Chapter VI: The Argument Built on Exceptions

So much for the enormous advance that the Mendelian principles already permit us to make. But what does Professor Weldon offer to substitute for all this? Nothing.

Professor Weldon suggests that a study of ancestry will help us. Having recited Tschermak’s exceptions and the great irregularities seen in the _Telephone_ group, he writes:

“Taking these results together with Laxton’s statements, and with the
evidence afforded by the _Telephone_ group of hybrids, I think we can
only conclude that segregation of seed-characters is not of universal
occurrence among cross-bred peas, and that when it does occur, it may
or may not follow Mendel’s law.”

Premising that when pure types are used the exceptions form but a small part of the whole, and that any supposed absence of “segregation” may have been _variation_, this statement is perfectly sound. He proceeds:--

“The law of segregation, like the law of dominance, appears therefore
to hold only for races of _particular ancestry_ [my italics]. In
special cases, other formulae expressing segregation have been
offered, especially by De Vries and by Tschermak for other plants,
but these seem as little likely to prove generally valid as Mendel’s
formula itself.

“The fundamental mistake which vitiates all work based upon Mendel’s
method is the neglect of ancestry, and the attempt to regard the
whole effect upon offspring, produced by a particular parent, as due
to the existence in the parent of particular structural characters;
while the contradictory results obtained by those who have observed
the offspring of parents identical in certain characters show clearly
enough that not only the parents themselves, but their race, that
is their ancestry, must be taken into account before the result of
pairing them can be predicted.”

In this passage the Mendelian view is none too precisely represented. I should rather have said that it was from Mendel, first of all men, that we have learnt _not_ to regard the effects produced on offspring “as due to the existence in the parent of particular structural characters.” We have come rather to disregard the particular structure of the parent except in so far as it may give us a guide as to the nature of its gametes.

This indication, if taken in the positive sense--as was sufficiently shown in considering the significance of the “mule” form or “hybrid-character”--we now know may be absolutely worthless, and in any unfamiliar case is very likely to be so. Mendel has proved that the inheritance from individuals of _identical ancestry_ may be entirely different: that from identical ancestry, without new variation, may be produced three kinds of individuals (in respect of each pair of characters), namely, individuals capable of transmitting one type, or another type, or both: moreover that the statistical relations of these three classes of individuals to each other will in a great number of cases be a definite one: and of all this he shows a complete account.

Professor Weldon cannot deal with any part of this phenomenon. He does little more than allude to it in passing and point out exceptional cases. These he suggests a study of ancestry will explain.

As a matter of fact a study of ancestry will give little guide--perhaps none--even as to the probability of the phenomenon of dominance of a character, none as to the probability of normal “purity” of germ-cells. Still less will it help to account for fluctuations in dominance, or irregularities in “purity.”

_Ancestry and Dominance._

In a series of astonishing paragraphs (pp. 241–2) Professor Weldon rises by gradual steps, from the exceptional facts regarding occasional dominance of green colour in _Telephone_ to suggest that the _whole phenomenon of dominance may be attributable to ancestry_, and that in fact one character has no natural dominance over another, apart from what has been created by selection of ancestry. This piece of reasoning, one of the most remarkable examples of special pleading to be met with in scientific literature, must be read as a whole. I reproduce it entire, that the reader may appreciate this curious effort. The remarks between round parenthetical marks are Professor Weldon’s, those between crotchets are mine.

