EFFECTS OF CHANGED CONDITIONS CONTINUED
THE CAUSES OF GENETIC VARIATION
In the last chapter we examined some of the evidence offered in support
of the belief that adaptation in highly organised forms is a consequence
of the inheritance of adaptative changes induced by the influence of
external conditions. The state of knowledge of this whole subject is, as
I have said, most unsatisfactory, chiefly for the reason that in none of
the cases which are alleged to show a positive result have two observers
been over the same ground, or as yet confirmed each other. In the wider
consideration respecting the causes of variation at large we find
ourselves still in the same difficulty. The study has thus far proved
sadly unfruitful. In spite of the considerable efforts lately made by
many observers to induce genetic variation in highly organised plants
or animals, and though successes have occasionally been announced, I
do not know a single case which has been established and confirmed in
such a way that we could with confidence expect to witness the alleged
phenomena if we were to repeat the experiment. Abundant illustrations
are available in which individuals exposed to novel conditions manifest
considerable changes in characters or properties, but as yet there is
no certain means of determining that germ-cells of a new type shall be
formed.
Of the direct effect of conditions the lower organisms, especially
bacteria, offer the best examples, the alterations of virulence which
can be produced in so many distinct ways being the most striking
and familiar. That attenuation of virulence can be produced by high
temperatures or by exposure to chemical agents, and that this diminution
in virulence may remain permanent is, from our point of view, not
surprising; but the fact that in many cases the full virulence can by
suitable cultivation be restored is difficult to understand. Similar
variations have been observed in power of pigment production and other
properties.
These phenomena naturally raise the question whether any cases of
apparent loss of factors in higher forms may be comparable.
The subject of variations in the lower organisms and their dependence on
conditions is a highly special one, and I have no knowledge which can
justify me in offering any discussion of them, but I understand that
hitherto little beyond empirical recognition of the phenomena has been
attempted. A useful summary of observations made by many investigators
was lately published by Hans Pringsheim,[1] who enumerates the different
agencies which have been observed to produce modifications, and the
various ways in which these changes are manifested. One of the most
comprehensive studies of the subject from the genetic point of view is
that made by F. Wolf.[2] In his extensive cultivations of _Bacillus
prodigiosus_, _Staphylococcus pyogenes_ and _Myxococcus_ he succeeded
in producing many strains with modified properties. In most of these
the modifications arose in consequence of the application of high or
low temperatures or of the addition of various chemical substances
to the culture-media. Some of the variations, which are for the most
part in the powers of pigment-formation, persisted when the strains
were returned to normal conditions, and others did not. In reference
especially to the variations witnessed in the Cocci the reader should
consult the critical account of variation in that group published by
the Winslows,[3] where much information on the subject is to be found.
The authors attempted to determine the systematic relationships of the
several forms, as far as possible, by the application of statistical
methods. The result is interesting as showing that the problem of
species in its main features is presented by these organisms in a form
identical with that which we know so well in the higher animals and
plants, whatever properties be selected as the diagnostic characters.
There are many types perfectly distinct and others which intergrade.
Some of the types change greatly with conditions while others do not.
This is exactly what we encounter whenever we study the problem of
species on an extended scale among the higher forms of life.
There is now practically complete agreement among bacteriologists
that the observations made first by Massini on the change in color of
_Bacterium coli mutabile_ grown in Endo's medium, associated with the
acquisition of the power to ferment lactose, are perfectly reliable
and free from possibilities of mistake. The work has been extended and
confirmed by many workers, especially R. Müller, who finds that this
bacterium can similarly acquire and maintain the power to ferment other
sugars. A careful account of the whole subject written by Müller for the
information of biologists will be found in _Zts. für Abstammungsl._,
VIII, 1912. After discussing the biological significance of the facts,
he concludes with a caution to the effect that bacteria are so different
from all other living things that generalizations from their behavior
must not be indiscriminately applied to animals and plants.
In all work with this class of material there is obviously danger of
error through foreign infection of the cultures, but there can be no
doubt that though some of the "mutations" recorded may be due to this
cause, the majority of the instances observed under stringent conditions
are genuine.
Another and equally serious difficulty besetting work with bacteria
and fungi cultivated from spores is that the appearance of variation
may in reality be due to the selection of a special strain previously
living masked among other strains. This possibility must be remembered
especially in those instances which are claimed as exemplifying the
effects of acclimatisation. Manifestly this consideration can be urged
with most force when the strain which gave rise to the novelty was
not raised from a single individual spore. Moreover, when once the
possibility of spontaneous variation is admitted, it must be difficult
to be quite confident that any given variation observed is in reality
due to the novel conditions applied, and as I understand the evidence,
the appearance of the mutational forms does not with any regularity
follow upon the application of the changed conditions.
