0
3
19.1
0
3
900
900
2700
900
2430000
Ji
May 26
0
10
21.2
0
9
110
900
8210
821
7302100
T*
June 2
0
11
19.1
0
9
370
900
8847
804
7569029
M
June 13
0
16
20.7
0
10
175
900
10424
695
8645366
• Selection was repeated from a later brood because of the loss of the earlier brood
0 JJeath Josses in this strain made this interval between generations unusually long
A PHYSIOLOGICAL CHARACTER.
95
TABLE 42. — Summary of selection data by broods for Line 757 minus — Continued.
J
g
3
.2
o
a
Q
.2
•5
g
•S
•s
.
g
.2
I!
1
1
1
i
1
~
1
i-j
1
I
Si
.2
.2
lividuala fi
end of the
1
j
.2
•8
0
|j
Hour of ea
Age of yo\
periment,
•8
6
fe
ftjl
fj
No. of neg
dividuals
.2g
"8 •§
Minimum
seconds.
| Maximum
| seconds.
fl
j
1
0 j
11
1914
Mt
June 22
4
0
7
19.2
0
4
150
900
4412
630
3482344
Ni
June 29
2
0
7
19.3
0
0
118
245
1191
170
214923
0,
July 7
5
0
10
18.3
1
7
380
900
8101
810
6850241
July 14
3
0
13
21.6
0
9
420
900
10580
814
8881400
1
July 20
July 27
3
4
0
0
7
4
21
21.7
0
0
2
225
80
900
900
4180
1270
597
318
2880150
859700
s>
Aug. 5
3
0
4
21.4
0
3
230
900
2930
733
2482900
Tt
Aug. 13
2
o
5
22.6
0
1
290
900
2553
511
1547969
U,
Aug. 21
3
0
10
23.1
0
7
300
900
7551
755
6213501
Vi
Aug. 27
3
0
7
21
0
3
150
900
4333
619
3205749
w,
Sept. 2
1
0
13
22.6
1
9
160
900
9603
739
8160549
X,
Sept. 8
4
0
15
18
0
85
900
4145
276
1789001
Sept. 19
11
0
13
18
0
J
110
900
3825
294
1896125
z,'
Sept. 25
2
0
11
19
0
9
370
900
8975
816
7681925
At
Oct. 5
3
1
13
19.8
0
12
175
900
10975
844
9750625
B«
Oct. 13
3
0
18
19.1
0
17
105
900
15405
856
13781025
C4
Oct. 21
11
0
16
18.9
0
16
900
900
14400
900
12960000
D4
Oct. 29
4
0
15
18
0
15
900
900
13500
900
12150000
Nov. 6
2
0
19
17.9
0
14
260
900
15761
830
13536465
F!
Nov. 13
2
0
16
17.9
0
16
900
900
14400
900
12960000
G4
Nov. 20
3
0
14
20.8
0
14
900
900
12600
900
11340000
H4
Nov. 28
11
0
10
18
0
10
900
900
9000
900
8100000
Dec. 7
3
8
19.8
0
7
215
900
6515
814
5716225
J4
Dec. 16
3
0
1
17
0
0
310
310
310
310
96100
K4
Dec. 24
2
0
8
18
0
8
900
900
7200
900
6480000
1915
I*
Jan. 1
11
0
12
17.9
0
9
204
900
9014
751
7087716
M«
Jan. 8
3
0
11
19.5
0
10
825
900
9825
893
8780625
N<
Jan. 15
2
0
15
19.9
0
15
900
900
13500
900
12150000
04
Jan. 25
1
15
17.2
0
14
312
900
12912
861
11437344
p4
Feb. 3
0
16
15.3
1
5
57
900
8025
502
5341163
Q<
Feb. 13
0
10
16.5
0
2
210
900
4145
415
2373475
IU
Feb. 22
0
11
19.6
1
4
145
900
5390
490
3725000
S,
Mar. 2
0
10
20
0
0
150
555
2370
237
698150
T4
Mar. 10
0
15
20
1
7
125
900
9076
605
6748964
U4
Mar. 19
2
0
7
18.2
0
1
358
900
3547
507
2006369
V4
Mar. 30
2
0
10
16.8
0
1
210
900
5025
503
3112775
W4
April 7
3
0
12
17.5
0
10
100
900
9345
779
8170025
X4
April 15
3
0
16
17.3
0
12
180
900
12172
761
10295594
Y4
April 23
3
0
14
18.7
0
14
900
900
12600
900
11340000
ll
May 1
2
0
15
19.4
0
9
282
900
11507
767
9479349
At
May 10
6
0
12
19.2
0
12
900
900
10800
900
9720000
Bt
May 21
3
0
13
18.3
0
0
135
475
3523
271
1104857
C.
May 31
3
0
14
15.5
0
13
620
900
12320
880
10914400
D,
June 9
4
0
12
18.4
0
10
125
900
9555
796
8300525
E»
June 16
3
0
20
21
0
6
120
900
9855
493
6737025
F,
June 24
5
0
7
19
0
0
80
288
1118
160
203844
G.
July 3
3
0
8
21.2
0
2
250
900
5152
644
3860034
Ht
July 10
3
0
7
21.2
0
7
900
900
6300
900
5670000
I.
July 17
3
0
9
22.1
0
8
110
900
7310
812
6492100
J<
July 25
Rand
m distrib
ution.
Ki
Aug. 2
4
1
8
26
0
5
184
900
5484
686
4407906
L.
Aug. 10
4
0 .
3
22.6
0
2
470
900
2270
757
1840900
M,
Aug. 16
3
0
8
24.1
0
8
900
900
7200
900
6480000
N.
Aug. 23
3
0
10
21.2
3
1
150
900
5340
534
3433800
0,
Aug. 31
2
0
9
18.9
0
9
900
900
8100
900
7290000
P,
Sept. 8
5
0
6
22.2
0
5
460
900
4960
827
4261600
Sept. 14
6
0
10
21.9
0
8
150
900
7550
755
6542500
RB
Sept. 20
5
1
13
20.1
0
11
270
900
10440
803
9055800
s,
Oct. 4
4
0
7
17.8
0
7
900
900
6300
900
5670000
Tf
Oct. 13
4
0
17
17.8
0
14
510
900
14255
839
12255625
U.
Oct. 21
3
0
13
18.8
0
13
900
900
11700
900
10530000
V,
Nov. 1
5
1
13
17
1
8
90
900
9215
709
7685425
w.
Nov. 9
4
0
20
15.5
1
6
175
900
10780
539
7539150
X,
Nov. 19
5
0
18
16
0
12
150
900
12510
695
10311300
Y»
Nov. 29
5
0
23
14.5
0
16
160
900
17620
766
14847950
Zi
Dec. 9
5
0
18
14
0
11
150
900
12225
679
9752925
A«
Dec. 18
7
0
19
16.5
0
19
900
900
17100
900
15390000
B.
Dec. 28
4
1
17
14.2
0
17
900
900
13900
900
13770000
96
SELECTION IN CLADOCERA ON THE BASIS OF
TABLE 42. — Summary of selection data by broods for Line 757 minus — Continued.
a
8'"
|
h
s
C
_a
a
J
A
1
•o
1
'i
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§
•
0
1
1
1
'•3
1!
1
D
.
1
.S
i
*£3
fl
*•£ tn
E
M
E
JO'S
o
O
§
a
*o
1
8 .
1
i-l
.5
.a
2
i
S's
11
1
1
1
I
s
1
21
"3
If
|
I
!i
Si
S «;
3 *ri
Q •
3T3
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1
5
2
° -XI
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Ld
*£. ^
*o -^
S «
5 c
I
° s
1
!J
W
6
||
&
|l
'S °
§ S
|i
ll
1*
1916
c,
Jan. 11
6
i
11
18
0
11
900
900
9900
900
8910000
Di
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
Gg
Feb. 19
3
0
14
15.5
0
4
140
900
6695
478
4428675
H,
Mar. 1
4
0
13
15
0
11
420
900
10920
840
9446400
I.
Mar. 11
4
0
12
16.4
0
7
90
900
7325
610
5909325
J«
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
L.
April 18
•71
M.
May 2
'"3"
'"6"'
15
ie'.s' '
6
is
'226
'966
12210
"814"
'16662566'"
N,
May 13
10
0
18
16
0
18
900
900
16200
900
14580000
0,
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
JU
June 22
4
0
12
18.6
0
0
60
450
3305
275
1141325
8.
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
Wjb
July 28
0
11
21
0
7
X,
Aug. 4
"ii"
0
3
19.3
1
2
'426
'966
"2226
"746"
"1796466"'
Y,
Aug. 11
2
0
7
19
0
3
50
900
4110
587
3169150
Aug. 17
3
0
8
20.6
0
7
260
900
6560
820
5737600
AT
Aug. 24
9
1
8
21.3
0
8
900
900
7200
900
6480000
BT
Aug. 30
1
0
9
17
0
4
360
900
6470
719
4984500
CT
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
ET
Oct. 20
Randc
m distribution.
FT
Oct. 29
21
Randon
distr
butic
n.
GT
Nov. 8
"2"
'"6"'
16
16
0
14
300
900
13200
825
11520000
HT
Nov. 17
2
0
14
13
0
12
64
900
11014
787
9746596
IT
Nov. 25
3
0
8
10.5
0
8
900
900
7200
900
6480000
it
Dec. 7
2
0
7
13.2
0
1
50
900
2160
309
1208800
KT
Dec. 16
2
0
20
11.5
0
8
60
900
8990
450
6774100
L*
Dec. 25
Rand
m distrit
ution.
1917
MT
Jan. 4
2
0
6
12.5
0
0
100
210
795
133
113725
NT
Jan. 13
4
0
10
11.2
0
2
80
900
2670
267
1717750
Or
Jan. 20
4
0
14
12
0
0
100
480
2970
212
735700
PT
Jan. 30
4
1
26
15.5
0
21
380
900
21150
814
18094900
Or
Feb. 10
3
1
4
10.5
0
2
450
900
2830
708
2158900
RT
Feb. 19
3
1
11
11
0
2
150
900
4560
415
2691)800
S,
Feb. 28
2
1
7
10.5
0
3
110
900
3000
429
1996200
TT
Mar. 9
3
0
9
17
0
7
50
900
6440
716
5680600
UT
Mar. 17
Random distrib
ution.
VT
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
XT
April 11
3
0
15
13.5
0
4
270
900
8220
548
5382400
YT
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 not
included here as not strictly comparable to other data of this table.
"> 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
1 4 8 0 6 0 7 I0 0
O O O
O O |0 O O
O .3 3 06
O O g
O
0
'
I 2 1
0 0
J
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
1912
1913
1914
1915
1916
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.
I
*i
fcf
i
S
J
|
A
i
1
a
1 g
a %
.2
si!
'1
1
•
a
1
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s 1
1 "
1
aj§
£
£
i
£
"o
1
• 9
a
Time period.
"5
1
•8
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0
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°U
1
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1-s
H a
1
1
maximu
j
1
1
i
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ti
•3
i
i
£
O
^
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>^-o
*
*
^"
< *"
<*
i
S*
i
£
Nov. 1912
1-2
2
6
3.0
13.7
0.22
0
4
495
900
4115
685
318.01
87.57
Dec. 1912-Jan. 1913
3-9
7
82
11.7
9.8
.19
0
59
245
900
61250
747
274.69
20.36
Feb.-Mar. 1913...
10-15
6
72
12.0
9.7
.24
0
55
407
900
57805
803
198.86
15.81
Apr.-May 1913...
16-22
6
82
13.7
8.4
.63
0
66
496
900
65632
800
208.50
15.53
June-July 1913...
23-29
8
78
9.6
7.4
.30
1
63
679
900
63440
813
196.47
15.00
Aug.-Sept. 1913...
30-38
B
120
12.3
6.4
.92
1
75
389
900
81986
683
301.07
18.54
Oct.-Nov. 1913. ..
39-45
6
50
9.1
8.2
.11
0
30
309
862
35109
702
270.01
25.76
Dec. 1913-Jan. 1914
46-52
8
82
9.7
8.8
.10
0
33
279
811
48045
586
309.38
23.04
Feb.-Mar. 1914 . . .
53-59
104
14.9
8.3
.80
2
57
348
900
67229
646
296.25
19.59
Apr.-May 1914...
60-65
8
72
12.0
8.2
.46
0
46
423
900
51828
720
266 . 32
21.17
June-July 1914...
66-73
8
76
9.9
7.6
.30
0
61
395
900
60373
794
227.95
17.64
Aug.-Sept. 1914...
74-81
7
80
13.3
7.1
.87
2
29
241
791
44890
561
298.18
22.47
Oct.-Nov. 1914 . . .
82-89
8
119
15.0
8.3
.81
2
69
243
859
78240
657
303.13
18.74
Dec. 1914-Jan. 1915
90-96
69
10.2
8.2
.24
1
44
493
900
49288
714
269.78
21.91
Feb.-Mar. 1915...
97-102
6
66
11.4
9.5
.20
0
16
121
742
27182
412
305.76
25.38
Apr.-May 1915...
103-108
5
53
11.0
9.7
.13
1
24
261
863
34563
652
281.37
26.07
June-July 1915...
109-116
8
85
9.9
7.6
.30
0
34
304
815
51316
604
281.06
20.56
Aug.-Sept. 1915...
117-123
7
60
10.0
7.4
.35
0
26
187
900
35235
587
323.40
28.16
Oct.-Nov. 1915. . .
124-130
7
132
17.8
9.2
.93
5
31
132
763
57260
434
270.87
15.90
Dec. 1915-Jan. 1916
131-136
C
95
17.8
9.8
.82
9
7
255
689
29685
312
200.72
13.89
Feb.-Mar. 1916...
137-142
G
99
15.2
10.3
.48
1
42
251
847
51895
524
336.60
22.82
Apr.-May 1916...
143-145
3
45
17.8
9.8
.82
0
22
190
900
26125
581
255.24
25.66
June-July 1916...
146-152
7
74
10.6
8.3
.28
0
15
143
805
31188
421
276.37
21.67
Aug.-Sept. 1916...
153-160
8
82
13.7
6.9
.99
0
23
109
815
36460
445
309.23
23.03
Oct.-Nov. 1916. . .
161-165
1
11
12.8
11.0
.16
0
0
70
390
2180
198
78.92
16.05
Dec. 1916-Jan. 1917
166-171
4
36
10.4
10.0
.04
0
7
123
720
15070
419
298.80
33.59
Feb.-Mar. 1917...
172-178
a
74
12.3
8.8
.40
0
7
87
600
17806
241
232 . 34
18.22
Apr. 1917
179-181
a
42
14.0
9.2
.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 11 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.
I
•Si
JS i
cj V
it
3
g
i
jj
a
8
i
.
1
|
II
Is
3*
I
te
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1
-.3
8
1
i
•g
a
1
IJ
I
h
1
1?
1
o EC
•s|i
£
«j
8
|
g
I
|l
1
S
•i
§
"3
"ol
•o^S
•1
3-5
3
d
"V
"3
•j
•o
| «
•8
•o
Time period.
•o
K
>>
d
0 JD
d^a
>
1-a
•p
.J
i?
'£
g
«1
g
'?
1
|
"o
6
£
£
:i
*11
o £ g>
-i
11
o>
S
0>
i
1
1
1
•° a
i*
1
§g
1
•3
6
£
!
i
P
Zsl
&
9-°s
!»i
a
"s-8
6 o
55 **
II
II
Sum of
II
1
1
H
1
U
Nov. 1912
1-2
2
8
4.0
14.3
0.28
0
8
900
900
7200
900
0.00
0.00
-214
87.56
2 45
Dec. 1912-Jan. 1913
3-8
6
50
8.3
9.9
.84
0
32
307
900
35880
718
274.60
26.19
+ 29
33.18
0.87
Feb.-Mar. 1913 . . .
9-14
6
58
9.7
9.3
1.04
0
50
485
900
48015
828
185.83
16.46
- 25
22.82
1.09
Apr-May 1913. . .