“Mendel treats such characters as yellowness of cotyledons and the
like as if the condition of the character in two given parents
determined its condition in all their subsequent offspring[147]. Now
it is well known to breeders, and is clearly shown in a number of
cases by Galton and Pearson, that the condition of an animal does
not as a rule depend upon the condition of any one pair of ancestors
alone, but in varying degrees upon the condition of all its ancestors
in every past generation, the condition in each of the half-dozen
nearest generations having a quite sensible effect. Mendel does
not take the effect of differences of ancestry into account, but
considers that any yellow-seeded pea, crossed with any green-seeded
pea, will behave in a certain definite way, whatever the ancestry
of the green and yellow peas may have been. (He does not say this
in words, but his attempt to treat his results as generally true of
the characters observed is unintelligible unless this hypothesis be
assumed.) The experiments afford no evidence which can be held to
justify this hypothesis. His observations on cotyledon colour, for
example, are based upon 58 cross-fertilised flowers, all of which
were borne upon ten plants; and we are not even told whether these
ten plants included individuals from more than two races.

[147] Mendel, on the contrary, disregards the “condition of the
character” in the parent altogether; but is solely concerned with the
nature of the characters of the _gametes_.

“The many thousands of individuals raised from these ten plants afford an admirable illustration of the effect produced by crossing a few pairs of plants of known ancestry; but while they show this perhaps better than any similar experiment, they do not afford the data necessary for a statement as to the behaviour of yellow-seeded peas in general, whatever their ancestry, when crossed with green-seeded peas of any ancestry. [Mendel of course makes no such statement.]

“When this is remembered, the importance of the exceptions to dominance of yellow cotyledon-colour, or of smooth and rounded shape of seeds, observed by Tschermak, is much increased; because although they form a small percentage of his whole result, they form a very large percentage of the results obtained with peas of certain races. [Certainly.] The fact that _Telephone_ behaved in crossing on the whole like a green-seeded race of exceptional dominance shows that something other than the mere character of the parental generation operated in this case. Thus in eight out of 27 seeds from the yellow _Pois d’Auvergne_ ♀ × _Telephone_ ♂ the cotyledons were yellow with green patches; the reciprocal cross gave two green and one yellow-and-green seed out of the whole ten obtained; and the cross _Telephone_ ♀ × (yellow-seeded) _Buchsbaum_[148] ♂ gave on one occasion two green and four yellow seeds.

[148] Regarding this “exception” see p. 146.

“So the cross _Couturier_ (orange-yellow) ♀ × the green-seeded _Express_ ♂ gave a number of seeds intermediate in colour. (It is not clear from Tschermak’s paper whether _all_ the seeds were of this colour, but certainly some of them were.) The green _Plein le Panier_ [_Fillbasket_] ♀ × _Couturier_ ♂ in three crosses always gave either seeds of colour intermediate between green and yellow, or some yellow and some green seeds in the same pod. The cross reciprocal to this was not made; but _Express_ ♀ × _Couturier_ ♂ gave 22 seeds of which four were yellowish green[149].

[149] See p. 148.

“These facts show _first_ that Mendel’s law of dominance conspicuously fails for crosses between certain races, while it appears to hold for others; and _secondly_ that the intensity of a character in one generation of a race is no trustworthy measure of its dominance in hybrids. The obvious suggestion is that the behaviour of an individual when crossed depends largely upon the characters of its ancestors[150]. When it is remembered that peas are normally self-fertilised, and that more than one named variety may be selected out of the seeds of a single hybrid pod, it is seen to be probable that Mendel worked with a very definite combination of ancestral characters, and had no proper basis for generalisation about yellow and green peas of any ancestry” [which he never made].

[150] Where was that “logician,” the “consulting-partner,” when this
piece of reasoning passed the firm?

Let us pause a moment before proceeding to the climax. Let the reader note we have been told of _two_ groups of cases in which dominance of yellow failed or was irregular. (Why are not Gärtner’s and Seton’s “exceptions” referred to here?) In one of these groups _Couturier_ was always one parent, either father or mother, and were it not for Tschermak’s own obvious hesitation in regard to his own exceptions (see p. 148), I would gladly believe that _Couturier_--a form I do not know--may be an exceptional variety. _How_ Professor Weldon proposes to explain its peculiarities by reference to ancestry he omits to tell us. The _Buchsbaum_ case is already disposed of, for on Tschermak’s showing, it is an unstable form.