Researches into the variation of these lower forms will, no doubt, be
continued on a comprehensive scale. So long as the instances recorded
are each isolated examples it is impossible to know what value they
possess. If they could be coordinated in such a way as to provide some
general conception of the types of variation in properties to which
bacteria, or any considerable group of them, are habitually liable, the
knowledge might greatly advance the elucidation of genetic problems.
Of mutational changes directly produced with regularity in
micro-organisms by treatment, the experiments with trypanosomes provide
some of the clearest examples. A summary of the evidence was lately
published by Dobell,[4] from which the present account is taken. The
most definite fact of this kind established is that certain dyes
introduced into the blood of the host have the effect of destroying
the small organ known as the "kinetonucleus" in the trypanosomes. The
trypanosomes thus altered continue to breed, and give rise to races
destitute of kinetonuclei. This observation was originally made by
Werbitzki and has been confirmed by several observers. The exact way
in which this alteration is effected in the trypanosomes is not quite
definitely made out, but there is good reason for supposing that
the dyes have a direct and specific action upon the kinetonucleus
itself, and circumstances make it improbable that in some division a
daughter-organism without that body is produced, or that any selection
of a pre-existing defective variety occurs.
Ehrlich has suggested with great probability that the dyes which possess
this action owe it to the fact that they have the particular chemical
linkage which he calls "ortho-quinoid." In outward respects, such as
motility and general appearance, the modified organisms are unchanged,
but their virulence is diminished. As regards the possibility of the
defective strain reacquiring the kinetonucleus, Werbitzki states
that in one case passage through 50 animals and treatment with dyes
left the strain unaltered; but that in another case at the sixteenth
passage 7 per cent. of the trypanosomes were found to have re-acquired
the organ, and in subsequent passages the percentage increased, until
at the twenty-seventh passage practically all had re-acquired it.
Kudicke, however, in similar experiments did not succeed in causing
re-acquisition by transplantation.
By the action of various drugs and anti-bodies races of trypanosomes
resistant to those substances have been obtained. These breed true, at
least when kept in the same species of animal in which the resistance
was acquired. As to whether change of virulence is produced by passage
through certain animals or not, there is as yet no general agreement.
Other changes, especially in size and some points of structure, are said
to occur when certain trypanosomes proper to mammals are passed through
cold-blooded vertebrates (Wendelstadt and Fellmer), and it is stated
that these changes persist, but the observations have not yet been
confirmed.
Experiments lately conducted by Woltereck with _Daphnia_ are interesting
as having given a definite positive result, in so far, at least, as the
ova were affected by conditions before leaving the bodies of the parent
individuals. The observations relate to the offspring resulting from
_parthenogenetic_ eggs. Females bearing ephippia (fertilised eggs) were
isolated until the ephippia were dropped, and in this way the offspring
of fertilisation were excluded. Males, of course, appeared from time
to time in the cultures, but as fertilised eggs were rejected, their
presence did not disturb the result. The most remarkable observations
related to _Daphnia longispina_.
This species as found in the lower lake at Lunz had the front end of
the body blunt and nearly round in profile; but on being cultivated in
a warm temperature and with abundant nourishment the front end of the
body became produced into an elongated "helmet," as Woltereck calls it.
Experiment showed that the change was primarily due to the abundance of
food, and owing to temperature in a subordinate degree.
This distinction arose as soon as the species was taken into the
hothouse, but when the modified individuals were put back into the
original conditions, a lower temperature and scanty food-supply,
the next generation returned to their original form. After being
cultivated for two years and about 40 generations in the more favourable
conditions, when similarly put back into the lower temperature with
scanty food the _first generation_ born in these conditions was helmeted
like the modified parents. Woltereck is of opinion that the ova were
still unformed at the time the parents were put back, and the influence
of the favourable conditions upon the unformed ova he speaks of as a
"prae-induction." The effect never extended beyond the one generation,
after which the strain returned to its original state.
The fact that the influence on the offspring was not manifested at
first led Woltereck to expect that by more prolonged cultivation in the
favourable conditions a further extension of this influence would be
produced, but this expectation was never fulfilled, though the attempt
was made again and again.
Similar experiments were made with _Hyalodaphnia cucullata_, which is
far more sensitive to cultural influences, and in nature manifests
a considerable elongation of the helmet as a seasonal modification,
but the results were essentially the same as in the preceding case,
no modification extending beyond the first generation born after the
restoration to _normal conditions_.[5]
The only criticism of these extremely interesting results which suggests
itself is that perhaps the original appearance of the modification was
not in reality due to an _accumulated_ effect of the conditions, but
to some change in the conditions themselves which was not noticed. It
is difficult to see how length of time or even the lapse of several
generations could have so specific an effect on the race. It is no
doubt often vaguely supposed by many that a long period of time may be
necessary for the effect of climate or of other environmental conditions
to be produced in an organism which does not thus respond at first. I
have never been able to see any reason for this opinion nor how it is
to be translated into terms of physiological fact, and I imagine that
in those cases in which the lapse of time is really required for the
production of an effect, the influence of the prolongation is rather
on the conditions than on the organisms. The response of the organisms
thus probably indicates not that the creature is at length feeling the
effects because of their accumulated action on itself, but that the
conditions have at length ripened.