15-21
6
63
10.5
9.5
1.11
0
58
755
900
54907
872
101.83
8.65
- 72
17.78
4.05
June-July 1913 . . .
22-28
9
82
8.3
7.4
1.12
1
79
827
900
72950
890
55.42
4.13
- 77
15.56
4.94
Aug.-Sept. 1913...
Oct.-Nov. 1913. . .
29-37
38-44
9
7
103
(if)
11.4
9.4
6.9
8.3
.65
.13
1
1
93
47
812
454
900
900
87860
49984
853
757
157.60
242.21
10.47
20.11
-170
- 55
21.29
32.67
7.98
1.68
Dec. 1913-Jan. 1914
45-51
7
75
10.7
8.3
.29
0
70
719
900
65540
874
100 . 95
7.86
-288
24.35
11.82
Feb.-Mar. 1914...
52-57
5
50
10.0
9.1
.10
1
33
585
900
35437
709
291.54
27.81
- 63
34.02
1.85
Apr-May 1914 ...
58-62
5
44
8.3
8.7
.95
0
35
425
900
34765
790
236.26
24.02
- 70
32.02
2.18
June-July 1914 . . .
63-70
8
74
7.7
7.5
.03
1
42
240
818
49005
662
306.09
24.00
+ 132
29.79
4.43
Aug.-Sept. 1914...
71-78
8
78
11.8
7 6
.55
1
34
212
900
43915
563
325.30
24.84
- 2
33.49
0.05
Oot.-Nov. 1914 . . .
79-86
8
121
16.0
s!i
.98
0
114
630
900
106041
876
117.19
7.19
-219
20.07
10.91
Dec. 1914-Jan. 1915
87-93
7
70
10.3
8.1
.27
0
63
524
816
59276
847
171.25
13.81
-133
25.89
5.13
Feb.-Mar. 1915...
94-100
7
79
11.7
9.3
.26
3
20
179
851
37578
476
278.70
21.15
- 64
33.04
1.93
Apr-May 1915. ..
101-107
7
96
12.9
9.6
.34
0
70
445
839
72267
753
262.89
18.10
— 101
31.74
3.18
June-July 1915 . . .
108-114
6
63
10.8
7.8
.38
0
33
264
798
39290
624
327.67
27.85
- 20
34.62
0.57
Aug.-Sept. 1915. . .
115-122
S
67
8.4
7.1
.18
3
49
436
900
51344
766
243.39
20.06
-179
34.57
5.17
Oct.-Nov. 1915. . .
123-129
7
111
17.1
9.7
.76
2
76
412
900
82380
742
263.55
16.87
-308
23.18
13.28
Dec. 1915-Jan. 1916
130-134
5
75
16.0
10.3
0
68
750
900
62125
828
244.04
19.01
-516
23.54
21.92
Feb.-Mar. 1916...
135-141
7
92
11.9
10.2
'.17
0
75
607
900
72640
790
241.59
16.99
-266
28.45
9.34
Apr .-May 1916 . . .
142-145
3
55
18.3
11.0
.66
0
52
423
900
47460
863
155.04
14.10
-282
29.28
9.63
June-July 1916 . . .
146-153
7
86
12.8
8.2
.56
0
56
236
836
58350
678
317.39
23.08
-275
31.66
8.11
Aug.-Sept. 1916...
154-160
1
57
8.1
6.7
.21
0
41
363
900
43570
764
240.07
21.45
-319
31.47
10.13
Oct.-Nov. 1916. ..
161-165
3
38
15.1
9.0
.68
0
34
421
900
31414
827
216.57
23.70
-629
28.62
21.97
Dec. 1916-Jan. 1917
166-172
8
83
13.3
8.9
.49
0
32
128
715
38735
467
356.93
26.42
- 48
42.74
1.12
Feb.-Mar. 1917. . .
173-178
5
41
8.2
9.5
.86
0
21
174
900
23970
585
330.26
34.79
-344
39.27
8.75
April, 1917
179-181
3
45
15.0
8.8
1.71
0
23
142
900
27559
612
315.97
31.77
-488
32.78
14.88
severe losses by death in both strains and particularly in the plus
strain. In general, however, there has been no obvious relation
between losses by death and reaction-time, and it is doubtful if that
was an influence at this time.
The test series conducted during September of this year-period
contained 14 broods of each strain, 322 individuals of the plus strain
and 367 individuals of the minus strain. The means were 409.0
seconds for the plus strain and 859.7 seconds for the minus strain
(table 45). Thus the mean for the minus strain was more than twice
that for the plus strain. The difference (-450.7 ±12.49 seconds)
100
SELECTION IN CLADOCEKA ON THE BASIS OF
was 36.08 times its probable error. In every case in this test series
the plus brood had a lower reaction-time than the corresponding
minus brood.
For the year August 1914-July 1915 (43 generations and 472
individuals, and 44 generations and 507 individuals, respectively)
TABLE 45. — Selection summary for Line 757.
Time period.
Nov. 16, 1912-
July 31, 1913
Aug. 1, 1913-
July 31, 1914
Test s e ri e s ,
Sept. 1913. .
Aug. 1, 1913-
Oct. 31, 1913
Aug. 1, 1914-
July 31, 1915
Aug. 1, 1915-
July 31, 1916
Test series,
Apr. 1916...
Mar. 1, 1916-
MaySl, 1916
Aug. 1, 1916-
May 1, 1917.
Strain.
Plus. .
Minus
Plus. . .
Minus
Plus..
Minus
Plus..
Minus
Plus. .
Minus
Plus. .
Minus
Plus...
Minus
Plus. ..
Minus
Plus. ..
Minus
1-29
1-28
30-73
29-70
34
33
30-42
29-41
74-116
71-114
117-152
9.0
11.5
41 10.0
1423.0
.2
12.1
1310.4
.5
11.8
14.0
116-163 37 12.9
143 1254.3
140-145
138-146
153-181
164-181
142 12 56
29 11.0
7 16
7 16.6
20 12.3
2411.0
4>;<
683.7
753.0
409.0
559. 7
691.2
553.4
604.8
706.5
458.2
770.2
697.0
564! 3
542.2
313.1
625.9
226.8
294.7
260.7
309.3
8.57
171.9
290.4
235.;
301.7 11.34
6.26
292.0 15.72
166.4
305.4
290.5
8.72
7.9;
9.05
9.48
5.70
8.84
7.
8.17
-60.6 11.18
-460.7 12.
-162.218.16
'-ib'g.bi'e.87
-167.6
'-£77'. 9
'-Slie'.S 18.37 17.01
9.1218.38
is'.4sii'.86
TABLE 46. — Same-day broods. Summary of data for Line 757.
Time period.
Nov. 16, 1912-
July 31, 1913
Aug. 1, 1913-
July 31, 1914
Aug. 1, 1914-
July 31, 1915
Aug. 1, 1915-
July 31, 1916
Aug. 1, 1916-
May 1, 1917
Strain.
Plus....
Minus . .
Plus....
Minus . .
Plus
Minus . .
Plus....
Minus . .
Plus
Minus . .
9 12.7
9 11.0
13.2
// 11. £
8 12.9
814.1
818.1
8 16.0
1 6.0
1 26.0
1.3
798.4213.38
8 133.47
3306.29
505.4 219.47
299.79
09.76
100.89
'IS. 56
50.28
5 184.86
13.49
9.05
17.16
13.36
19.92
9.86
16.85
.13.09
13.85
24.45
-.735
-200'
16. 24
21 '.74
tl'.ll
21 '.34
3.69
c'.si
9'.46
t'.66
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
1 5
10
00
10
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
4-5 8-9 12-1 4-5 8-9 12-1 4-5 8-9 12-1 4-5 8-9 I2~l
1913 1914 1915 1916
B
S
-2.00
HI .ii]i *%in "J un.I ]
- .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 -r i Q-
I
1.50
1.00
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
.75
.50
.25
00
.25
.50
900
750
600
450
300
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
:TT!TT I
n
T T
11
150
00
ll-l
1912
4-5 8-9
12-1
4-5 8-9
1914
12-1 4-5 8-9
1915.
4-5 8-9
I9IQ
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
450
150
5-10 11-4 5-10 11-4 5-IO
1914 1915 1916
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 thejfplus^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 1 1 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
18s) 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.
OTHEB FEATURES OP 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 occurre^ 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).
1 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.
A PHYSIOLOGICAL CHARACTER.
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108 SELECTION IN CLADOCERA ON THE BASIS OF
Thus, for the moment ignoring the data for the less reactive
(over-time) individuals, one finds that the more reactive individuals
of the plus strain showed greater reactiveness than the more reactive
individuals of the minus strain. These differences are not large and
are not all in the same direction, but on the basis of this portion of
the data alone there is evidence for an effect of selection, though
naturally it is not as decisive as that obtained from the complete
data.
RELATION BETWEEN RELATIVE VIGOR AND MEAN REACTION-TIME.
There remains to be considered the relation, if any, between the
relative vigor of the two strains and their relative reactiveness.
This is an important point. While it has already been shown that
with the other lines subjected to selection in connection with which
this point was considered, there is little, if any, relation between
reactiveness and the general vigor of the stock, it still remains
desirable to know whether such is certainly the case with Line 757.
There is to be noted with the minus strain of Line 757, as with
the minus strains of many other lines of Cladocera subjected to
selection, a somewhat lower reproductive vigor than with the
corresponding plus strain. This is presumably the result of the
individuals selected in the minus strains being somewhat below the
average in vigor. That this at the same time has not operated
materially to increase the mean reaction-time of the minus strain is
evident enough in nearly every line. The data for vigor and mean
reaction-time considered for the other individual lines (and for all
the lines as a whole) seem to show conclusively that there is no
relation between vigor and reaction-time. While this suggests that
in Line 757 the result of selection is not due to less vigor in the minus
strain, yet it does not preclude the possibility that it may have been
a factor in the divergence in reactiveness in Line 757. Some attention
will now be directed toward an analysis of the data on mean reaction-
time as related to vigor in this line. The matter of concern is whether
there is a relation between the relative vigor of the two strains and
their mean reaction-times, and, if there is a relationship, to what
extent it is operative. Several more or less disconnected considera-
tions will be taken up in regard to this point before referring to
the statistical correlations.
The two strains were compared as to general vigor as revealed
by the measures applied to them (see page 42) and stated in terms
of the reproductive index (tables 43 and 44).
1. The plus strain was on the whole the more vigorous of the two,
as will be seen by reference to tables 43 and 44 and figure ISA. Of
the 27 periods of 2 months each (except the first period of 3 months
and the last period of 1 month) the plus strain was of superior vigor
during 16 periods and the minus strain during 11 periods. The
A PHYSIOLOGICAL CHARACTER. 109
quantitative differences in favor of the plus strain averaged greater
than those for the minus strain, so that on the whole the plus strain
averaged somewhat higher in reproductive vigor. For the first 11
periods (23 months) this superiority in vigor in the plus strain was
marked; but, during the remainder of the experiment (16 two-month
periods) during which the divergence in mean reaction-time became
very large, the average superiority of the plus strain in reproductive
vigor was slight indeed, and, in fact, during 9 of these 16 periods the
minus strain was the more vigorous. This at once throws serious
doubt upon the possibility of explaining any portion of the diver-
gence in mean reaction-time as a result of a difference in vigor.
2. Near the middle of the curve for 5 successive two-month
periods the minus strain showed (see figure 18, A and B) a slight
superiority in reproductive vigor over the plus strain, and during
this time the minus curve twice approached the plus curve, although
in spite of its greater vigor the minus strain had a consistently
higher reaction-time. Examination of the curves at this point might
at first sight seem to suggest, however, that there was actually some
effect upon mean reaction-time of the relatively greater vigor of
the minus strain. At two points the curves covering this limited period
are separated by spaces representing differences in reaction-times of
only 64 and 20 seconds, but at the other three points the curves are
separated by 219, 133, and 101 seconds (table 44). These last three
divergences are larger than had occurred except in 3 of the 11 preced-
ing periods of the experiment. The mean reaction-time for all the
data for the plus strain during this ten-month period of superior
vigor on the part of the minus strain was 613.7 seconds, 119.2 seconds
(19 per cent) less than that for the minus strain, whose reproductive
index during this time averaged 0.10 higher than that for the plus
strain. This may be compared (a) with similar averages for all
data for Line 757 obtained previous to October 1914, during which
the mean reaction-times differed by 68.7 seconds (10 per cent), the
minus strain having the higher reaction-time and a less vigor by
0.25; (6) with the period of 10 months immediately following the
ten-month period of superior reproductive ability on the part of the
minus strain, in which the plus strain was the superior in vigor by
0.19 and its reaction-time averaged 325 seconds (71 per cent) less;
and (c) with the final 6 periods (11 months), during which the plus
strain had a lower reproductive index by only 0.08 and a lower mean
reaction-time by 300.6 seconds (nearly 89 per cent). This shows
that the rapidly increasing percentage difference between the mean
reaction-times was quite independent of the relative vigor of the two
strains.
Further, in the 2 two-month periods immediately preceding the
5 periods of superior vigor on the part of the minus strain just
referred to, the minus strain showed a greater reactiveness to light
110 SELECTION IN CLADOCERA ON THE BASIS OF
than the plus strain in one case and a practically equal reactiveness
in the other case — a thing which did not occur elsewhere in the
experiment — and yet the plus strain was markedly superior in vigor.
3. There were 10 two-month periods during which the vigor of
the plus strain was quite above the average (figure 17c) the repro-
ductive index being more than 1.50. The mean reaction-time for
these periods of high vigor was 546.8 seconds, 36 seconds less than
the mean for the plus strain for the entire period of the experiment.
There were 9 corresponding periods of high vigor for the minus
strain, for which the mean reaction-time was 768 seconds, 23 seconds
greater than the mean for the entire period for this strain. There
were 9 two-month periods during which the plus strain had a repro-
ductive index under 1.25, somewhat below the average. For these
periods the general average reaction-time was 634.9 seconds, 52
seconds above the average for the entire experiment. For 12 periods
of correspondingly low reproductive indices in the minus strain the
mean reaction-time was 770.8 seconds, 26 seconds above the average
for the whole experiment. Hence in 3 of these 4 comparisons the
mean reaction-times varied in the direction to indicate some relation
between actual vigor and mean reaction-time. In one case the
reverse is true.
4. A fairer method of making a comparison between relative
vigor and mean reaction-times is a detailed period-by-period exami-
nation of the data for reproductive index and the mean reaction-
time. Comparing the mean reaction-time for the plus strain for the
10 periods of high reproductive indices (above 1.50) with the reaction-
time means for the adjacent points in the curve (each representing
two-month periods), it is seen (figures 17c and 18s) that in 2 cases
the reaction- time means are higher than in the adjacent points in
the curve, in 3 cases intermediate, and in 5 cases lower. For the 9
periods of like high reproductive indices for the minus strain, the
mean reaction-times are higher than in the adjacent portions of the
curves in 5 cases, intermediate in 1 case, and lower in 3 cases. A
similar comparison for 9 periods of low reproductive index (below
1.25) in the plus strain shows that in 5 cases the mean reaction-time
is higher than in adjacent points in the curve, and in 4 cases it is
lower. For the 12 periods of like low reproductive indices in the
minus strain the mean reaction-times are higher than in adjacent
portions of the curve in 3 cases, intermediate in 5 cases, and lower in
4 cases. These comparisons indicate a possible slight relation be-
tween mean reaction-time and reproductive vigor for the plus strain,
high vigor being more often associated with greater reactiveness and
a lower reaction-time; but the reverse relation is more strongly
indicated for the minus strain, higher reproductive vigor being more
often associated with less reactiveness and a higher reaction-time.