Happily, thanks to Professor Weldon, we know rather more of the third case, that of _Telephone_, which, whether as father or mother, was frequently found by Tschermak to give either green, greenish, or patchwork-seeds when crossed with yellow varieties. It behaves, in short, “like a green-seeded pea of exceptional dominance,” as we are now told. For this dominant quality of _Telephone’s_ greenness we are asked to account _by appeal to its ancestry_. May we not expect, then, this _Telephone_ to be--if not a pure-bred green pea from time immemorial--at least as pure-bred as other green peas which do _not_ exhibit dominance of green at all? Now, what is _Telephone_? Do not let us ask too much. Ancestry takes a lot of proving. We would not reject him “_parce qu’il n’avait que soixante & onze quartiers, & que le reste de son arbre généalogique avait été perdu par l’injure du tems_.”

But with stupefaction we learn from Professor Weldon himself that _Telephone_ is the very variety which he takes _as his type of a permanent and incorrigible mongrel_, a character it thoroughly deserves.

From _Telephone_ he made his colour scale! Tschermak declares the cotyledons to be “yellowish or whitish green, often entirely bright yellow[151].” So little is it a thorough-bred green pea, that it cannot always keep its own self-fertilised offspring green. Not only is this pea a parti-coloured mongrel, but Professor Weldon himself quotes Culverwell that as late as 1882 both _Telegraph_ and _Telephone_ “will always come from one sort, more especially from the green variety”; and again regarding a supposed good sample of _Telegraph_ that “Strange to say, although the peas were taken from one lot, those sown in January produced a great proportion of the light variety known as _Telephone_. These were of every shade of light green up to white, and could have been shown for either variety,” _Gard. Chron._ 1882 (2), p. 150. This is the variety whose green, it is suggested, partially “dominates” over the yellow of _Pois d’Auvergne_, a yellow variety which has a clear lineage of about a century, and probably more. If, therefore, the facts regarding _Telephone_ have any bearing on the significance of ancestry, they point the opposite way from that in which Professor Weldon desires to proceed.

[151] “_Speichergewebe gelblich--oder weisslich--grün, manchmal auch
vollständig hellgelb._” Tschermak (36), p. 480.

In view of the evidence, the conclusion is forced upon me that the suggestion that “ancestry” may explain the facts regarding _Telephone_ has no meaning behind it, but is merely a verbal obstacle. Two words more on _Telephone_. On p. 147 I ventured to hint that if we try to understand the nature of the appearance of green in the offspring of _Telephone_ bred with yellow varieties, we are more likely to do so by comparing the facts with those of false hybridisation than with fluctuations in dominance. In this connection I would call the reader’s attention to a point Professor Weldon misses, that Tschermak _also got yellowish-green seeds from Fillbasket (green) crossed with Telephone_. I suggest therefore that _Telephone’s_ allelomorphs may be in part transmitted to its offspring in a state which needs no union with any corresponding allelomorph of the other gamete, just as may the allelomorphs of “false hybrids.” It would be quite out of place here to pursue this reasoning, but the reader acquainted with special phenomena of heredity will probably be able fruitfully to extend it. It will be remembered that we have already seen the further fact that the behaviour of _Telephone_ in respect to seed-shape was also peculiar (see p. 152).

Whatever the future may decide on this interesting question it is evident that with _Telephone_ (and possibly _Buchsbaum_) we are encountering a _specific_ phenomenon, which calls for specific elucidation and not a case simply comparable with or contradicting the evidence of dominance in general.

In this excursion we have seen something more of the “exceptions.” Many have fallen, but some still stand, though even as to part of the remainder Tschermak entertains some doubts, and, it will be remembered, cautions his reader that of his exceptions some may be self-fertilisations, and some did not germinate[152]. Truly a slender basis to carry the coming structure!