As this sheet is passing through the press Agar has published[6] an
abstract of evidence as to another comparable case in a parthenogenetic
strain in the daphnid, _Simocephalus vetulus_. When fed on certain
abnormal foods the shape of the body is changed, the edges of the
carapace being rolled backwards so as to expose the appendages. The
offspring of animals thus modified showed similar modification in the
first, and to a very slight degree, in the second generation, though the
original mothers were removed to normal conditions before their eggs
were laid. In the third generation there was "a very pronounced reaction
in the opposite direction." Agar suggests that the change may be due to
some toxin-like substances, carried on passively by the egg into the
next generation, against which the protoplasm eventually produces an
anti-body.
The experiments which have been in recent years regarded by evolutionary
writers as the most conclusive proof that direct environmental action
may produce germinal variation are those of Professor W. L. Tower, of
Chicago, on _Leptinotarsa_, the potato beetles. This work has attained
considerable celebrity and has been generally accepted as making a
definite extension of knowledge. After frequently reading Tower's papers
and after having been privileged to see some of the experiments in
progress (in 1907) I am still in doubt as to the weight which should be
assigned to this contribution.
The work is described in two chief publications, the first of which
appeared in 1906.[7] This treatise contains a vast amount of information
about numerous species and varieties of these beetles which the author
has observed and bred in many parts of their distribution throughout
the United States, Mexico and Central America. The part of the book
which has naturally excited the greatest interest is that in which Tower
states that by subjecting the beetles to change in temperature and
moisture, he caused them to produce offspring quite unlike themselves,
which in several cases bred true.
It is much to be regretted that the author did not happen to become
acquainted with Mendelian analysis at an earlier stage in the
investigation. The evidence might then have been handled in a much more
orderly and comprehensive way, and a watch would have been kept for
several possibilities of error.
The headquarters of the genus is evidently as Tower states, in Mexico
and the adjoining countries. In this region there is a great profusion
of forms, some very local, some as for instance the well-known
_decemlineata_,[8] more widely spread. The distinctions are almost all
found in peculiarities of colour and pattern, and the limits of species
are even more indefinable than is usual in multiform animals. Tower
arranges the various types into seven groups of which the one most
studied is that which he calls the _lineata_ group. To this group belong
all the forms to which reference is here made, and, as I understand,
they differ among themselves entirely in size, colour and pattern.
There is no suggestion of infertility in the crosses made between the
several forms of the _lineata_ group; in fact they present, like many
Chrysomelidae, a good example of what most of us would now call a
polymorphic species, consisting of many types, some found existing in
the same locality, others being geographically isolated.
A series of experiments was devoted to the attempt to fix strains
corresponding to the extremes of continuous variations. For example,
those with most black pigment and those with least black taken from
a population continuously varying in this respect, were separately
bred; but almost always the selection led to no sensible change in the
position of the mean of the population. The variations in these cases
were evidently fluctuational. In some instances, however, real genetic
differences were met with, and strains exhibiting them were, as usual,
rapidly fixed.
Tower points out that several of the varieties (or species, as he
prefers to call them) were obviously recessive to _decemlineata_. This
is most clearly demonstrated in the case of the form called _pallida_,
which is a pale depauperated-looking creature, with the orange of
the thorax almost white and the eyes devoid of pigment.[9] This form
behaved as an ordinary Mendelian recessive, breeding true whenever it
appeared in the cultures, or when individuals found wild were studied
in captivity. A black form which Tower names _melanicum_ was similarly
shown to be a Mendelian recessive. Wild specimens of this variety of
opposite sexes were not found simultaneously in nature, and there was
thus no opportunity of breeding them together, but the hereditary
behaviour was seen in the F_{2} generation from a _melanicum_ found
coupled with _decemlineata_. Experiments also occurred giving indication
that a variety with the stripes anastomosing in pairs (_tortuosa_), was
another recessive, and that a variety--called "_rubri-vittata_"--gave an
intermediate F_{1} with subsequent segregation. All these are forms of
_decemlineata_ Stål.
Similar observations were made regarding forms recessive to
_multitaeniata_ Stål. Of these two were thrown by _multitaeniata_
itself, namely a form named by Stål _melanothorax_, and regarded by him
as a species, and one which Tower names _rubicunda_ n. sp. The facts
proving the recessive behaviour of their several forms will be found in
the following places in Tower's book:
_pallida_, pp. 273-278.
_melanicum_, p. 279.
_tortuosa_, p. 280.
_rubrivittata_, pp. 280-281.
_melanothorax_ and _rubicunda_, pp. 283-285.