A PHYSIOLOGICAL CHARACTER. Ill
5. Arbitrarily selecting the 6 periods of highest reproductive
indices for the plus strain, they have, as compared with adjacent
parts of the curve, a higher reaction-time twice, an intermediate
reaction-time once, and a lower reaction-time 3 times. The minus
strain for its 6 highest reproductive indices has a higher reaction-
time 4 times, an intermediate reaction-time once, and a lower reac-
tion-time once. The 5 periods of lowest reproductive indices for
the plus strain have, as compared with adjacent points in the curve,
a higher reaction-time 3 times and a lower reaction-time twice.
The similar periods for the minus strain have a higher reaction-time
in one case, an intermediate reaction-time in 3 cases, and a lower
reaction-time once.
The comparisons under (4) and (5) show some evidence of a
relation between vigor and reaction-time in the plus strain, the mean
reaction-time more often varying in the direction anticipated, if
greater vigor is associated with greater promptness in reaction (and
hence lower reaction-time) than in the reverse direction. But with
the minus strain the differences are such as to indicate the reverse
relation, i. e., greater vigor associated with less promptness in re-
action (higher reaction-time). These small differences abo*ut neu-
tralize each other and lead one to conclude, £rom this portion of
the evidence, that there is probably no relation between vigor and
reaction-time.1
6. For the two-month period during which the minus strain
showed its greatest vigor (October-November 1914) its mean reaction-
time was greater than at any other period except one, and much
greater than the average for the succeeding 5 periods, during which
its reproductive index was about 35 per cent lower.
7. It is worthy of note that 4 of the 6 high points in the curve
of the reaction-times of the minus strain occur when the minus
strain was superior in vigor to the plus strain. This, if considered
alone, would seem to suggest a reverse relation between reproductive
vigor and reaction-time, a relation which would seem not to have
biological significance.
8. Attention may be directed to the two-month periods during
which (even after the effect of selection appeared already to have
become established) the minus strain had a reaction-time approxi-
mating or lower than that for the plus strain. For the period June-
July 1914, during which the minus strain was on the average the
more reactive of the two, and the following two-month period during
which the minus strain was the less reactive by only 2 seconds, the
plus strain was markedly the more vigorous. For the February-
March 1915 period, when the minus strain was within 64 seconds
of the plus strain, the minus strain was slightly the more vigorous.
1 These data, however, might be taken to offer slight corroboration of the puzzling results
obtained from the correlations based upon this data (see page 113).
112 SELECTION IN CLADOCERA ON THE BASIS OF
For the following June- July period, during which the minus strain
was within 20 seconds of being as reactive as the plus strain, it again
was slightly the more vigorous. But it is noteworthy that in the
period intervening between the two periods just referred to the
minus strain was superior in vigor to the plus strain by a difference
3 times as great as in the other two periods, and yet the reaction-time
for the minus strain was 101 seconds greater than that for the plus
strain. For the last two-month period (December 191 6- January
1917), in which the reaction- time mean for the minus approached
that for the plus strain, the minus was considerably the more vigorous.
It is noteworthy here, however, that in the period just preceding1
the minus strain showed greater superiority in vigor than at any
other time, and yet its reaction-time differed from that for the plus
by a larger margin (629 seconds) than for any other period of the
experiment. This analysis might be prolonged further, but enough
has been seen to make clear that there is at most a very slight rela-
tion between the reproductive index and the mean reaction-time.
Of the points discussed above, (3) and doubtfully (4) and (5)
indicate a possible effect of vigor upon reaction-time in the plus
strain (but not in the minus strain) — i. e., the greater the vigor the
more reactive the individuals and the lower the reaction-time. But
this bit of evidence is not very convincing, since the numerical differ-
ences in the comparisons made are small and the differences in
averages are not all in the right direction to bear out such a relation-
ship. On the other hand, (1), (2), and (8) fail to show the appearance
of an effect of mean vigor upon mean reaction-time, while (5) (in
part), (6), and (7) seem to indicate the reverse effect, that greater
vigor is associated with lessened reactiveness and a higher reaction-
time.
Obviously, the best method of determining the relation, if any,
between reproductive vigor and mean reaction-time is by statistical
correlation. The mean reaction-times and mean reproductive
indices, both by two-month averages, were correlated for the data for
the Line 757 strains, using as the terms of the correlation the repro-
ductive index of the mother and the mean reaction-time of her first
brood of offspring. The correlation is —0.1 109 ±0.127 for the plus
strain and 0.0612 ±0.129 for the minus strain. In order to have a
larger series, the data for both strains of Line 757 were thrown
together.2 The resulting correlation is -0.12845 ±0.0902. These
correlations are low, and, judged by their probable errors, are not of
statististical significance.
JThe numbers of individuals tested during this period were abnormally small, so that these
averages are not as trustworthy as most of the others, but the major part of this difference can
not be regarded aa accidental.
1 Since the two strains are different in their reactiveness to light, this combination of data is
open to criticism, although it is reasonable to suppose that variations in vigor should influence
the reaction-time in the two strains similarly.
A PHYSIOLOGICAL CHARACTER. 113
Correlations involving the individual reaction-times and the re-
productive indices of the mothers were made. For the plus strain
this data included every individual of Line 757 whose reaction-time
was obtained in making the selections and the reproductive index
of whose mother was known — a total of 1,992 individuals. The
correlation was —0.000828 ±0.0151. For the minus strain (with
1,842 individuals) the corresponding correlation was 0.2154 ±0.0149.
A negative correlation between vigor and reaction-time would
have obvious biological significance, while a positive correlation would
not, for it is difficult to see how less vigor would be causally asso-
ciated with greater reactiveness (lower reaction-time). Both the
correlations for the 757 plus strain are negative, but although sug-
gestive, they are much too small to be statistically significant. The
correlations for the 757 minus strain are both positive and the
second, while not large, is 14 times its probable error. This is not
convincing evidence, but on its face it indicates that in the minus
strain of Line 757 the greater the vigor the higher the reaction- time;
that is to say, a greater vigor is associated with less reactiveness and
a lower vigor with greater reactiveness. Some possible corroboration
of this anomalous state of affairs is seen under (4) and (5) above
(page 110),1 but it is difficult to understand how this relation is
possible.
Further analysis of the data for Line 757 was attempted. Cor-
relations were made between age of the mother at the time of pro-
ducing her first brood and the reaction- times of her young; between
the number in the brood (size of brood) and their reactiveness, and
between the age of the mother and the size of her first brood.
The correlations between the age of mothers and reactiveness
of young were -0.03865 ±0.0205 for the plus strain (1,992 young)
and —0.01529 ±0.0157 (1,842 young) for the minus strain. Between
the number in the brood and their reactiveness the correlations
were -0.07619 ±0.0150 for the plus strain and 0.01606 ±0.0157 for
the minus strain.
Other things being equal (temperature, food, etc.), the more
vigorous mothers reproduce earlier and the less vigorous mothers
later. Hence a correlation of obvious biological significance between
age of mothers and reactiveness of young should be positive. Both
the correlations obtained were negative, but they were very low, and
neither was of statistical significance. A biologically significant cor-
relation between size of brood and reaction-time should apparently
be negative. The correlation obtained for the data for the plus
strain was negative and was 5 times its probable error, but it is
numerically so small (-0.07619) that it at most indicates an ex-
tremely slight relation between size of brood and reaction-time. The
1 Likewise, this fits with the observation recorded under (7) stated on page 111.
114 SELECTION IN CLADOCERA ON THE BASIS OF
correlation obtained for the same data for the minus strain is positive
and is too small to be of statistical significance.1
Considered on all points, there is little in the data or the course
of the curves to indicate any appreciable relationship between the
relative vigor of the two strains and their mean reaction-times. The
comparisons made and the correlations calculated show that there
is at most only a possible slight relation between average vigor
(reproductive index) and mean reaction-time. The relation (if
actually significant) is so small, however, that when the data are
examined in many detailed ways it usually quite fails to appear and
statistical correlations do not establish it. This possible relation is
large enough at most to account for only a small portion of the fluc-
tuations of the curves and at best for only a small fraction of the
divergence between the mean reaction-times of the two strains of
Line 757. Hence it becomes clear that fluctuations of the mean
reaction-time curves of the two strains were controlled neither in
direction nor in amount (to any appreciable extent at any rate)
by the relative vigor of the two strains.2
While the writer is convinced that in Line 757 there is at most
only a very slight relation between vigor and reaction-time and has
already laboriously presented the evidence on this point from the
data for Line 757 itself, this conclusion is perhaps even better justi-
fied from consideration of the data for the plus and minus strains
of the other Cladocera lines subjected to selection. Inspection of the
curves showing mean reaction-times and reproductive indices for the
other lines of Simocephalus exspinosus3 brings some crucial evidence
to bear directly on this point.
The data for Line 794 (tables 33 and 34, and figures 11, A, B,
and c) show considerably greater reproductive superiority on the part
of the plus strain (as compared with the corresponding minus strain)
than in the plus strain of Line 757, yet the reaction-time means for the
two strains of Line 794 run much nearer together than in Line 757.
The data for Lines 795 (figures 12, A, B, and c) and 796 (figures 13,
A, B, and c) show still greater differences in mean reproductive vigor of
their plus and minus strains, yet there are no corresponding differences
in mean reaction-time. The differences in mean reaction- time which
occur in different parts of the curves for these lines do not at all lend
1 The correlations between age of mother and size of brood have no apparent bearing upon
reactiveness, but they may be mentioned for their biological interest. The correlations obtained
are -0.05785 ± 0.0525 for the plus strain and - 0.01732 ± 0.0527 for the minus strain. If one
eliminates the data for mothers which did not produce their first broods until more than 11 days
of age, thus eliminating the obviously weak mothers (since more than 1 1 days is an abnor-
mally late reproductive age), the correlations are 0.11618 ±0.0531 for the plus strain and
0.12409 ± 0.0533 for the minus strain.
2 It is obvious that if the vigor of the stock were extremely low the mean reaction-times
should become higher, but this stock was always maintained at the highest possible vigor and
the varying reproductive indices merely indicate various degrees of the always (except for very
limited periods) successful maintenance of high levels of reproductive vigor.
3 See pages 44-45, 60 and 65 and figures 2, B and c, 3, B and c, 7, B and c, and 8, B and c,
for treatment of this data for the D. pulex lines.
A PHYSIOLOGICAL CHARACTER. 115
themselves to explanation as due to differences in relative vigor. For
example, during the period from October 1915 to May 1916, inclusive,
the plus strains of Lines 795 and 796 Vere both greatly superior in
reproductive vigor to the minus strains of the same lines (figures
12, A and B, and 13, A and B), yet during this time the plus strain of
Line 795 was much less reactive than its minus strain, while the plus
strain of Line 796 was more reactive than its minus strain.
The curves for Line 740 (figures 14s and 15) are also instructive
on this point. In general, the minus strain averaged slightly less
reactive than the plus, yet it was on the whole consistently the more
vigorous.
It is obvious that reaction-time and reproductive vigor are on the
whole, very slightly, if at all, related in S. exspinosus, and that
differences in vigor do not explain the divergence in mean reaction-
time so pronounced in Line 757.
Further, there is no basis for an assumption that the reaction-
time differences which arose in Line 757 are due to differences in
swimming ability. If the difference in reaction-time in Line 757
arose from differences in swimming ability, the results are fully as
interesting from a genetic standpoint as though they were due to
differences in reactiveness to the light per se. But differences in
swimming ability were not the source of the differences in reaction-
time between the two strains of Line 757. As compared with the
plus strains, individuals of the minus strain less often showed any
tendency to react to light, i. e., many more were immobile throughout
the time of the test, and individuals of the minus strain which did
reach an end of the tank in general did so after more and longer inter-
ruptions of swimming than occurred for individuals of the plus
strain. It is not believed that anyone who observed the behavior
of individuals of these two strains in the experimental tank and in
the culture-bottles would countenance any suggestion that the dif-
ferences between the two strains were due to differences either in
general activity or in swimming ability.
SPECIAL FEATURES OF THE REACTION-TIME CURVES.
The approximation of the curves (figure ISA) for mean reaction-
times in Line 757, during 1914 and 1915, at first sight appears more
significant than it really is. For 1 two-month period, June-July
1914, the minus strain was more reactive than the plus strain, and
for the following two-month period the difference was only —2
seconds, while for the June- July 1915 period the plus strain averaged
only 20 seconds lower in reaction-time than the minus strain; but in
the intervening February-March period, for wkich the curves appear
so nearly to approach, there is a difference of 64 seconds, or 16 per
cent. That such fluctuations are to be expected occasionally would
seem to be indicated by the fact that during the last longer (nine-
116 SELECTION IN CLADOCERA ON THE BASIS OF
month) period of the experiment the mean for the minus strain for 1
two-month period (December 1916-January 1917) fell to within 48
seconds of that for the plus strain, yet, from consideration of the
data for this line for the 21 months at the end of the experiment, no
one would be inclined to question the validity of a real and large
difference in reactiveness between the two strains. Further, periods
of local shifts in reactiveness — changes which are believed to be
purely environmental and non-genetic — are seen in other lines, e. g.,
Lines 695 and 740 (see pages 44 and 86), in which there is not evi-
dence of an effect of selection. The approximation of the curves
for the plus and minus strains during 1914 and again in 1915
seems to have been due to differential local environmental influences
(see also pages 140-142).
There is an additional suggestive feature of the general course
of the curve for the Line 757 selection data. It is in the marked
general lowering of the mean reaction-time for both the plus and
minus strains of Line 757. The curve starts at 743 seconds for the
plus strain1 and at the close of the experiment ends at 124 seconds.2
The average reaction-time for the plus strain for the first 9
months of selection was 788 seconds and for the last 9 months 313
seconds, while the intervening 3 year-periods of data show a pro-
gressive and consistent lowering of this reaction-time as follows:
684, 605, 458. The form of the curve suggests that this lowering
had not ceased, but was continuing when the experiment closed.
The curve shows a slight lowering of the reaction-time for the
757 minus strain, but the amount is small and the mean reaction-
times for the longer periods of the experiment do not indicate as
large and consistent lowering in this as with the plus strain. The
means for the minus strain for the longer periods are 839, 783, 707,
770, and 626 seconds.
EXTENT OF THE CHANGE IN REACTIVENESS OF THE LINE 757 STRAINS.
The lowering of the mean reaction-time for the 757 plus strain
may be examined further. When it is recalled that a mean reaction-
time of 788 seconds (that for the plus strain for the first longer period)
means that by far the greater part of the individuals failed to react8
to the light stimulation during the test (actually 77.2 per cent of the
individuals of the plus strain during the first nine-month period
failed to reach an end of the tank) and that a mean reaction-time
of 313 seconds indicates that a great preponderance of the individuals
responded to the light stimulation (only 15.1 per cent, less than one-
tenth as many as during the earlier period, failed to respond during
the final nine-month period) the decline in reaction-time from 788
seconds to 313 seconds takes a still greater significance. Not a
1 Average for 2><j months' data, 88 individual records.
2 Average for 1 month's data, 42 individual records.
* That is, reach an end of the tank during the 15-minute period of the test.
A PHYSIOLOGICAL CHARACTER. 117
single brood in the first longer period failed to contain individuals
which did not respond during the 15 minutes to the test. In the
final longer period nearly half the broods tested contained no over-
time individuals. Of the last 10 broods tested, only 3 contained
over-time individuals and the last 5 broods contained none. The
change in reactiveness in this strain is also strikingly indicated by
again referring to the average minimum reaction-times in the first
and last periods. For the first nine-month period it was 468 seconds;
in the last period 94 seconds. These figures, it will be recalled, refer
only to the most reactive individuals of each brood, but when these
have become 5 times as reactive as during the early period of the
experiment, the fact can not fail to have considerable significance.