Following this evidence of recessive nature of the six forms
enumerated, Tower describes experiments showing, as he believes, that
some of them may be caused to appear by applying special treatment
to the parents during the "growth and fertilisation" (p. 287) of the
eggs. The most striking example is that in which 4 males and 4 females
of _decemlineata_ were kept very hot (average 35° C.) and dry, and at
low atmospheric pressure (19-21 inches). The eggs laid were restored
to natural conditions. These gave 506 larvae, from which emerged 14
normal, 82 _pallida_ and 2 "_immaculothorax_," viz., without pigment
on the pronotum. The account of the rest of the experiment is somewhat
involved, but I understand that the _pallida_, of which two only
survived, behaved as normal recessives when bred to the type: also that
the parents, after having laid the eggs whose history has been given,
were restored to normal conditions and laid 319 eggs which gave 61
normals.
In another case normal parents laid 409 eggs in the hot and dry
conditions, and on restoration to normal conditions, the same parents
laid 840 eggs. Then 409 eggs gave 64 adults as follows:
_Males_ _Females_
_decemlineata_ 12 8
_pallida_ 10 13
_immaculothorax_ 2 3
_albida_ 9 7
--- ---
33 31
The 840 eggs laid in normal conditions gave 123 normal _decemlineata_.
Similar experiments were made with _multitaeniata_ and gave comparable
results, the two recessives (_melanothorax_, _rubicunda_) being produced
in large numbers when the parents were subjected to heat, but in this
case the atmosphere was kept _saturated_ with moisture, instead of dry,
as in the previous instance. The same parents transferred to normal
conditions gave normals only.
Lastly the form _undecimlineata_ was exposed "to an extreme stimulus of
high temperature, 10° C. above the average," and a dry atmosphere, with
the result that from 190 eggs there emerged 11 beetles, all of the form
_angustovittata_ Jacoby, which subsequently bred true to that type (see
p. 295).
In the results of these experiments, as described, there is one feature
which I regard as quite unaccountable. Tower makes no comment upon
it. Indeed, from the general tenour of the paper, I infer, not only
that he does not perceive that he is recounting anything contrary to
usual experience, but rather that he regards the result as conforming
to expectations previously formed. The point in question is the
genetic behaviour of the dominant normals produced under the abnormal
conditions. These normals were the result of the breeding of parents
declared to be at the same time giving off many recessive gametes. Some
of these normals must be expected therefore to be heterozygous unless
some selective fertilisation occurs. Nevertheless in every case they and
their offspring are reported to have continually bred true. I allude
especially to the tables given on pp. 288, 289, 292, and 293. Tower does
not mention any misgiving about this result, and I think he regards
himself as recounting phenomena in general harmony with the ideas of
mutation expressed by De Vries. This they may be; but to anyone familiar
with analytical breeding the course of these experiments must seem so
surprising as to call for most careful, independent confirmation.
In 1910[10] Tower published an account of further experiments with
_Leptinotarsa_. The work described related to two subjects. Crosses
were made between three forms, _undecimlineata_ Stål, _signaticollis_
Stål and "_diversa_" named by Tower as a new species. The distinctions
between these three depend partly on characters of the adults and
partly on those of the larvae. The adults of _undecimlineata_ and
_diversa_ have the elytra striped, but the elytra of _signaticollis_
are unstriped. The larvae of _signaticollis_ and of _diversa_ are
yellow, but those of _undecimlineata_ are white.[11] Moreover, in
_signaticollis_ and _diversa_ the black increases in the third
stage of the larvae to form transverse bands which are absent in
_undecimlineata_. The general course of the experiments shows that these
differences may be approximately represented as due to the action of
three factors, any of which may be independently present or absent. The
stripings of the elytra and of the larvae are each due to a separate
factor. As regards the distinction between the yellow and the white
larvae the evidence does not prove that there is decided dominance of
either colour and I infer that the heterozygotes are often intermediate.
The chief contribution which this new paper claims to make relates to
differences in the results which ensue from crosses effected between
these three types at different average temperatures.
We are first concerned with four experiments which I number (1), (2),
(3), (4):
1. _Signaticollis_ [F] × _diversa_ [M] bred at an average temperature
of 80º F. by day and 75° F. by night, gave two groups in about equal
numbers. The first (49) was pure _signaticollis_ and bred true. The
second (53) was of an intermediate type, which on being bred together
gave the typical Mendelian result--1 _sig._: 2 _intermediate_: 1 _div_.
2. Next, as the account originally stood in the published paper, we
are told that _sig_ [F] × _div_ [M] bred together at a day-temp.
average 75° F. and night average 50° F. gave an _intermediate_ only,
which subsequently produced a normal 1:2:1 ratio. The two crosses were
repeated eleven times with identical results.
In a further experiment (3) _signaticollis_ [F] × _diversa_ [M] were
bred under the same conditions as those used in expt. (1). They again
gave _sig._ and intermediates as before in fairly equal numbers. The
_sig._ as before bred true, and the intermediate gave 1:2:1, all exactly
as in expt. (1).