The change in responsiveness in the plus strain of Line 757 was
in no other way as striking to an observer as the change in the general
behavior of the animals in the experimental tank. Early in the
history of this strain the young daphnids, on being released in the
experimental tank, ordinarily behaved as follows: first they settled
to the bottom in a rather close group; after 3 to 5 minutes a few
(usually less than 30 per cent) moved a little — ordinarily toward the
source of light, though many movements were indifferent to the
light. (In about a third of the earlier selection tests of entire broods,
there was no movement at all after the animals settled to the bottom
of the tank, though these were vigorous individuals.) Of the small
number swimming toward the light, about two-thirds, in spite of
many pauses, made their way to the end of the tank within the
period (15 minutes) of the test. Toward the close of the selection
experiment with Line 757, the behavior of the 757 plus strain was
essentially as follows : the animals, on being released, ordinarily settled
to the bottom as previously, but within a few seconds some of
them began to swim toward the light and within a minute or two all
were usually under way toward the positive end of the tank, which
the majority reached with few interruptions of their swimming
movements. In the earlier part of the experiment the animals
frequently did not move a second time after once swimming for
a moment and then settling to the bottom or holding fast to the
surface film or the sides of the tank. At the close of the experiment
the movements in the plus strain were not only more prompt and
general, but were less often interrupted, and when interrupted were
generally resumed rather promptly.
There was to the eye of the observer possibly some increase in
reactiveness of the 757 minus strain as exhibited in its general be-
havior, but it was slight as compared with the change which occurred
in the 757 plus strain.
The lowered mean reaction-times for the strains of 757 during
the course of the selection experiment are in partial accord with
the general lowering of the mean which occurred for all of the strains
118 SELECTION IN CLADOCERA ON THE BASIS OF
of Simocephalus during the course of the selection experiment. This
general lowering of the mean is probably attributable to environ-
mental conditions (see pages 137-1 39). l But in the case of the
757 minus strain, and particularly during the final 11 months of the
experiment, the mean was lowered much less than that for each of
the other 9 selected strains of this species. Selection here seems to
have operated to hold the mean much higher than it otherwise
would have been.
REACTIVENESS OF BOTH STRAINS OF LINE 757 MODIFIED THROUGH SELECTION.
Another method of checking up the effect of selection in Line
757 is by a detailed comparison of the mean reaction- times of the
plus and minus strains of Line 757 with corresponding data for
Line 740, with which it is most similar in point of time of beginning
the selections and in duration of the selection experiment; and by a
further comparison of the reaction-time means for the two strains
of Line 757 with corresponding mean reaction-times for the plus and
the minus strains of the other lines of the same species, Lines 794, 795,
and 796.
This comparison is given in detail in tables 48 and 49 and serves
to show whether both the plus and minus strains of Line 757 became
modified during the selection or whether the divergence between the
.two strains of this line resulted from a genetic change in reactiveness
of the minus strain alone. This would seem to have an important
bearing upon the question as to whether the results came from
mutation or gradual change in Line 757. It would seem less easy
to explain the result as due to larger mutations if both the plus and
minus strains of Line 757 were affected, i. e., departed from the
means for corresponding strains of the other lines of the same species,
than if only one of the two strains of Line 757 was affected.
The comparison between the mean reaction-times of the Line 757
plus strain and the plus strains of the other S. exspinosus lines is
the more crucial test, for it is obvious, by mere inspection and com-
parison of the figures (figures lie, 12c, 13c, 15, and 18c) showing the
reaction-time curves for Line 757 and the other S. exspinosus lines,
that the Line 757 minus strain became much less reactive than any
of the other S. exspinosus strains. The following analysis seems to
show, however, that the plus strain of Line 757 likewise became
more reactive than the plus strain of Line 740 or than the plus
strains of the other S. exspinosus lines.
The curves showing reaction-time means for the plus strain of
Line 757 (figures 18s and 19) are somewhat irregular, but their
1 Possibly the general lowering of the means in both the plus and minus strains of all the
lines of S. exspinosus is to be accounted for through a better handling of the material as the exper-
iment progressed. No intentional changes in the handling of the material were made after about
July 1912, however, and we were not conscious of any better manipulation as the experiments
progressed.
A PHYSIOLOGICAL CHARACTER.
119
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SELECTION IN CLADOCERA ON THE BASIS OF
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A PHYSIOLOGICAL CHARACTER.
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SELECTION IN CLADOCERA ON THE BASIS OF
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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 highly 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.
lrThe 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 18s).
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
1This 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/Ef
Time
period.
No. of
tested
young.
r
W
T/&
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 10D)
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 OP 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.
*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 OF 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 10o) during December
1It 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 later 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 18s) 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 10D) 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 18s. 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 10D 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 lOo) 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
temperature is referred to frequently because the data include temperature records, whib
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 3K 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 S. exsjnnosus
is in line with physiological differences between the two species observed in other connections.
A PHYSIOLOGICAL CHARACTER. 137
Lines 740 and 757 had been in the laboratory for more than 2 years
before these fluctuations occurred in the S. exspinosus lines; (3)
that no such improvement is shown in the reproductive indices.1
While with the older lines of D. pulex subjected to selection the
reaction-time means decreased markedly almost from the beginning
of selection, and the means for these lines reached a low point in
the curves (see figures 10D,2c, 3c,4, 5, 6, 7c, and 8c) within 8 months,
they afterward fluctuated upward and downward through the same
general range, so that there is no ground for assuming that this early
fluctuation was anything other than one of the wide general fluctua-
tions of the curves. This decrease in mean reaction-time was due
to environmental conditions, i. e., factors in the environment which
made for changed conditions under which the animals were tested
in the experimental tank.
Further, comparison of the curves for mean reaction-times for
Line 751 (figure 9) with the curves for the other D. pulex lines shows
that the reduced reaction-time means, which occurred with all the
older D. pulex lines simultaneously soon after selection was begun,
were not attributable to any improvement in the general vigor or
quality of the daphnid stock due to its possibly being subjected to
more advantageous conditions in the laboratory than in the ponds
from which the stock was obtained. This is clear because the curves
for the two strains of Line 751 not only do not show a drop soon after
laboratory culture was begun, but start at very nearly the same
level as was attained during the same months (December 1912-
January 1913) by the other D. pulex lines (figures 2 to 8) which had
previously been subjected to laboratory culture for about 11 months
and selection for approximately 7 months; and these curves for Line
751 follow the same general course as those for the older lines (during
the same two-month periods) regardless of the fact that Line 751
had been under laboratory culture for a period 11 months shorter
than that for the older lines.
GENERAL INCREASE IN REACTIVENESS OP SIMOCEPHALUS EXSPINOSTJS.
With the S. exspinosus lines, Lines 740 and 757,2 there is a
slight simultaneous increase in reactiveness of both the plus and the
minus strains beginning after the June- July 1913 period (figure 15)
and continuing, with fluctuations, throughout the rest of the exper-
iment. This increase is more marked after the August-September
1915 period (figures 18s, lie, 12c, 13c, and 15), where it appears
in all the 10 S. exspinosus strains, though to a less degree in the
minus strain of Line 757. The curves for S. exspinosus strains
1 Not only is there no improvement reflected in the reproductive indices, but those indices
for the D. pulex lines are abnormally low (indicating reduced vigor) for the very months (in the
autumn of 1912) during which this increase in reactiveness is most marked.
2 No other Simocephalus lines were then in the laboratory. Lines 794, 795, and 796 were
begun in December 1914.
138 SELECTION IN CLADOCERA ON THE BASIS OF
previous to August-September 1913 period show fairly consistent
rises in reaction-time, but henceforth, in spite of fluctuations, some-
what lower levels are usually maintained.
It is extremely improbable that these general increases in re-
activeness in the S. exspinosus strains are due to improvement in
the stock due to better conditions in the laboratory than in the out-
door ponds from which the material was obtained. The increased
reactiveness does not appear soon enough after selection was begun
with Lines 740 and 757 to render this interpretation probable, and
there was an actual decrease in reactiveness for approximately 12
months (Line 740) and 9 months (Line 757) after laboratory culture
was begun. Then, too, the increases in reactiveness for Lines 740
and 757 are simultaneous, in spite of the fact that Line 740 had been
in the laboratory for about 12 months and Line 757 for only 9 months.
Further, Lines 794, 795, and 796 were in the laboratory for 10
months before a pronounced general tendency to increased reactive-
ness is evident (figure 18s).
Again, with the 3 lines of Simocephalus (Lines 794, 795, and
796), with which selection was begun more than 2 years later than
with the older S. exspinosus lines (Lines 740 and 757), the reaction-
time curves (figures lie, 12c, and 13c) start at near the same levels
as were attained by the two older lines (figures 15 and 18s) for the
same months, regardless of the fact that the older lines had already
lived under laboratory conditions and undergone selection for ap-
proximately 28 and 25 months.1 The striking decrease in reaction-
time means which occurred during the last few months of selection
(except to a lesser degree in the minus strain of Line 757) is likewise
as pronounced with the newer as with the older S. exspinosus lines.
A further and (to the writer's mind) a fatal criticism to the
assumption (to explain decreased reaction-time) of a progressive
improvement in the stock due to favorable laboratory conditions
lies in the fact that there are not changes in reproductive indices
coincident with changes in reaction-time and that there is no pro-
gressive increase in reproductive index.
To explain the result within Line 757 as due to a differential
physiological improvement requires several assumptions: (1) that
such improvement is not shown in the reproductive index; (2) that
Line 757 was more susceptible to such improvement than the other
S. exspinosus lines; (3) that not only was the plus strain of Line
757 subjected to greater improvement than the minus strain, but to
greater improvement than the other S. exspinosus plus strains; (4)
'Omitting the minus strain of Line 757 — which because of the effect of selection had a
considerably higher mean for this period, 829 seconds (47 individual records) — the mean re-
action-time for the period December 11, 1914, to January 15, 1915, for the older S. exspinosua
strains was 645 seconds (124 individual reaction-time records). For the same period, the period
during which the selection was begun with the newer lines of S. exspinoaua, the mean for the
newer lines was 658 seconds (319 individual records).
A PHYSIOLOGICAL CHARACTER. 139
that the minus strain of Line 757 through the selection was very
little, though somewhat improved; (5) that this improvement was
permanent, or at least persisted for 32 months after the close of
selection.
The writer challenges the first of these assumptions as contrary
to fact.1 Further, at no time was there any observable difference
in the general activity of the two strains of Line 757, except when
subjected to directive light stimulation in the experimental tank.
To the second, third, and fourth assumptions the writer makes no
objection, except the general objection that there is no obvious ad-
vantage derived through them and they seem uncalled for; further,
there is no obvious reason for assuming an obscure physiological
effect rather than genetic changes. The last of these assumptions
appears tantamount to the assumption of a permanent extra-chro-
mosomal inheritance or at least an extra-genetic transmission — an
extremely precarious assumption.
It is possible to assume that the increase in reactiveness as the
experiment progressed was due to a better handling of the material,
such as to affect the reactiveness (but not the reproductive index), but
the author was not conscious of any better manipulation.
The logical explanation would seem to be that the pond-water
which was used in testing these animals was undergoing a progressive
change. This pond is fed by a spring and has considerable overflow.
It is a dying pond, rapidly filling with silt. At the close of the experi-
ment the spring-water was flowing through a pond which then had
probably less than one-third the water-content which it had at the
beginning of the experiment. The water was always taken at the
outlet. Consequently, as the experiment progressed, the water used
in the experimental tank had remained for shorter and shorter periods
of time in the pond from which it was obtained. It is reasonable to
suppose that this water was progressively changed in some of its
constituents (carbon dioxide, oxygen, substances obtained from the
silt, or in other constituents). This explanation also seems plausible
in view of the fact that reaction-time is clearly closely related to the
condition of the water in which the young daphnids are tested.2
Whatever may have been the cause for the generally reduced
reaction-time for S. exspinosus strains during the latter part of the
experiment, it should be borne in mind that except for the practice
of selection all strains were treated as nearly alike as possible and
that there is apparently no ground for the assumption of a possible
continued differential effect due to selection other than such as came
about through genetic changes.
1 The validity of the reproductive index as a measure of vigor (including general muscular
activity) is discussed on page 143.
2 Local influences, presumably due to changes in the water, have been frequently referred to
(see table 2, pages 17-18, and elsewhere).
140 SELECTION IN CLADOCERA ON THE BASIS OF
INDEPENDENT SHIFTS IN REACTION-TIME MEANS.
Throughout this experiment every effort was made to treat alike
all the lines, and particularly the two strains of a line, as regards food,
temperature, light, method of handling stock, making of transfers,
etc. However, the two strains of the same line did not usually
reproduce on the same day, and consequently transfers were generally
made into culture-water of different collections from the outdoor
pond. Such variations as occurred in food and chemical content of
the culture-water, and temperature, light, etc., in the laboratory,
impinged upon different periods of the life of the new generations.
Such unavoidable differential treatment can scarcely be conceived to
be without influence upon the physiology of these organisms.1
In spite of the somewhat differential treatment of the two strains
of a line, there has been seen in general a fairly close correlation
between the reaction-time means for the two strains of most of the
lines throughout their entire history. In so far as there are differ-
ences in reactiveness between the two strains of the same line, where
they are temporary and hence are presumably non-genetic, they are
believed to be due to this unavoidable differential environmental
treatment.
Pronounced independent shifts in the reaction-time curves cover-
ing considerable periods for the two strains of the same line are seen in
several cases. Three such shifts relative to each other are seen in the
reaction-time curves for the two strains of Line 695: (1) September
1912 to January 1913, in which the minus strain is consistently higher
than the plus strain and higher than the composite curve (figure 10o)
for the minus D. pulex strains for this period; (2) September 1914 to
May 1915, in which the minus strain is consistently, though only
slightly, lower than the plus strain and lower than the composite
curve for the period; (3) February-September 1916, during which the
two strains differed consistently by a widening margin due to the
plus curve running sharply upward. None of these three periods
of consistent differences in reactiveness for periods of 6 months or
longer are considered to have genetic significance (see pages 44-45).
Two shifts are seen in the curves for Line 691, September 1912-
January 1913, during which the plus curve runs lower than the minus
curve and lower than the composite curve (figure lOo), and June-
September 1913, during which the minus curve runs abnormally high.
This last shift may have genetic significance, since the minus curve
tends to run slightly higher than the Line 691 plus and the composite
curves during the remainder of the experiment, but its genetic sig-
nificance is extremely doubtful.
In Line 713 the plus-strain curve is particularly fluctuating,
there being 3 main periods of differences in reaction-time : December
1 It is obvious that temperature and character of the food affect the reproductive index.
A PHYSIOLOGICAL CHARACTER. 141
1912-October 1913, when in general the plus curve was low (see
figures 6 and 10D); December 1913- July 1914, when the plus curve
was relatively high; and October 1914- July 1915, when the plus
curve again was relatively low, compared in each case with the com-
posite curve (figure lOo). The minus curve fluctuated similarly, but
to a less degree..
In Line 714 the minus curve shows 3 parts where it is quite
separated from the plus curve (figure 7c) : (1) April-September 1912,
when it is high; (2) June-September 1913, when it was low; April-
July 1914, when it was high. None of these cases has apparent
genetic significance.
Line 794 shows (figure lie) 2 fluctuations, each consisting of 3
two-month periods. During the first (December 1914-May 1915)
the minus strain was low, and during the second (June-November
1915) it was high, both relative to the plus strain of Line 794 and the
composite curves for the other S. exspinosus strains (figure 18s).
Line 795 shows two similar periods for the minus strain (figure
12c): February-May 1915 and December 1915- July 1916, during
which this strain was lower relative to the plus strain of this line and
the composite curves for the other S. exspinosus strains (figure 18s).
Line 740 shows 3 periods of 6, 10, and 10 months, during which
the two strains differed consistently in reaction-time. The first 2
periods, February 1914-July 1914 and September 1914-May 1915,
are apparently due principally to fluctuations in the plus strain.
There were no sufficient other strains of this species in the laboratory
during this time to make adequate comparisons. During a third
period, April 191 6- January 1917, the minus strain ran higher than
the composite curve for the other minus strains (except Line 757, see
figure 18B). For the latter part of this period the plus strain of Line
740 ran abnormally low. The two fluctuations produced a relatively
large difference between the reaction-time curves for the two strains
of this line. The later history of the two strains, especially the
test-series data obtained at the close of selection with this line, indi-
cated that this divergence had no genetic basis.
Line 757 shows a period, June 1914-July 1915, in which 4 of the
7 two-month periods show relatively low points in the curve for Line
757 minus (figure 18s). While the effect of selection within this line
is such that the minus curve drops below the plus curve at only one
point, it is believed this clearly represents a period, or two periods
(the October 1914- January 1915 period shows the minus curve rela-
tively high), which are in every way similar to these just referred to
(for the other lines) and that non-genetic influences served to mask
the effects of selection during this portion of the experiment.
The outstanding fact regarding local fluctuations in reaction-
time means of the two strains in Line 757 is that they produce
relatively so little reduction or interruption of the difference between
142 SELECTION IN CLADOCERA ON THE BASIS OF
those means. This fact would seem a further confirmation of the
effect of selection within Line 757.
Fluctuations for single two-month periods are common in the
data. They are presumably due to the same factors which produce
the fluctuations in reaction-time for longer periods. It is also true
that the number of reaction-time records for single two-month
periods is not sufficient to obtain highly reliable averages, and hence
fluctuations for single two-month periods are less significant than
those covering 3 or more such periods.
Purely local differences in reactiveness might be expected to be
seen occasionally in results obtained from the different test series,
and such proves to be the case. The first test series for Line 695
gave a mean reaction-time for the plus strain 32 seconds greater than
that for the minus strain. The second test series (9 months later)
gave a mean for the minus strain 62 seconds the greater. The other
2 test series for Line 695 were in consecutive months, yet the differ-
ence in one was —1 second and the other +51 seconds. Such local
differences lend caution to one's interpretation of the result of test
series, and the results of test series are given credit only when they
show pronounced agreement and there is other confirmatory evidence.
In the first test series for Line 757 the mean for the plus strain is
282 seconds lower than the mean for the selection data for the three-
month period during which this test series was conducted (table 45) ;
while during the second test series the mean for the plus strain was
1 33 seconds higher than that for the selection data for the contingent
three-month period. It is extremely probable that if another test
series had been conducted within a few weeks after the first one, the
mean for the plus strain would have been more nearly within the
range (5f the selection data for the plus strain for that period, and that
another test series conducted a few weeks after the second test series
would have given a much lower mean for the Line 757 plus strain.
The reaction-time differences between the two strains of Line 757 are
so large, however, that a wide departure from the usual reactiveness
of a strain still leaves a large difference in mean reactiveness between
the two strains.1
WHETHER "DEPRESSION PERIODS" OCCUR.
These periods of relatively temporary drops or rises in the
reaction-time curves are probably in no case due to genetic influences.
In cases in which the reaction-time curves rise, the large and some-
what continuous lifts in the level of the curves may be ascribed by
1 For the benefit of any who (in view of the fact that local environmental effects are seen
even in test series) might be inclined to wonder if additional test series would have shown such
large divergences between the two strains of Line 757, it may be added that in July 1919 and
again in December 1919 (27 and 32 months after selection was discontinued) extensive test series
were conducted in Line 757, each covering a period of and consisting of material from three
generations. In extremely few cases in these large series did a brood of the plus strain fail to
show markedly greater reactiveness than the corresponding brood of the minus strain.
A PHYSIOLOGICAL CHARACTER. 143
some workers to "depression periods" in the Cladocera stock. This
explanation is rejected for the following reasons:
1. These periods of local drops or rises in individual reaction-
time curves are extremely irregular in their occurrence and do not
affect all the strains or the two strains of the same line equally.
2. There is no coincidence between changes in reaction-time and
reproductive index and no demonstrable relation between reaction-
time and the vigor of the stock, as measured by the reproductive
index.
3. Cases such as Lines 714, 794, and 795, in which these non-
genetic differences appeared immediately upon the beginning of
selection (figures 7c, lie, and 12c), can scarcely be attributed to
depression periods, inasmuch as the two strains of each of these
lines had just been derived from the same progenitor.
4. Males or ephippial females were not found in this stock at
any time during the selection experiments, either in periods of
unusually low or high reproductive indices or unusually slight or
great reactiveness ; yet if males or ephippial females occurred in any
appreciable numbers (particularly the latter) they would have been
detected.
5. Depression periods as such are not seen in the stock bred in
the laboratory.
The writer believes that these fluctuations are merely reflections
of relatively local environmental influences upon the individual
strains concerned.
6. ENVIRONMENTAL CONDITIONS AS AFFECTING VIGOR OF STOCK.
Obviously vigor is closely related to environmental factors. As
to the Cladocera reared in this laboratory, there is reason to -think
that their vigor, with most of the species at any rate, is completely
controlled by environmental influences and is not affected by internal
factors, such as the effect of long-continued parthenogenetic repro-
duction.
RELIABILITY OF THE REPRODUCTIVE INDEX.
The reproductive index is a safe criterion to use as a measure of
general vigor and, it is believed, of general muscular activity as well.
During times of lowered reproductive activity (appearing in decreased
numbers in first broods and in increased age of mothers when first
broods are produced), such as frequently affected several or all of our
strains of the same species at the same time, there was high mortality,
more sterility, more cases of failure to produce a second brood, and
frequently, with the Daphnia species, a noticeable decrease in general
muscular activity so far as swimming movements were concerned.1
1 It seems surprising that this is not reflected in increased reaction-time means, but (as is
shown elsewhere) there is extremely little evidence of a relation between vigor and reaction-time.
During part of the time, when the stock was low in reproductive vigor, selections were omitted.
144 SELECTION IN CLADOCERA ON THE BASIS OF
Conversely, when the reproductive index is high, mortality is low,
sterility is extremely rare, there are few failures to produce a second
brood, and swimming activity is at its maximum.
COINCIDENT FLUCTUATIONS IN VIGOR.
If vigor is related to environmental conditions, coincident fluc-
tuations in reproductive indices for the two strains of the same line
and for the strains of different lines should be in evidence. They are
present and most conspicuous when the two strains of the same line
are compared.
In the data for Line 757 (figure 17c) for example, the reproduc-
tive indices for the two strains, compared in each case with the repro-
ductive index for the preceding period, rise or decline together 16
times and move in reverse directions only 10 times. Of these 16
coincident fluctuations, 13 are large enough to have a probable real
meaning, while only 4 of the 10 independent fluctuations are large
enough to be considered significant.
There is not more coincident fluctuation in reproductive index
in Line 757 than with most of the other lines (see figures Ic, 3 A, ?A,
SA, HA, 12A, 13A, and 17c), so that in general there is a rather close
correlation between the reproductive indices of the two strains of a
line. This was clearly seen to be a reflection of environmental in-
fluences in some cases, e. g., the August-November 1912 period
(see page 135), and it is believed to have been equally due to environ-
mental influences where the precise character of the environmental
influences was not recognized.
Coincident fluctuations in reproductive indices are likewise seen
between the different lines of the same species and also between lines
of D. pulex and S. exspinosus.
With the D. pulex lines there is a remarkable agreement in the
occurrence of low reproductive indices for the August-November
1912 period (figures Ic, SA, TA, and SA). Line 740 (S. exspinosus)
likewise shows extremely low points for this period (figure 14A).
There are some early data for Line 757 (figure 17c), another line of S.
exspinosus, culture of which was begun during the month of Novem-
ber 1912, which show very low reproductive indices. This is a striking
fact, as showing that whether the lines had been in the laboratory for
nearly a year (as in the case of the Daphnia pulex lines), for 4 months
(Line 740), or had just been introduced into the laboratory (Line
757), low reproductive indices occurred. As has been stated before
(page 136), this was a period of particularly poor food conditions and
it involved the loss of much laboratory stock.
During June-July 1913, 7 of 8 strains of D. pulex (figures Ic, SA,
7A, and SA) show lowered reproductive indices. This was again a
period of not very successful feeding and of considerable loss of
A PHYSIOLOGICAL CHARACTER. 145
strains of the D. pulex stock, but it scarcely affected the two S.
exspinosus strains (figures 14A and 17c).x
During August-September 1913 all the D. pulex strains and all
the S. exspinosus strains show relatively high reproductive indices.
Again, for October-November 1914 all the strains of D. pulex and
2 (of 4) strains of S. exspinosus have high reproductive indices.
These are particularly marked for the D. pulex strains. For the
period immediately following, December- January 1915, most of the
strains of both species show considerably lowered though not particu-
larly small reproductive indices (figures Ic, 7 A, SA, HA, 12A, 13A, 14A,
and 17c). In October-November 1915 all the 4 strains of S. exspino-
sus and 3 of the 4 strains of D. pulex have high reproductive indices.
Still other periods of more or less coincident fluctuations in repro-
ductive indices may be seen in the curves.
Mean reproductive indices for all the D. pulex strains and for all
the S. exspinosus strains were determined, and it was found that the
coincident high and low points in reproductive index as given above
were clearly reflected in these means.
These fluctuations in reproductive indices are in two cases
(August-November 1912 and June— July 1913) clearly related to
poor food, and perhaps to other unfavorable conditions, and it is
believed the other coincident fluctuations are likewise associated with
environmental factors.
INDEPENDENT FLUCTUATIONS IN VIGOR.
Vigor is closely related to environmental conditions and many
coincident fluctuations in vigor have just been referred to; but be-
cause of the necessarily somewhat differential treatment (see page 140)
of the two strains of the same line (and of the different lines), inde-
pendent fluctuations are much in evidence in the figures showing
reproductive index (figures Ic, 3A, ?A, SA, HA, 12A, 13A, 14A, and
17c).
For example, in figure 17c, showing reproductive indices for the
two strains of Line 757, conspicuous instances in which the changes
in reproductive index for the two strains of this line are in reverse
directions (as compared with the preceding period) are seen in the
February-March 1914, August-September 1916, October-Novem-
ber 1916, and February-March 1917 periods. Six other less con-
spicuous fluctuations (in reverse directions) are seen in this same
figure. Some of these small fluctuations are probably due to the
reproductive indices for the two-month period, having been derived
from a relatively small amount of data; but this is not believed to
be the case in general and can scarcely be assumed to account for
1 It has been found in other observations that while the physiology of these two species of
Cladocera is fundamentally very similar, yet physiological differences not infrequently appear.
The lack of a pronounced lowering of the reproductive indices for the two S. exspinosus lines for
this period is an illustration of these physiological differences between the two species.
146 SELECTION IN CLADOCERA ON THE BASIS OF
the wide fluctuations in the four periods enumerated above; cer-
tainly it is not true for consistent differences in reproductive index
between the two strains of a species for several months in succession,
such as are seen in four different periods for Line 757 (figures 17c
and ISA) and in parts of the data for nearly every line for which
reproductive indices are given.
Such independent fluctuations are believed to be due to the
somewhat differential treatment resulting from the transfer of young
of the two strains of a line on different days and into culture-water
of different collections. The cumulative effect of continued trans-
fers into somewhat different culture media seems very readily to
account for local differences in reproductive vigor between the two
strains of a line.
Some of these independent fluctuations are capable of explana-
tion as due to variations in an internal cycle which most workers
believe exists in Cladocera. But differences in environmental con-
ditions explain these cases so readily, some of the contemporaneous
changes are associated with recognized changes in environmental
conditions, and it is well known that such culture media do have
great variation, so that any assumption of an internal cycle seems
quite unnecessary. Further, in Line 689 (figure SA and B) the plus
strain, which had much the higher reproductive index for the
first two months of selection, had a considerably lower reproductive
index for the following 2 months. This would seem a short time for
the internal cycles of these two sister strains to have diverged. The
minus strain of Line 714 (figure 7, A and B) had a reproductive index
nearly three times as large as that for the plus strain for the first
two-month period. Can these strains just derived from the same
individual have immediately passed into different periods of an
internal cycle? There are other cases (less striking, however) of
significantly different reproductive indices immediately or very soon
after the origin of related strains.
However, in spite of the unavoidably somewhat dissimilar treat-
ment of the two strains of a line and the consequent local differential
changes in reproductive indices, there is in general a very close
correlation between the reproductive indices of kindred strains, a
result to be ascribed, in large part at least, to the general influence of
similar environmental conditions.
DIFFERENCES BETWEEN REPRODUCTIVE INDICES DURING DIFFERENT PARTS OF EXPERIMENT.
The reproductive indices for the two strains of the different
lines in general differed somewhat less at the beginning of selection
(for the first 4 months), when the two strains had just been separated,
than during the later parts of the experiment, after the two strains
had been separate for considerable periods of time (figures Ic, SA,
7A, SA, HA, 12A, 13A, 14A, and 17c). This is explainable as due to
A PHYSIOLOGICAL CHARACTER. 147
the fact of the identity of environmental conditions before selection
was begun and the diversity of environmental conditions as affecting
the two strains later (or as due to their being in different portions of
their cycles at the same time later). However, some of the strains,
e. g., Lines 695 and 689, on the whole show greater differences in
reproductive indices during the first 4 months than later (see figures
Ic and 3 A).
Is THE INCREASED REACTIVENESS FOR LINES OF S. EXSPINOSUS ASSOCIATED WITH
INCREASED VIGOR?
In view of the increase in reactiveness in the S. exspinosus lines
during the course of the experiments, it is of interest to examine the
reproductive indices to see if similar or other changes are seen in
them.
With the 4 lines of Daphnia pulex (Lines 689, 695, 714, and 719)
for which reproductive indices are worked out, there are in general
reproductive indices somewhat below the average for the first 2
two-month periods, very low reproductive indices for the following
2 periods (during the obviously poor food conditions), and there-
after the reproductive indices are somewhat fluctuating, but in
general fall about slightly higher means throughout the remainder
of the experiment (figures Ic, 3 A, 7 A, and 8 A). Hence with the lines
of D. pulex the reproductive index increased somewhat during the
course of the experiment.
The reproductive indices for Line 740 show marked increases
for the two-month periods just following the first 4 months of selec-
tion. These early low reproductive indices are clearly related to
the poor food conditions prevalent during the early months of
selection with this line and reflected even more strongly in the D.
pulex lines which had been in the laboratory for several months
previously. There is also a rise in reproductive index for Line 757
after the first 2 months, and this rise is only partly coincident with
recognized differences in food conditions in the laboratory. This
rise is capable of explanation as due to the cumulative effect of
better food conditions in the laboratory than in the outdoor ponds,
but practically the maximum rise in reproductive index for this line
was realized at once (figure 17c)x and there is no evidence for a
cumulative effect of better food conditions. The reproductive
indices for Line 795 are much higher during the latter part of the
experiment than earlier (figure 12A), but the reverse is true for Line
796 (figure 13 A). On the whole, the reproductive indices for S.
exspinosus do not show any general increase or decrease during the
course of the experiment.
Thus it is seen that reproductive indices for the D. pulex strains
increase somewhat during the course of the experiment, while re-
1The general level for the plus strain was not higher for the remainder of the experiment,
though the general level for the minus strain increased somewhat further.
148 SELECTION IN CLADOCERA ON THE BASIS OF
action-time means are neither increased nor decreased. On the other
hand, reaction-time means are decreased for S. exspinosus lines,
while there are no consistent general changes in reproductive indices.
Perhaps no better evidence bearing on the relation between
reproductive vigor and reaction-time can be had (or desired) than
that obtained from the following data: The total numbers of indi-
viduals tested in making the selections in all the plus strains of D.
pulex for the entire series of the experiments was 6,991; the mean
reaction-time of these 6,991 individuals was 343.76 seconds. The
corresponding figures for all the minus strains of D. pulex were 6,973
individuals, with a mean reaction-time of 345.92 seconds. The
difference was only 2.16 seconds. For the S. exspinosus plus strains
(except Line 757 plus), the total number of individuals was 4,722
and the mean reaction-time 531.56 seconds. For the S. exspinosus
minus strains (except 757 minus) the figures were 3,943 individuals
and 532.55 seconds. The difference is 0.99 second.