In expt. (4) _the same parents used_ in (3) were again mated under
conditions of expt. (2) at the lower temperature, and this time gave
_signaticollis_ exclusively, which bred true for four generations. This
experiment was repeated seven times with uniform results.
Diagrams are given representing all these histories in graphic fashion.
From these observations, Tower concludes that the determination of
dominance, and the ensuing type of behaviour, is clearly a function of
the conditions incident upon the combining germ plasms.
It will be observed that expts. (1) and (3) gave identical results
but (2) and (4), though much the same conditions were applied, are
at variance, for (2) gave all intermediates, while (4) gave all
_signaticollis_. In _Amer. Nat._, XLIV, 1910, p. 747, Professor T.
D. A. Cockerell commented on this paper of Tower's and pointed out
that there must be an error somewhere, for when he discusses these
experiments Tower speaks of (2) and (4) as confirming each other. To
this Tower replied[12] that there had been a mistake. He states that
in preparing the paper "certain minor experiments were taken from
a larger series and combined to illustrate a general point in the
behaviour of alternative characters in inheritance," and that expt. (2)
was introduced inadvertently in place of another which he desires to
substitute. In this, which I number (5), _signaticollis_ [F] × _diversa_
[M] from exactly the same stocks as those used in (1), were mated at the
lower temperatures specified for (2), day average 75° F., night average
50° F. These gave all of the _signaticollis_ type with a narrow range of
variability, which bred true, in some cases to F_{6}. Tower says he has
repeated this experiment six times with identical results.
Nevertheless he proceeds to say that the description of expt. (2), which
was repeated eleven times with identical results, was correct "as far as
given." That experiment was "from a second series of cultures parallel
to the one given, but in which there are other factors involved, which
in H. 410 [my (2)] are productive of a typical Mendelian behaviour." He
adds he does "not care at this time to make any statement of what these
factors are, nor of their relations to the behaviours given in the H.
409, H. 411, H. 409/11 series [my (1), (5) and (3)--(4)] which are the
simplest and most easily presented series obtained in the crossing of
_signaticollis_ and _diversa_."
Professor Cockerell's intervention has thus elicited the fact that
we have as yet only a small selected part of the evidence before us,
even as concerning the effect of temperature on the cross between
_signaticollis_ [F] × _diversa_ [M]. We learn that at the lower
temperatures the result was eleven times the expected one, and six times
an unexpected one; further, that we owe it to the author's inadvertence
that we have come to hear of the expected result at all, and that though
he knows the factors which determine the discrepancy, he declines for
the present to name them. In these circumstances we can scarcely venture
as yet to estimate the significance of these records.
The paper goes on to recount somewhat comparable, but more complex
instances in which the descent of the colour of adults and of larvae
was affected by temperature in crosses between _undecimlineata_ and
_signaticollis_. As they stand the results are very striking and
unexpected, but I think, in view of what has been admitted respecting
the former part of the paper, full discussion may be postponed till
confirmation is forthcoming.
One feature, however, calls for remark. This second paper is written
apparently without any reference to the discoveries related by Tower in
his previous book, to which no allusion is made. This is most noticeable
in the case of an experiment in which (p. 296, H. 700A) _undecimlineata_
[F] (the dominant) was mated to _signaticollis_ [M] with the result
that all the offspring were _undecimlineata_ and bred true to that
type (Parthenogenesis was tested for, but never found to occur). This
experiment was made at a temperature averaging 95° F. ± 3.5° by day
and 89° F. ± 4.8° by night, and in a humidity given as 84 per cent. by
day and 100 per cent. by night; but in the previous book (p. 294) we
are told that pure _undecimlineata_ bred together "under an extreme
stimulus of high temperature, 10° C. above the average" and a relative
humidity of 40 per cent. gave 11 beetles only, all _angustovittata_.
But reference to the Plate 16, Fig. 2, shows that _angustovittata_
must be exceedingly like _signaticollis_, having, like it, the elytral
stripes obsolete, and if there is any marked difference at all, it can
only be in the larvae. It seems strange that if _undecimlineata_ really
gives off ova of this recessive type at high temperatures, the fact
should not be alluded to in connection with expt. H. 700A, where, as
the father was _signaticollis_, having the same recessive character,
their appearance might have been expected not to pass unobserved. The
temperature in the older experiment is, of course, not given with the
great accuracy used in the second, and it may have been higher still.
The humidity also was widely different. Still, in discussing the
phenomena we should expect some reference to the very remarkable and
closely cognate discovery which Tower himself had previously reported in
regard to the same species.[13]
The hesitation which I had come to feel respecting these two
publications of Tower's has been, I confess, increased by the appearance
of a destructive criticism by Gortner[14] who has examined the parts
of Chapter III of Tower's book, in which he discusses at some length
the chemistry of the pigments in _Leptinotarsa_ and other animals. As
Gortner has shown, this discussion, though offered with every show of
confidence, exhibits such elementary ignorance, both of the special
subject and of chemistry in general, that it cannot be taken into
serious consideration.