Since in both species the plus strains in general were somewhat
more vigorous than the corresponding minus strains, these negligible
differences in reaction-time means for the entire series of experiments
indicate a lack of relation between reproductive index and reaction-
time.
POSSIBLE DIVERGENCE IN VIGOR BETWEEN THE Two" STRAINS OF A SELECTED LINE.
The effect of selection on the basis of reaction to light might
conceivably result in plus strains of greater vigor and minus strains
of reduced vigor. It would indeed seem possible that the minus
strains might become very much reduced in vitality and perhaps
eventually die out if selection were continued long enough. In view
of this theoretical possibility it is interesting to note that of the
selection experiments discontinued because of loss of one or both of
the strains (see table 1, column 8), 2 lines were discontinued because
of the loss of both strains within a few days of each other, 1 line
suffered the loss of the minus strain only, and 5 lines suffered the
loss of the plus strain only.1 Hence this significant bit of data does
not indicate reduced vitality in the minus strains as compared with
the corresponding plus strains.
Column 9 of table 1 likewise throws light upon a point inti-
mately associated with the general vitality of corresponding plus and
minus strains, the relative numbers of generations of descent of the
plus and corresponding minus strains. This column shows the total
number of generations of descent of the different strains at the close
of selection within the different lines. In 8 lines the plus strain had
descended for a greater number of generations than the corresponding
minus strains; in 7 the minus had descended farther; and in 1 line
1 Two of the last were lost through accident, however, rather than because of general weak-
ness or poor food conditions, and one of the minus strains suffered the accidental loss of part
of its stock, except for which it might have survived.
A PHYSIOLOGICAL CHARACTER. 149
the 2 strains had descended an equal number of generations. The
plus strains which had descended a greater number of generations
averaged an advance of 2.9 generations ahead of their respective
minus strains; the minus strains which had descended farther than
their corresponding plus strains averaged an advance of 4.3 genera-
tions beyond the plus strains. The D. pulex and S. exspinosus lines
were pretty well divided as to whether the plus or minus strains had
descended the greater number of generations. In all the D. longispina
lines, however, the plus strain had descended farther — by a single
generation in two of the lines, but by 5 generations (almost 11 per
cent) in one. Hence, there was possibly some real loss of vigor in
the minus strains of D. longispina as compared with their respective
plus strains (though three lines are too small a number from which to
reach a safe conclusion). But with S. exspinosus and D. pulex there
was no loss in vigor of the minus as compared with their respective
plus strains, so far as was revealed by their total numbers of general
tions of descent. In fact (so far as this point is concerned), the minus
strains in 5 of the 8 D. pulex lines descended farther and by larger
margins (an average of 5.2 generations as compared with 3.7 genera-
tions for the plus strains), which affords almost as much ground for
assuming that the plus strains of the D. pulex lines became of inferior
vigor compared with the corresponding minus strains as is afforded for
the opposite conclusion with reference to the D. longispina lines.
Some effect of the selections upon the general vigor of the
corresponding plus and minus strains is evident, however, in some
cases in the generally lower reproductive indices for some of the
minus strains as compared with their corresponding plus strains. In
only 1 (Line 740) of the 8 lines for which the reproductive indices
have been worked out was the minus strain in general conspicuously
the more vigorous of the 2 strains of the same line (figure 14s). It
is significant, however, that (except possibly for parts of the data
for Lines 795 and 796, and these differences were not maintained)
the differences in vigor of the two strains of the same line were not
cumulative; and in no case were they greater during the later than
during the early portions of the experiments, except for the very
first months of selection. It would seem, then, that in some cases
there was a tendency for the selected individual (the most reactive
individual) in the plus strain to be more vigorous than the selected
individual (the least reactive individual) in the corresponding minus
strains, but that this difference is neither universal, uniform, nor
cumulative in its effect. As is shown elsewhere, differences in re-
productive indices, even when relatively large, are not associated with
corresponding differences in reaction-time.1
xThe lack of relation between reproductive index and reaction-time has been referred to
in several places (see pages 40-45, 49, 60, 65, 72, 76, 80, 83, 88-89, and 108-115) and need
not be further discussed here.
150 SELECTION IN CLADOCERA ON THE BASIS OF
STATEMENT OF RESULTS OF SELECTION.
. The results of this series of experiments may be stated briefly:
With two lines, Lines 689 and 719, there were found reaction-
time differences in the reverse of selection. These differences were
so consistent as to suggest real genetic differences affecting reactive-
ness in the two strains of these two lines, though Line 689 was carried
for a relatively short period of time. In the case of Line 719 the
reaction-times differed less toward the end of the experiment, and it
is suggested that selection may have operated to produce this effect.
The results with Line 689 and Line 719 suggest that in addition .
to genetic differences in the direction sought in selection genetic
changes may occur in the opposite direction. This is strongly sug-
gestive of mutation. It does not, however, obviate the theoretical
difficulty of explaining by mutation the plural genetic changes in
Line 757.
Lines 695, 713, 714, 751, 766, 768, and 796 show no evidence
(or slight evidence as in Line 713) of significant differences in reactive-
ness between their plus and minus strains. Some of these lines were
carried for relatively short periods of time, but Lines 695, 713, 714,
and perhaps 768 and 796 were subjected to selection for what would
seem adequate periods of time.
Line 691 shows a possible effect of selection, though such an
effect is not fully supported by the last year's data and may therefore
be disregarded. Selection was conducted for 27 months, 84 and 85
generations in the two strains.
Line 711 apparently shows an effect of selection, but the differ-
ences are small and the evidence is not entirely conclusive, though
the case is as convincing as the result in the opposite direction for
Lines 689 and 719. Selection was continued for 45 and 46 genera-
tions, covering a period of about 16 months.
Line 762 gave a possible effect of selection during the last two
months of the experiment. In view of the wide fluctuations in the
reaction-time curves of many of the lines, this is not sufficient data
upon which to base a conclusion, but the entire consistency in the
differences between broods of the plus and minus strains during this
late portion of the experiment is unusual enough to be highly sug-
gestive. However, in view of the local differences in reactiveness
seen in other lines (see pages 140-142), an effect of selection in Line
762 must be considered problematical. The plus strain was lost,
thereby terminating the experiment after 36 and 35 generations of
selection.
Line 794 may have undergone a divergence in reactiveness during
selection. For the first 6 months the differences were in the reverse
of selection, but the remaining 12 two-month periods gave differences
in the opposite direction, with 3 exceptions, and 8 of these differences
A PHYSIOLOGICAL CHARACTER. 151
were relatively large. The single test series, however, quite fails to
lend support to a difference in reactiveness between the two strains,
and this case likewise remains doubtful.
Line 795 clearly shows a lack of selective effect during most of
the experiment. The last month's data is very suggestive of signi-
ficant reaction-time differences, but the data are too slight in amount
to receive serious consideration.
A mutation possibly occurred in the minus strain of Line 740,
causing it to be more reactive to light. The case was critically
examined (page 89) and the conclusion was reached that the unusual
reactiveness of a few broods of this strain was probably due to
peculiar local environmental conditions.
None of the above possible cases of differences in reactiveness
between the two strains of a line is well enough supported by the
evidence to be considered proven. The cases best supported by the
evidence are in Lines 689, 719, and 711. In Lines 689 and 719 the
differences are in the reverse of selection. With Line 719 the
difference was slight during the latter portion of the experiment.
Line 689 was lost before enough evidence was obtained to convince
one that the difference was necessarily genetic. Line 711 shows con-
sistent differences in accordance with an effect of selection, but, like
Line 689, was lost before enough evidence was obtained to convince
one that the difference was certainly genetic.
With Line 757, on the other hand, a marked effect of selection is
attested by every significant bit of evidence which the data for this
line provide. The divergence in mean reaction-times began to appear
after the first 3 months at the beginning of the experiment (for which
the two strains had the same reaction- time means). For the second
(two-month) period the means differed by —25 ±22. 82 seconds, and
the means for the following two-month period differed by -72 ±17.78
seconds, a difference of statistical significance (4.05 times the
probable error), in spite of the small numbers and consequently large
probable error. This is followed by 6 two-month periods, during
which the differences are all in the same direction and 3 of which
(even for the shorter periods) are of large statistical significance,
though the differences of —55, —63, and —70 seconds were not of
statistical value because of large probable errors. For the two-month
period June- July 1914 the minus strain was the more reactive and
the difference was 132 ±29. 79 seconds, a difference 4.4 times the
probable error. For the following two-month period the difference
is only —2 seconds. From this period on, all the 16 differences (by
two-month periods) indicate greater reactiveness in the plus strain.
With 3 exceptions the differences were large and of statistical value,
even when considered by two-month periods. One of these smaller
differences, which was not of statistical value, occurred within 3
152 SELECTION IN CLADOCERA ON THE BASIS OF
months of the close of the experiment, when the effect of selection
was at its height.1
Considering the data by longer periods, those from the beginning
of the experiment to July 1914 show significant differences in mean
reaction-time, though the early differences were generally not large.
Following this was a period during which both strains, and particu-
larly the minus strain, fluctuated widely, though the difference for
this longer period is significant of a real difference in reactiveness
between the two strains. During the latter part of 1915 the dis-
turbances due to environmental factors became less marked and
the divergence in mean reaction-time appeared to increase greatly
and, with a single fluctuation, continued very large during the
remainder of the experiment. During the final 9 months of the
experiment the mean for the plus strain was only one-third that for
the minus strain. The form of the curves (figure 18s) suggests that
the divergence in reactiveness was still increasing at the close of the
experiment.
Points of importance in connection with the results with Line
757 are that both the plus and minus strains became modified during
selection and that the divergence was permanent or at least persisted
for 112 generations. It is also important to note that there is nothing
to distinguish the two strains other than the difference in behavior
of the strains in the experimental tank in response to directive
stimulation by light, and a somewhat higher reproductive index for
the plus strain during most but not during the concluding months of
the experiment.
DISCUSSION OF RESULTS.
Consistent differences in reactions to light of animals of the same
or closely related stocks appear not to have been frequently found.
McEwan (1918) presents a conclusive case of differences in
phototropism which are clearly genetic. A certain mutant strain,
"tan," of Drosophila melanogaster, unlike most of this species,
showed very little response to light. Tan is a very variable character
and can not always be distinguished by inspection of the flies. The
differences in phototropism between tan and normal flies were so
striking that it was found (and proven by breeding tests) that the
slight phototropism of tan stock was a more reliable means of diag-
nosing tan than inspection of the flies. The character was shown to
be a recessive sex-linked character. McEwan found differences in
1 Such a marked reduction in the difference between the means, coming at a time when
according to all criteria the two strains were so different in their reactiveness to light, seems
certainly due to unequal local environmental influences, which may also account for the relatively
wide fluctuations in the reaction-time curves for the period from June 1914 to July 1915 (see
pages 141 and 159-160). While during this earlier period the minus strain for 1 two-month
period had the lower reaction-time mean, this fact need perhaps excite no more surprise than
the fact that the mean for the minus strain was so near that for the plus strain at the later period
when the general reactiveness of the two strains was so widely different.
A PHYSIOLOGICAL CHARACTER. 153
phototropism of other types of Drosophila, but these were associated
with differences in the color or other conditions of the eye itself. In
the tan mutant, on the other hand, no peculiarities could be dis-
covered in the eye.
The writer has considerable (unpublished) data tending to show
that the amphipod Eucrangonyx gracilis, living in the surface streams
near Bloomington, Indiana, is less negatively phototropic than the
same species which lives in Mayfield's Cave, near at hand. The
differences in reaction are not large and consistent enough to be
very conclusive. In this species those inhabiting caves and those
inhabiting the open are indistinguishable morphologically, except for
differences in pigmentation, the former having fully pigmented eyes,
but no body pigment.
In asexually reproducing forms1 Jennings (1908, 1909) with
Paramecium, Agar (1913, 1914) with Cladocera, Ewing (1914) with
aphids, Hanel (1908) and Lashley (1915, 1916) with Hydra, and
other workers have failed to obtain hereditarily diverse strains by
selection within the uniparental progeny of single individuals.
On the other hand, Middleton (1915) in Stylonychia, Stout (1915)
with Coleus, Jennings (1916) with Difflugia, Root (1918) in Centro-
pyxis, and Hegner (1918, 1919) with Arcella seem to have found
genetic variation wherever they sought it.
Stocking (1915), studying abnormalities in Paramecium, found
that of 122 strains containing abnormal individuals, in which the
character was clearly hereditary, 97 were persistently abnormal
races, i. e., they presumably were not subject to genetic variation
in this regard; and 25 strains were subject to genetic variation, i. e.,
selection was effective in 25 strains. That is to say, about 20 per cent
of the strains studied by her, in which the character was inherited,
were subject to genetic variation.
The results of selection with Protozoa are open to certain objec-
tions which Jennings (1916, p. 523) outlines somewhat as follows:
(1) that the results may be due to cytoplasmic influences and hence
may not be of real genetic significance; (2) that the result may be
due to nuclear recombination between the active nucleus and nuclear
material within the cytoplasm (chromidia); and (3) that nuclear
recombination may occur between different nuclear masses in the
same individual. Professor Jennings (1916, p. 525) believes that
these possibilities have "no claim to greater probability than their
negatives, so far as our knowledge of the facts is concerned." The
opinion of different workers as to the validity of these objections
differs greatly.
1 In view of the many excellent historical accounts which have appeared it does not seem
desirable to review any considerable portion of the vast amount of work on selection within the
pure line. Excellent discussions of this literature are given by Jennings (1910, 1917), Pearl (1917),
and Sturtevant (1918).
154 SELECTION IN CLADOCERA ON THE BASIS OF
Cladocera material has the advantages for a selection experiment
possessed by protozoa in that it is (under controlled conditions)
exclusively uniparental in inheritance. It has additional advantages
in that the germ-plasm is less bound up with the soma than in
protozoa and the very great advantage that (so far as our knowledge
goes, at any rate) there are no complications of chromidia or separate
nuclear masses within the germ-cell, recombination of which might
conceivably produce the results attained by selection in protozoa.
So far as known, there is no mechanism within the germ-cells
of Cladocera through which a segregation or recombination of nuclear
material is to be expected in parthenogenesis. In the production
of the parthenogenetic egg there is a single maturation division
(Weismann, 1886; Kiihn, 1908), in which the chromosomes behave as
in any ordinary cell-division, i. e., each chromosome is merely divided
into two theoretically equal parts, equal quantitatively and quali-
tatively. Hence the objections offered to results of selection with
protozoa do not apply to the present material. Any results obtained
from selection in such material (parthenogenetic and without reduc-
tion) should have a crucial bearing on the problem of selection.
In experiments having to do with the variability of organisms
it does not seem necessary to suppose that all apparently similar
material should be found similarly subject to genetic variation.
In Stocking's (1915) Paramecium, about 20 per cent of the strains
in which an abnormality was hereditary showed genetic variation.
In the Cladocera material used in selection, Line 757 alone has
certainly shown such genetic variation with regard to the character-
istic studied, though the possible (but doubtful) mutation in the
minus strain of Line 740 and the somewhat consistent differences
in mean reaction- times between the two strains of Lines 689, 691,
711, 719, 762, 794, and 795 are suggestive of differences of genetic
significance. Allowing that the case is proven for Line 757 alone
(though some of the other cases are very suggestive), genetic variation
was found in only one of the 15 distinct lines of Cladocera studied.
This is approximately only 7 per cent.
The effects of selection in Line 757 may be assumed to be
either (1) of a nature of general physiological changes, possibly
changes in metabolism or some kindred effect, or (2) direct genetic
changes.1
If the former interpretation is favored, the question arises as
to what is the basis of such physiological effects. If they are con-
sidered not strictly genetic, the apparently permanent nature of the
change is a matter of especial importance. Cases of transmission
1 In this discussion genetic variations, in metazoa at least, are assumed to concern genetic
factors confined to the chromosomes. The carrying over from one generation to another of
cytoplasmic inclusions and the transmission of variations or modifications which affect only one
or two generations are assumed to concern the cytoplasm alone and are considered not strictly
genetic.
A PHYSIOLOGICAL CHARACTER. 155
not really genetic and some of which are clearly cytoplasmic are well
known. If Sudan III is fed to fowls some of the dye is deposited in
the egg-yolk (Riddle, 1908) and reappears in certain of the tissues
of the chick (Gage and Gage, 1908). This is merely the carrying
over of a cytoplasmic inclusion. Other cytoplasmic inclusions, the
plastids of plants, parasitic or symbiotic bacteria and protozoa, are
carried over in the cytoplasm of the egg; but these are mere inclusions
and this is not true inheritance. Starving occasionally causes the
production of young of small size. The effect would seem probably
due to reduced quantity of food material (or to deleterious sub-
stances) in the egg cytoplasm. Later generations are not affected.
The effects of immunization are transmitted by the mother and not
by the father. The effects are not permanently transmitted and
are presumably cytoplasmic. Agar (1913) was able to produce
young Cladocera with carapace gaping open by keeping the mothers
in a particular food medium. This peculiarity was transmitted for
one generation by affected stock when reared in the usual culture
medium, but the abnormality rapidly decreased and ceased to appear
after one generation. This transmission may readily have been
cytoplasmic, but it is clearly not a case of real inheritance. The
substances carried over in the egg cytoplasm (if there were such)
were rapidly dissipated and lost.
These are typical cases of cytoplasmic transmission, and one
may conclude with Conklin (1920) that "in the present state of
our knowledge there is not sufficient evidence to conclude that
modifications of the cytoplasm of germ-cells are ever really inherited
or that they are ever the initial steps in evolution. "
To assume that the changes in phototropism in the present case
were not really genetic would seem to place them in the class of
cytoplasmic transmissions and to assume a permanent cytoplasmic
or extra-genetic transmission which in our present state of knowledge
would seem unwarranted.
Aside from its theoretical difficulty, there does not seem to be
any evidence favoring the assumption of general and purely physio-
logical changes as accounting for the divergence in reactiveness of
the two strains of Line 757, since these two strains are indistinguish-
able, except in their reactiveness to light.
However, it is of course possible to assume that a non-chromo-
somal inheritance occurs and to apply this interpretation to the
present results, though the writer can see no reason for favoring such
an interpretation. If, however, this should prove to be the correct
interpretation a practically new phase of inheritance is opened up,
and in any case the importance of the present results as bearing
upon selection within the pure line is not lessened.
If, on the other hand, physiological changes having a genetic
basis be assumed to explain the present results of selection, the nature
of the factorial changes involved is still the pertinent question.
156 SELECTION IN CLADOCERA ON THE BASIS OF
If the variations are considered direct genetic changes (or
general physiological changes having a genetic basis), they may be
thought of as having been due to segregation,1 to larger mutations,
or to many small genetic changes.
It would seem that the effect of selection here obtained can not
be explained as due to. recombination or segregation unless segrega-
tion occurred rather generally in Line 757 under conditions in which
it is not known and is theoretically not supposed to occur. It is
conceivable, however, that segregation may occur on rare occasions
in Cladocera in the germ-tract previous to maturation, in the one
maturation division (even though that is a non-reduction division),
or in cleavage or later somatic divisions. But this explanation for
the result with Line 757 encounters certain difficulties: (1) Such
factorial recombination, if large quantitatively, should at once pro-
duce an obvious effect. (2) If, on the other hand, the factorial re-
combination produces only a slight effect, the result obtained with
Simocephalus Line 757 is explicable only as the result of several
of these exceptional factorial changes. (3) The effect is in both high
and low selection strains.
Since with Line 757 the effect is extensive, is in both strains, and
appears gradually and not by a sudden increment (except possibly,
but doubtfully, at one point in the minus strain), this sort of explana-
tion would call for more than a single factorial recombination.
Probably many more genetic changes occurred, but at least three
such genetic recombinations are required to explain the case on this
basis. Such might possibly be located as follows: one in the minus
strain early in its laboratory history, one in the plus strain, possibly
about January 1915, and one in the minus strain about August 1915
(figure 18s). But the points at which these three or more hypothet-
ical recombinations occurred are not indicated in the data by broods
(tables 41 and 42) and can be doubtfully and only approximately
located (assuming that they were masked for a time by environ-
mental conditions) by reference to the averages by two-month periods
(tables 43 and 44 and figure 18s). Yet presumably these, at least
the third one2 (assuming a minimum of three genetic modifications),
must have been genetic changes of large moment, considering the
wide differences in reactiveness between the two strains during the
latter portion of the experiment. Since these factorial changes must
have come about under exceptional conditions, three or more such
occurrences affecting two strains of the same line within a limited
period seem unlikely of attainment.
NOTE. — There is another possible suggestion concerning segregation in this material:
that notwithstanding parthenogenesis and the presumed lack of segregation in maturation
there may be a mechanism in Cladocera by which changes in genetic constitution may,
1 Including the phenomena of crossing-over and non-disjunction.
*As shown on page 159, there is strong probability that no large genetic change occurred
at this point.
A PHYSIOLOGICAL CHARACTER. 157
in certain cases, be readily brought about in parthenogenetic reproduction. This is not
indicated by the cytological evidence, so far as known, and would seem to call for quite
a novel type of cytological behavior for which there is no direct genetic or cytological
evidence in parthenogenetic organisms. But should there prove to be such a segregation
in Simocephalus, then the effects of selection in this experiment with Cladocera readily
yield to explanation in conformity with the chromosome interpretation of the pure-line
hypothesis. Otherwise the writer does not see how the results can be satisfactorily made to
conform with this theory. The Cladocera material deserves a critical cytological analysis.
The writer would gladly cooperate in providing material for such a study by a cytologist
who could give ample time to the problem.
It is just possible, however, that there may be an unusual or at any rate virtually
unknown type of chromosomal behavior in Cladocera, permitting segregation more fre-
quently than would otherwise be expected in material in which segregation seems improb-
able. Wenrich's (1916) observations on Phrynotettix are most suggestive in this connection.
He found reduction occurring in a single chromosome and apparently not affecting the
other chromosomes in that division. Crossing-over and non-disjunction (which Bridges
and others have demonstrated and utilized so successfully in interpreting genetic results
with Drosophila and other forms) may be appealed to as affording possible vehicles for such
genetic changes, assuming that several genetic factors are involved in the reactiveness to
light in Cladocera and that Line 757 was originally heterozygous for certain of these factors.
In view of such known irregularities in chromosome behavior, one is led to consider the
possibility that a partial segregation may occur in the one maturation division in partheno-
genetic reproduction in Cladocera. Nabours (1919) found segregation in parthenogenetic
reproduction in one of the grouse locusts, Apotettix. However, judging from the very
limited cytological evidence presented, it would seem that in this case reduction was fully
accomplished, as in sexual reproduction.
However, there is no cytological evidence that reduction or segregation of any sort
occurs in the one maturation division of parthenogenetic eggs of Cladocera and, even
assuming that such a partial segregation may occur in the single maturation division in
Cladocera, there is still the theoretical difficulty of accounting for such frequent genetic
change as seems to have occurred in Line 757. For it is hard to see how, even with assumed
partial reduction in Cladocera (which lack the complicated features of protozoan nuclei),
the rate of genetic change should so greatly have exceeded that so carefully worked out
for Drosophila. This is not necessarily a fatal objection, however. Even though Cladocera
are probably very little subject to genetic change, once genetic change occurs, for any
thing we know to the contrary, it may proceed at a rapid rate.
This possible explanation has a certain appeal to the writer, not only because it offers
an explanation (in accord with the explanation offered for most results of selection) for
the results attained in the present study of the effects of selection on the basis of reactive-
ness to light, but also because certain other (unpublished) results of study of Cladocera
material may in part be explained on this basis.
Explanation of the result with Line 757 by larger mutations
encounters considerable difficulty: (1) the necessity for assuming
at least three mutations, at least one of which must have been quite
large; (2) the lack of definite points at which mutations may be
presumed to have occurred; and (3) the occurrence of an additional
mutation or mutations effective in the same direction as an earlier
one. Further, the mutations in the two strains must have occurred
in opposite directions, rendering the plus strain more reactive and
the minus strain less reactive.
In general, in uniparental inheritance mutation may perhaps be
expected to be only half as frequent as in biparsntal inheritance,
since in the former a single germ-cell is involved in the production of
each individual offspring. Bridges (1919) states evidence tending to
show that in Drosophila mutations may occur in oogonial (or sperma-
togonial) divisions and in the zygote "immediately after fertiliza-
158 SELECTION IN CLADOCERA ON THE BASIS OF
tion," although he states that in the much-mutating Drosophila the
bulk of the mutations occur "at or very near the maturation stage."
In parthenogenetic reproduction in Cladocera,' with only one matura-
tion division (and that a non-reduction division) the probability of
mutation would seem further greatly reduced. Hence, so far as one
may judge, mutations in Cladocera should be expected to be much
less frequent than in Drosophila. In the latter, Muller and Alten-
burg (1919) state that mutation in a particular factor is infrequent.
From their data,1 mutation in a particular factor occurs about once
in 50,000 generations. Yet this is the much-mutating Drosophila.
Hence, if one may judge from the evidence from Drosophila, mutation
in parthenogenetic reproduction in Cladocera should be very rare.
The selection experiment with Line 757 covered 181 generations.
If larger mutations (affecting reactiveness to light) occurred, there
must have been one (at least) in the plus strain and two (at least)
in 181 generations in the minus strain, a surprisingly high incidence.
It is possible that several genetic factors are concerned in the photo-
tropism of Simocephalus, though McEwan (1918) found that a single
mutant factor in "tan" Drosophila profoundly modified its photo-
tropism. But even if several factors are concerned in the present
case, at the rate of one mutation in a particular factor in 50,000
generations (for which Muller and Altenburg give data for Droso-
phila), the rate of assumed mutation in Line 757 would seem most
excessive. If one disregards the possible, though improbable, muta-
tion in the plus strain of Line 740, and the possible, though unproven,
genetic modifications in Lines 689, 691, 711, 719, 762, 794, and 795,
of the Cladocera lines subjected to selection, Line 757 alone showed
mutation. If larger mutations are to account for the result with
Line 757, mutation must have been extremely frequent in this line
and rare in the other Cladocera lines studied.
It seems, then, that there is difficulty in explaining so many
genetic changes as must have occurred in Line 757 during a relatively
short period as due to larger mutations, such as have been worked out
for Drosophila.
If one seeks in the curves (figure 18s) for possible points of
occurrence of larger mutations, three points at which such seem
more nearly possible than elsewhere are seen in the curves, about
April 1913 for the minus strain, about January 1915 for the plus
strain, and about August 1915 for a second possible mutation in the
minus strain. But the data by broods do not indicate points at
which any mutation seems to have occurred. It is possible, of
course, that the rather widely fluctuating averages by broods might
tend to mask the occurrence of a mutation, but even so one would
1 Conklin criticizes Muller and Altenburg's statement regarding the frequency of mutation,
but even if this statement is unwarranted it will still readily be granted that mutation in a
particular factor is infrequent.
A PHYSIOLOGICAL CHARACTER. 159
expect a mutation of considerable import to appear evident very
quickly in the brood averages and one would seem justified in expect-
ing a mutation of considerable import to produce a marked and
immediate change in the two-month curve for the strain affected.
But such marked and immediate shifts are not found in the curves
(figure 18s), except possibly in the Line 757 minus curve following
August 1915, and even there a mutation seems not to have occurred
(see below).
Since under the influence of the first two larger mutations pro-
visionally assumed to have occurred (but for which no satisfactory
locations can be found) the curves do not show great divergence, in
comparison with their later divergence, the wide divergence between
the curves for the two strains during the latter part of the experiment
seems to call for a mutation of large moment if the result is to be
explained by larger mutations. The divergence in the curves after
the third point (August 1915), at which a larger mutation may
possibly have occurred, seems somewhat pronounced, but does not
appear abrupt enough to have been due to the influence of a single
genetic change, unless the influence of this mutation was masked for
a time by a pronounced influence of environmental factors. It is,
of course, possible that this may have been the case.
If, however, it is necessary to concede a marked influence of
environmental factors, a more logical explanation of the curve for
the minus strain would seem to lie in the interpretation that the irreg-
ularities in the curve for the minus strain during the period June
19 14- August 1915 were due entirely to marked local environmental
influences. There seems no logical objection to this explanation.
Evidence that this is the correct interpretation is seen in the wide
fluctuations, parallel for the most part to those for the minus strain,
in the Line 757 plus strain for the same period.1 Further evidence
that this may be the correct interpretation is seen in the fact that
fluctuations similar to those found in the strains of Line 757 during
the period under discussion are seen during certain periods in other
strains, e. g., in strains of Lines 695 and 740, in which genetic changes
obviously had not occurred and in which only local environmental
influences seem to account for the facts.
During this time of great fluctuation in mean reaction-times
in the two strains of Line 757, there were three relatively very low
points in the Line 757 minus curve. Except for these low points,
the curve for the minus strain is seen to have held to a fairly
consistent course throughout the entire experiment (except for one
two-month period, December 1916- January 1917, in which local
1 Except for the June-July 1914 period the fluctuations in the curves for the two strains of Line
757 are remarkably similar, though it is obvious that the Line 757 minus strain was somewhat
more influenced by environmental factors than the plus strain (figure 18s). It is interesting to
note that this June-July 1914 period is the only period of the entire experiment in which the
minus strain of Line 757 appeared more reactive than the plus strain.
160 SELECTION IN CLADOCERA ON THE BASIS OF
environmental influences obviously were the determining factor). If,
assuming that environmental influences are responsible for the
fluctuations during parts of 1914 and 1915, one momentarily dis-
regards entirely the curves for the Line 757 strains during this period
of irregularities, the curves for the two strains following this period
are seen to be mere resumptions of the courses of the curves previous
to this period.
Hence it is seen that if it is necessary to assume that environ-
mental factors exercised a determining influence, they may account
in full for the deviations in the curves from their general courses
during parts of 1914 and 1915, and there seems no necessity for, nor
ground for, the assumption of a large mutation in the minus strain
of Line 757 about August 1915; and, although this is the only point
in the curves at which a mutation of considerable moment may
seem to have occurred, the writer believes that another interpreta-
tion more logically accounts for the facts of the case.
It therefore appears that the point of occurrence of any really
large mutation can not be satisfactorily located. Since the divergence
in reactiveness between the two strains of Line 757 became very
large and mutations of considerable moment can not be located,
larger mutations do not seem to account for the result in Line 757.
When one considers the further fact that a minimum of three larger
mutations would be required to explain the case on this basis, and
that, by analogy with the Drosophila case, this would call for an
extremely high rate of mutation, the plausibility of this explanation
still further decreases.
The data for Line 757 readily permit of the interpretation of
the result as having been due to imperceptibly1 small changes2
resulting, through selection, in cumulative effects in definite direc-
tions. The result as shown in the curves (figures 18s and 19) readily
lends itself to this explanation, assuming that the writer's inter-
pretation is correct and that ihe irregular portions of the curves
(during parts of 1914 and 1915) are really due to disturbing en-
1 At any rate, their occurrence was unperceived. Reaction-time in this study is read off in
seconds. The character studied, reactiveness to light, would seem ideal for detecting minute
genetic changes in that the smallest changes in reactiveness to light should presumably be de-
tected. But the profound influence of environment upon reactiveness proved extremely dis-
turbing and only by averaging large numbers of individual reaction-times could modifications
in reactiveness be detected. This was a distinct disappointment. In this study, because of
these fluctuations in reaction-time, every case in which there seems any reasonable doubt as to
an effect of selection is refused credit in conclusions favoring an influence of selection, but in
no case are the full experimental results withheld from the reader.