Some observations made by Dr. W. T. Macdougal[15] have also been
interpreted as showing the actual causation of genetic variation by
chemical treatment. Of these perhaps the least open to objection
were the experiments with _Raimannia odorata_, a Patagonian plant
closely allied to _Oenothera_. The ovaries were injected with various
substances and from some of the seeds which subsequently formed in them
a remarkable new variety was raised. This varying or mutational form
was strikingly different from the parental type, with which it was not
connected by any intergradational forms, and it bred true. It made
no rosette, growing to a much smaller size than the parent, and was
totally glabrous instead of being very hairy as the parental type is.
I was shown specimens of these plants by the kindness of Dr. Britton
in the Bronx Park Botanic Garden in 1907 and can testify to their very
remarkable peculiarities. They had a somewhat weakly look, and might
at first sight be thought to be a pathological product, but they had
bred true for several generations. From the evidence, however, I am by
no means satisfied that their original appearance was a consequence
of the treatment applied. This treatment was of a most miscellaneous
description. Two of the mutants came from an ovary which had been
treated with a ten per cent. sugar solution. Ten came from one into
which a 0.1 per cent. solution of calcium nitrate had been injected. One
was from a capsule which "had been exposed to the action of a radium
pencil." Macdougal speaks of these results as decisive, but clearly
before such evidence can be admitted even for consideration it must be
shown by control experiments that the individual plants which threw
the mutant were themselves breeding true in ordinary circumstances.
Nothing is more likely than that the mutant was an ordinary recessive.
I may add that Mr. R. H. Compton made a number of experiments with
_Raimannia odorata_, raised from seeds kindly given me by Dr. Britton,
injecting the ovaries with a variety of substances, including those
named by Macdougal; but though a numerous progeny was raised from the
ovaries treated, all were normal. Macdougal relates also that some
mutational forms came from ovaries of _Oenothera Lamarckiana_ exposed
to radium pencils, and also from _Oenothera biennis_ injected with
zinc sulphate a peculiar mutant was raised, but taking into account
the frequency of these occurrences in those species, he very properly
regarded this evidence as of doubtful application. In a later paper,[16]
however, he has returned to the subject and affirms his conviction that
the appearance of a mutant among seedlings raised from an ovary of
_Oenothera biennis_ treated with zinc sulphate was really a consequence
of the injection, saying that the variation previously observed in
the species was afterwards shown to be due to fungoid disease. The
circumstances to which he mainly points in support of his view is
that the mutation bred true, but this is only evidence of its genetic
distinctness, which may, of course, be admitted by those who remain
unconvinced as to the original cause of its appearance. He adds that he
is making similar experiments with some twenty genera; but what is more
urgently needed is repeated confirmation of the original observation.
When it has been shown that this mutation can be produced with any
regularity from a plant which does not otherwise produce it on normal
self-fertilisation, the enquiry may be profitably extended to other
plants.
A curious and novel experiment, which however, led ultimately to a
negative result, was made by F. Payne. Many discussions have been held
respecting the blindness of cave animals. The phenomenon is one of the
well-known difficulties, and most of us would admit that the theory of
evolution by the natural selection of small differences does not offer a
really satisfying account of it. Those who believe in the causation of
such modifications by environmental influences and in their hereditary
transmission make, of course, the simple suggestion that the darkness
is the cause of the loss of sight, and that disuse has led to the
reduction of the visual organs. Payne bred _Drosophila ampelophila_,
the pomace-fly (which is easy to keep in confinement, fed on fermenting
bananas), for sixty-nine generations in darkness. At the end of that
period there was no perceptible change in the structure of the eyes, or
in any other respect. The number of generations may possibly be regarded
as insufficient to prove anything, but comparing them, as he does, with
the generations of mankind, we see that they correspond with a period of
about two thousand years, an interval far longer than those which many
writers in particular cases have deemed sufficient.
In his first paper Payne states that, though no structural difference
could be perceived, the flies which had been bred in the dark reacted
less readily to light than those which had been reared under normal
conditions, and he inclined to think that the treatment had thus
produced a definite effect. After more careful tests, however, he
withdrew this opinion. It proved that both individual flies and
individual groups of flies, both of those bred in the light and of
those bred in the dark, differed greatly in their reactions, which were
measured by counting the time that it took for a fly to travel to the
light end of a covered tube, various sources of error being eliminated.