1 These changes, one believes, arose quite independently of selection, but were utilized
through selection in building up the differences in reactiveness between the plus and minus
strains. Selection does not cause genetic change. It merely seizes upon modifications of the
character used in selection as they occur, and in the case of plural genetic changes may build
up differences between selected strains. However, retention (through selection) of an heredi-
tarily changed character, in so far as it preserves the material which may undergo further genetic
variation, makes possible a further change in the same direction in the same character. Selection,
although not a cause of variation, thus enables a deviation to be built up in the character utilized
in selection if genetic changes occur in the direction sought.
A PHYSIOLOGICAL CHARACTER. 161
vironmental influences as the peculiar results for the December 1916-
January 1917 period clearly are.
Except for the fluctuations in the curves for the period men-
tioned, the curves for the two strains of Line 757 diverge rather
gradually and somewhat uniformly. There are considerable irregu-
larities in the curve for the plus strain, both during the period when
irregularities occurred in the minus strain and elsewhere, but despite
irregularities the general appearance of this curve strongly suggests
that its true course is approximately a straight line. The general
course of the curve for the minus strain less strikingly appears to be
a straight line, but, aside from the irregularities already pointed out
and accounted for, this curve is as regular as most curves based upon
similar biological material.
The courses of the curves at the" end of selection are such as
to suggest that the divergence in reactiveness between the two
strains was still increasing when the experiment closed.
Many workers would prefer to call these small genetic changes
mutations, but they are indistinguishable from continuous gradual
changes and must have occurred with a frequency quite unknown in
forms in which the points of occurrence of definite mutations have
been recognized. Jennings (1916, p. 526) applies the term mutation
to such frequent small changes in morphological characters in
protozoa, and Hegner (1919) calls such small structural changes
(arising in his Ar cello) mutations. But the complicated and less
well-known conditions of the nuclear material in protozoa and the
direct continuity between the cytoplasm, which one may think of as
soma, of the parent and that of offspring lead one to feel that at
least there is no positive assurance that conditions in protozoa are
entirely comparable with metozoa.
In parthenogenetic Cladocera, without the reduction matura-
tion division and apparently lacking in unequal distribution of
nuclear elements, such as may occur in division in protozoa (Hegner,
1919), frequent mutation would not seem to be expected, since such
frequent mutation as would be called for in the present case has not
been recognized in metazoa and the mutations which have been most
frequent have generally appeared to be associated with maturation.
So many mutations through a lapse of only 181 generations would
call for mutations at a truly phenomenal rate. However, the matter
of a name for the genetic change merely turns upon one's definition
of a mutation. The facts of the present case are in nowise altered,
whether the small changes in character are termed mutations or
given other designation. It is possible that modifications of genes
are frequently of a low order and consequently not generally recog-
nized; further, that although they are not to be found wherever
sought, once such genetic change is initiated, such variations may
be frequent in occurrence.
162 SELECTION IN CLADOCERA ON THE BASIS OF
The explanation of the results of selection in this material as
having been due to many small genetic changes is suggested with
extreme caution. There is the objection that such an explanation
runs counter to the most acceptable explanation for most cases of
effective selection within the pure line and that many cases for
which this explanation has been invoked have later yielded to
analysis on the basis of demonstrable mutation or genetic segrega-
tion. But the objections to explanation of the present case as due
to segregation or larger mutations seem even more weighty (unless
one assumes a novel and quite unknown type of segregation in this
material), and it is believed that gradual modification of the factor or
factors involved in the present case must receive serious consideration.
Since changes in a character seem to occur with such frequency
in some clones of Cladocera, material in which because of uniparental
inheritance and lack of reduction frequent genetic changes would
seem not to be expected, one wonders if this type of organism is
unique among metazoa and if such genetic changes may not likewise
be found to occur elsewhere. In other words, may not selection be
found equally successful in some other material and may not the
case for selection be more hopeful than is generally supposed? In
any material in which improvement or change in a character is
sought, genetic variation may (or may not) be encountered. If
genetic variation occurs in the character studied and in the direction
sought, selection (though not a cause of the variations) may be a
means of utilizing the variations in accomplishing the end sought.
The pure-line concept, set forth by Jordan and elaborated by
Johannsen and his followers, has been extended until it has been
shown to have wide application. Whether it may become universal
in its application remains to be seen. The writer believes that such
cases as the present are crucial in their relation to the universal
validity of the theory, and that if they are not (or until they are)
brought into line this concept can not attain the dignity of a law.
It is for this reason that the present case has been presented in so
much detail.
Whatever explanation one finds acceptable, the facts are that
the effects of selection within Line 757 are due both to an increased
reactiveness in the plus strain of Line 757 and a decreased reactive-
ness in the minus strain of the same line (both relative to the corre-
sponding strains of the other lines of Simocephalus) , the two modifica-
tions working together to produce a large difference in reactiveness
between the two strains of Line 757; and that this difference was
permanent or at least persisted for 112 generations after selection
was discontinued.
A PHYSIOLOGICAL CHARACTER. 163
SUMMARY.
1. In experiments undertaken to get additional data on the
effects of selection within the pure line, Cladocera material was
chosen because of certain advantages: (a) it reproduces rapidly, (6)
is readily handled in the laboratory, (c) under controlled conditions
it is exclusively parthenogenetic, and (d) in its parthenogenetic
reproduction there is a single maturation division without reduction,
so that presumably it presents none of the complications present in
material in which segregation occurs during maturation and in
which fertilization occurs.
2. Selection was based upon a physiological character: reactive-
ness to directive light stimulation.
3. The material consisted of three species: Daphnia pulex, D.
longispina, and Simocephalus exspinosus, collected at different times
from three ponds at Cold Spring Harbor.
4. The animals were reared in 200-c. c. wide-mouthed bottles
with about 100 c. c. of culture-water which was ordinarily unchanged
from the time a young female was placed in the bottle until she
produced young.
5. Selections were made during the first day of life of the young
daphnids. The plus and minus strains of each line came from a
single progenitor.
6. The selections were conducted with light intensities of ap-
proximately 120 candle-meters.
7. Environmental factors modified reaction-time; but, presum-
ably, these were horizontal influences equally operative in both strains
and with all the individuals tested in making each selection, and
hence did not influence the selections per se.
The relation between environmental conditions and both re-
action-time and vigor is considered in some detail.
(a) There are shown to be low correlations between the tempera-
ture of the water in the experimental tank and reaction-time of
young daphnids. These correlations are generally positive, meaning
that reactiveness is decreased with increase in temperature. This
result was not anticipated. It is suggested that where this relation
holds it may be due to greater oxygen-content in the water of the
experimental tank with lower temperatures.
(6) Some substance exhaled from the observer's breath, pre-
sumably C02, influences reactiveness. Frequent changes of the
water in the experimental tank reduced this influence to a minimum.
(c) Relatively temporary chemical (?) differences in the water
used in the experimental tank sometimes influences reaction-time to
a marked degree.
164 SELECTION IN CLADOCERA ON THE BASIS OF
(d) Negatively reacting individuals appear under circumstances
suggesting that their occurrence is the result of peculiar local environ-
mental conditions affecting the water of the experimental tank.
(e) Broad up-and-down movements in the reaction-time curves
are presumably largely reflections of underlying (as distinguished
from more temporary) changes in content of the pond-water, though
in some cases temperature influences are contributory. More local
contemporaneous changes are clearly produced by factors other than
temperature, presumably changed constituents of the culture-water
and the water used in the experimental tank.
(/) Slight seasonal (winter and summer) changes in reactiveness
are attributed to direct temperature influences and to the indirect
influence of temperature as affecting the content of the culture-water
as well as the water used in the experimental tank.
(gr) Following July 1913, and extending throughout the re-
mainder of the experiment, there was a gradual increase in the re-
activeness of all the strains of Simocephalus. That this is not due
to the cumulative effect of favorable conditions in the laboratory is
indicated: (1) by the fact that for several months after selection was
begun with the older lines of Simocephalus there was a general
decrease in reactiveness; (2) that the increase in reactiveness affected
all strains at the same time, though they had been in the laboratory
for different periods of time; and (3) that the reaction-time curves for
the newer Simocephalus lines (Lines 794, 795, and 796) start at and
follow approximately the same levels as were attained by the older
lines for contemporaneous two-month periods.
(h) Because of the fact that the two strains of a selected line
ordinarily reproduce on different days, culture-water from different
collections was usually used with the two related strains. This
unavoidably somewhat differential treatment is believed to account
for some of the independent shifts in reaction-time means affecting'
the two strains of the same line. One such shift, for a time, materially
cut down the difference in reaction-time means between the two
strains of Line 757.
(i) Relatively local environmental influences are believed to
explain certain abrupt shifts in the reaction-time curves and in
reproductive indices which some workers with Cladocera might be
inclined to ascribe to "depression periods."
(f) Environmental conditions obviously influence the vigor of
Cladocera.
(A;) The "reproductive index" is believed to be a safe measure
of general vigor and probably of general muscular activity as well.
(I) Similar effects of environmental conditions are seen in many
coincident fluctuations in vigor of the two strains of the same line,
of all or most of the strains of the same species, and even of many of
the strains of both Simocephalus and Daphnia.
A PHYSIOLOGICAL CHARACTER. 165
(m) Independent fluctuations in vigor, like the independent
fluctuations in reaction-time affecting related strains, are believed to
be due to the somewhat differential treatment of the material.
(ri) This factor is also believed to account for the generally
greater divergence between the two strains of a line during later
periods than at the beginning of selection.
(o) There is not a generally increased reproductive index in
S. exspinosus to correspond with the increased reactiveness of this
species.
(p) Selection did not produce plus strains of increasingly greater
vigor as contrasted with minus strains of less vigor, either when
measured by the numbers of plus and minus strains which died out,
by the total numbers of generations of descent, or by the reproduc-
tive index.
8. Sometimes the plus and minus strains of the same line repro-
duced on and were tested on the same day. The "same-day-brood"
data is more trustworthy, because of more uniform environmental
conditions, than data not simultaneously obtained, and is given
separate treatment.
9. Reaction-time means were obtained of large numbers of
young from mothers of each of the two strains of a line reared simul-
taneously and tested on the same day. These "test" series gave
averages obtained from several hundred individuals reared and sub-
jected to stimulation by light under as nearly uniform conditions as
possible, and served to check up the effect or lack of effect of selection
within individual lines.
10. Reaction-time is slightly, if at all, correlated with reproduc-
tive vigor. This is a point of interest, since the minus strains in
general had a somewhat lower reproductive index than the corre-
sponding plus strains.
11. The possibility that the method of selection might have been
inadequate is discussed and evidence given which it is believed
vindicates the method used. Were the method of selection greatly
at fault the results obtained with some of the lines, particularly with
Line 757, could not have been possible.
12. In general, the plus strains showed somewhat greater repro-
ductive vigor (measured by reproductive indices) than the corre-
sponding minus strains. But this effect was not cumulative and
selection did not result in plus strains of increasingly greater vigor
than corresponding minus strains.
13. There are differences between the behavior of the different
species in the experimental tank. S. exspinosus was generally
relatively slightly reactive compared with the two species of Daphnia.
Many individuals of this species (and a few of the species of Daphnia)
failed to reach either end of the experimental tank during the 15-
minute period of the selection test.
166 SELECTION IN CLADOCERA ON THE BASIS OF
14. The data for the over-time individuals presented a serious
difficulty. In many cases, especially with the Simocephalus lines,
they constituted a considerable portion, and a most significant por-
tion, of the reaction-time data. Over-time individuals were arbi-
trarily assigned a reaction-time of 900 seconds and treated as though
they had reached an end of the tank within the period of the test.
This expedient really minimized the rightful influence of these
relatively slightly reactive but highly significant individuals.
15. Negatively reacting individuals were very irregular in occur-
rence and of doubtful significance. They were utilized in selection in
the minus strains under the assumption that a genetic change might
occur rendering an individual fundamentally negative in its reactions
to light.
16. Many of the data are published for the benefit of workers
who might wish to go over the analysis or make further analysis.
In the treatment of the data for the different lines the following
results are recorded:
17. For Line 695 there was not an effect of selection. A marked
general influence of environmental factors upon reaction-times is
observed, as is true with most of the other lines.
18. In Line 689 selection was not effective. On the contrary,
there was a persistent, though slight, difference tending to indicate
that the minus strain was the more reactive.
19. In Line 691 there was no certain effect of selection, though
part of the data suggests such an effect.
20. In Line 711 a selective effect is strongly suggested, though
not fully supported by the same-day-brood data. The evidence for a
genetic difference between the two strains is, however, as good or
better than that suggesting a genetic difference between the two
strains of Line 689.
21. The data in part suggest an effect of selection in Line 713,
but it is improbable that such actually occurred.
22. Some of the early data for Line 714 suggest an effect of
selection, but the later data show clearly that such had not occurred.
23. In Line 719, as in Line 689, there is a generally higher
reaction- time for the plus strain. The different periods of the data,
in spite of considerable fluctuations, rather consistently bear out this
interpretation, and genetic differences in opposition to selection are
believed to have arisen. Selection possibly operated to reduce this
difference, which was less marked during the latter part of the
experiment.
24. There was no effect *of selection in Line 751.
25. Some of the final data for Line 762 suggest an effect of
selection, but the data are not sufficient to be at all convincing.
A PHYSIOLOGICAL CHARACTER. 167
26. In the short experiment with Line 766 (derived from Line
762) there was no effect of selection.
27. There was no effect of selection in Line 768.
28. Most of the data for Line 794 suggest an effect of selection,
though the test series did not support this conclusion and effect of
selection must remain in doubt.
29. Effect of selection is strongly indicated for the final data
for Line 795, but in view of the earlier differences in the opposite
direction and the small amount of the data which indicate an effect
of selection, the result is questionable.
30. There is clearly no effect of selection in Line 796.
31. Certain portions of the data for Line 740 seem to indicate
consistent differences in reaction-time for the two strains. A possible
mutation early in the course of the experiment was examined, but
ruled out as improbable. For 14 months preceding the final 3
months of the experiment, the plus strain was somewhat more
reactive. However, the data for the final 3 months of the experiment,
together with a large test series, indicate a lack of selective effect.
32. A large effect of selection is clearly indicated for Line 757:
(a) This conclusion is consistently attested by every check
which was applied to the data.
(6) The effect was due both to an increased reactiveness of the
plus strain and a reduced reactiveness in the minus strain
(relatively to the other plus and minus strains of Simo-
cephalus) . .
(c) The divergence between mean reaction-times became large.
(d) The divergence in reactiveness was permanent, or, at any
rate, still existed 32 months (112 generations) after selection
was discontinued.
(e) Except for the difference in reactiveness to light, and a
slightly lower reproductive index (which was not in evidence
during the latter part of the experiment) in the minus
strain, there is nothing to distinguish the two strains.
33. It is not believed that general physiological differences or
cytoplasmic transmission can be appealed to as explaining the result,
particularly in view of the apparently permanent nature of the
changes in reactiveness.
34. The effect is presumably not due to genetic segregation
(unless there is assumed a novel method of segregation, for which
there is no evidence) inasmuch as at least three such segregations
seem necessary to explain the result on this basis and there is no
chromatic reduction in the maturation of parthenogenetic eggs of
Cladocera.
35. A possible novel method of segregation is suggested. Such
an assumption would explain the results with Line 757 in accord with
168 SELECTION IN CLADOCERA.
the pure-line hypothesis, but there is no evidence for the occurrence
in Cladocera of such unusual chromatic behavior.
36. The divergence in reaction-time may be explained as due
to larger mutations. But the data do not show points, with one
possible exception, at which such mutations may presumably have
occurred, and, assuming a minimum of 3 mutations, each mutation
must have produced its maximum effect long after its occurrence, a
fact not consistent with the current conception of a mutation in such
material. Further, the mutations must have occurred in both
strains, and since three and probably more mutations are required,
this would call for a high rate of mutation in this line.
37. The result may be explained as due to a gradual modifica-
tion, many small changes (mutations?), in the genetic factor or factors
involved.
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