He found further that these differences of behaviour were not inherited
in any simple way, but he is disposed to attribute them to accidental
differences in the nature of the food, an account which seems probable
enough.[17]
In several recent publications Blaringhem[18] has described the origin
of many abnormal forms of plants, especially of maize, which he
attributes to various mutilations practised upon the parents. Respecting
these the same difficulty which has been expressed in other cases
reappears, that before drawing any conclusion as to the value of such
evidence we require to know that the plants treated belong to a really
pure line, which if left to nature in the ordinary circumstances of its
life in that locality would have had normal offspring. Abnormalities
abound in the experience of everyone who examines pans of seedlings
of almost any species of plant, and in maize they are well known to
be exceptionally common. Some of those which we meet with when we
attempt to ripen maize in this country are very similar to those which
Blaringhem describes, consisting in irregularities in the distribution
of the sexes, in the shapes of the panicles, etc. Many of these are
doubtless imperfections of development, due to the dullness of our
climate, but others are presumably genetic and would recur in the
offspring however treated. If some one working in a climate where maize
could be raised in perfection would repeat these experiments, and show
that a strain which was thoroughly reliable and normal in its genetic
behaviour did, after mutilation, throw the miscellaneous types observed
by Blaringhem, that would be evidence at least that the development of
the seed could be so influenced by injury to the parental tissues that
its properties were changed. Such evidence could be used for what it is
worth; but pending an inquiry of this kind I am disposed to regard these
observations of variation following on parental injury as suggestive
rather than convincing.
Some evidence of a remarkably interesting kind has been collected by
J. H. Powers[19] respecting the structure and habits of _Amblystoma
tigrinum_, which led him to the conclusion that striking differences
in the form, anatomy, and developmental processes could be effected
directly by change in the conditions of life. It is well known that
a profusion of forms, distinct in various degrees, is grouped round
_Amblystoma tigrinum_. Some of these are believed to be geographically
isolated, others occur together in the same waters, and, as usual,
authorities have differed greatly as to the number of names to be given.
These forms were studied in detail by Cope who described them in the
_Batrachia of North America_. The view which he inclined to take was
that the individual variations of _Amblystoma tigrinum_ resulted from
variations in the time and completeness of the metamorphosis, and these
were regarded as due to external causes, such as differences in season,
temperature, and geographical conditions. Powers, however, states that
collecting within a radius of six or eight miles he found almost if
not quite the whole "gamut of recorded variation in this species."
Some, however, as he states, occurred rarely except under experimental
conditions, but considerable differences in temperature were not found
necessary in producing them. Every year, he says, he has been able to
add to the number of peculiar types found in the same small area in
nature, until the amount of natural variation at least equals that seen
by Cope in the collections of the National Museum and those of the
Philadelphia Academy.
Powers states that his observations by no means confirm Cope's view
that these differences are in the main referable to variation in
the completeness of metamorphosis, and on the contrary, he regards
metamorphosis as on the whole a levelling process, tending to obliterate
diversity. The enormous differences in size and proportions which he
describes can only be appreciated by reference to his figures. They
affect almost all features of bodily organisation. These striking
differences he looks upon as brought about by differences in nutrition,
"diversities in habitual locomotion," and diversity in the age at which
metamorphosis occurs, and to sexual difference. Apart from sexual
difference he regards the chief distinctions, in brief, as "acquired
variations of the larva."
As an example he gives the great elongation of some of the forms as
"due first to slow growth, second to the free-swimming habit, third
to the prolongation of larval life, and finally to the assumption of
sexual maturity as males," either in the branchiate or non-branchiate
condition. He describes the rapid growth of some and the slow growth
of others. A larva of intermediate type may grow about a centimeter a
month, but a rapidly growing specimen may grow more than four times
as much. The slower rate of growth may, he says, be induced by winter
feeding, and other treatment.[20]
When, however, he goes on to describe the influences which he regards
as exerted by the habit of freely swimming, I am led to wonder whether
after all in most of these illustrations, the primary distinctions
are not in reality genetic. "Specimens raised in the same aquarium or
in similar aquaria, side by side with all conditions as uniform as it
is possible to make them, seldom fail to furnish striking examples of
broad-headed, short-bodied, and short-tailed types which are habitually
found at the bottom, while others, slender and elongated, are free
swimmers, and maintain themselves in almost as continual suspension
and motion as does a gold fish." Later, again, he writes, "Yet despite
the uniformity of these favourable conditions, the larvae soon began
to split up into two noticeably distinct groups, the one of unusually
compact proportions, the other of uniform intermediate build, such
is most commonly met with." It is to my mind scarcely possible to
resist the inference that, though there may be definite responses
to certain conditions, yet the chief distinctions are genetic, and
that it is these distinctions which confer the power to respond. The
parts respectively played by cause and effect are always difficult to
assign; but when it is stated that "a weak-limbed, long-bodied and
long-tailed animal becomes well nigh perforce an undulatory swimmer,
while the strong-limbed, short-tailed, heavy-bodied specimen, when
these characteristics are rapidly forced upon it, is, under certain
circumstances, just as forcibly induced to become a crawler," we feel
how erroneous any estimates of causation are likely to be.
One of the most remarkable and interesting sections of Powers' paper
is that in which he describes the differences in bodily structure and
habits which he attributes to cannibalism, and the whole account of the
phenomena should be read in the original. It appears that there are
two extremely distinct types of larvae, those with narrow heads and
slender bodies which live for the most part on small Crustacea such
as _Daphnias_, and those with huge mouths and very wide heads, which
disregard such small animals altogether and live on amphibian larvae,
whether of their own or other species. As the illustrations show, the
differences between these two types are very great, and the differences
in instinct and behaviour are no less. The cannibals take no heed of the
pelagic crustacea, lying sluggishly at the bottom, rousing themselves
immediately to a violent attack on the larger living things which
approach them. Nothing but the most incontrovertible evidence based on
abundant control experiments should convince us that such differences
are not primarily genetic, and in the present state of knowledge I
incline to think that the families really consist of individuals which
are ready to assume the cannibal habit if opportunity offers, and
others which are congenitally incapable of it. It may readily be that
if all chance of cannibal diet be excluded, the full development of the
wide head and mouth, or the other peculiarities, would never become
pronounced, but I doubt whether such change could be induced in any
individual taken at random.
FOOTNOTES:
[1] Pringsheim, H., _Die Variabilität niederer Organismen_, Berlin, 1910.
[2] F. Wolf, Modifikationen u. Mutationen von Bakterien, _Zts. F.
indukt. Abstam. u. Vererbungslehre_, II, 1909, p. 90.
[3] Winslow, C. E. A. and A. R., _Systematic Relationships of the
Coccaceae_. New York. 1909.
[4] C. C. Dobell, _Jour. Genetics_, 1912, II, p. 201, where full
references are given.
Still more recently the same author has contributed an excellent summary
of the evidence relating to bacteria (_ibid._, II. 1913, p. 325).
[5] See Woltereck, _Verh. d. Deut. Zool. Ges._, 1909, p. 110; and 1911,
p. 142. This is a subject which can only be properly appreciated on
reference to the original papers. Several complications are involved to
which I have not here alluded.
[6] _Proc. Roy. Soc._, B, Vol. 86, 1913, p. 113.
[7] _An Investigation of Evolution in Chrysomelid Beetles of the Genus
Leptinotarsa_, Carnegie Publications, 1906, No. 48.
[8] This is the famous Colorado beetle or potato-bug, which has caused
such serious destruction in potato crops. There seems to be no doubt
that this insect, formerly unknown in the eastern States, made its way
east along the mining trails when the west was opened up.
[9] This is indicated in the coloured plate, but I have not found any
explicit statement to this effect in the text, and am not sure if the
absence of pigment was regarded as complete.
[10] _Biol. Bull._, XVIII, 1910, p. 285.
[11] This description does not quite agree with the representation of
the larvae in Pl. 17 of the book _Evolution in the Genus Leptinotarsa_
for there the larva of _undecimlineata_ is shown as white in the second
stage, but yellowish in the third stage; perhaps there is an error in
printing.
[12] _Biol. Bull._, XX, 1910, p. 67.
[13] As to the interrelations of these three forms, Tower states (1906,
p. 18) that _angustovittata_, which he reared from _undecimlineata_, is
intermediate between it and _signaticollis_. Compare Stål, "_Monogr.
des Chrysomélides_," 1862, p. 163; and Jacoby, _Biol. Centr. Amer.
Celeopt._, vi, Pt. 1, p. 234, Pl. xiii, fig. 20; Tab. 41, fig. 15;
_ibid._, Suppl., p. 253. All these forms are evidently very closely
related, and the delimitation of species is quite arbitrary. Jacoby
indeed suggests that _undecimlineata_ may be a variety of _decemlineata_.
[14] Gortner, _Amer. Nat._, Dec., 1911, XLV, p. 743.
[15] _Mutations, Variations, and Relationships of the Oenotheras_,
Carnegie Institution Publication No. 81, 1907, pp. 61-64.
[16] Macdougal, D. T., "Alterations in Heredity induced by Ovarial
Treatments", _Bot. Gaz._, vol. 51, 1911, p. 241.
[17] Payne, Fernandus, _Biol. Bull._, XVIII, 1910, p. 188, and _ibid._,
XXI, 1911, p. 297.
[18] See especially, _Mutation et Traumatismes_, Paris, Felix Alcan,
1908.
[19] J. H. Powers, "Morphological Variation and its Causes in
_Amblystoma tigrinum_." _Studies from the Zoological Laboratory. _ The
University of Nebraska, No. 71, 1907.
[20] In connexion with this case I would refer the reader to some
remarkable observations of Dr. T. A. Chapman on various types of larvae
which he reared from the moth _Arctia caja_ (_Ent. Rec._, IV, 1893,
p. 265, and following parts). From a single mother he raised a great
diversity of forms, some which fed up rapidly and passed through their
development without assuming certain stages, and others which were, as
he called them, "laggards," moulting more times than their brethren and
developing at a much slower rate. It is greatly to be hoped that such a
case may be critically investigated by analytical breeding.