[152] In his latest publication on this subject, the notes to the
edition of Mendel in Ostwald’s _Klassiker_ (pp. 60–61), Tschermak,
who has seen more true exceptions than any other observer, thus
refers to them. As to dominance:--“_Immerhin kommen vereinzelt
auch zweifellose Fälle von Merkmalmischung, d. h. Uebergangsformen
zwischen gelber und grüner Farbe, runder und runzeliger Form vor,
die sich in weiteren Generationen wie dominantmerkmalige Mischlinge
verhalten._” As to purity of the extracted recessives:--_Ganz
vereinzelt scheinen Ausnahmsfälle vorzukommen._"

Küster (22) also in a recent note on Mendelism points out, with
reason, that the number of “exceptions” to dominance that we shall
find, depends simply on the stringency with which the supposed “law”
is drawn. The same writer remarks further that Mendel makes no such
rigid definition of dominance as his followers have done.

But Professor Weldon cannot be warned. He told us the “law of dominance conspicuously fails for crosses between certain races.” Thence the start. I venture to give the steps in this impetuous argument. There are exceptions[153]--a fair number if we count the bad ones--there may be more--must be more--_are_ more--no doubt many more: so to the brink. Then the bold leap: may there not be as many cases one way as the other? We have not tried half the sorts of Peas yet. There is still hope. True we know dominance of many characters in some hundreds of crosses, using some twenty varieties--not to speak of other plants and animals--but we _do_ know some exceptions, of which a few are still good. So dominance may yet be all a myth, built up out of the petty facts those purblind experimenters chanced to gather. Let us take wider views. Let us look at fields more propitious--more what we would have them be! Let us turn to eye-colour: at least there is no dominance in that. Thus Professor Weldon, telling us that Mendel “had no proper basis for generalisation about yellow and green peas of any ancestry,” proceeds to this lamentable passage:--

[153] If the “logician-consulting-partner” will successfully apply
this _Fallacia acervalis_, the “method of the vanishing heap,” to
dominant peas, he will need considerable leisure.

“Now in such a case of alternative inheritance as that of human
eye-colour, it has been shown that a number of pairs of parents, one
of whom has dark and the other blue eyes, will produce offspring of
which nearly one half are dark-eyed, nearly one half are blue-eyed,
a small but sensible percentage being children with mosaic eyes,
the iris being a patch-work of lighter and darker portions. But the
dark-eyed and light-eyed children are not equally distributed among
all families; and it would almost certainly be possible, by selecting
cases of marriage between men and women of appropriate ancestry,
to demonstrate for their families a law of dominance of dark over
light eye-colour, or of light over dark. Such a law might be as
valid for the families of selected ancestry as Mendel’s laws are for
his peas and for other peas of probably similar ancestral history,
but it would fail when applied to dark and light-eyed parents in
general,--that is, to parents of any ancestry who happen to possess
eyes of given colour.”

The suggestion amounts to this: that because there are exceptions to dominance in peas; and because by some stupendous coincidence, or still more amazing incompetence, a bungler might have thought he found dominance of one eye-colour whereas really there was none[154]; therefore Professor Weldon is at liberty to suggest there is a fair chance that Mendel and all who have followed him have either been the victims of this preposterous coincidence not once, but again and again; or else persisted in the same egregious and perfectly gratuitous blunder. Professor Weldon is skilled in the Calculus of Chance: will he compute the probabilities in favour of his hypothesis?

[154] I have no doubt there is no universal dominance in eye-colour.
Is it _quite_ certain there is no dominance at all? I have searched
the works of Galton and Pearson relating to this subject without
finding a clear proof. If there is in them material for this decision
may perhaps be pardoned for failing to discover it, since the
tabulations are not prepared with this point in view. Reference to
the original records would soon clear up the point.

_Ancestry and purity of germ-cells._

To what extent ancestry is likely to elucidate dominance we have now seen. We will briefly consider how laws derived from ancestry stand in regard to segregation of characters among the gametes.

For Professor Weldon suggests that his view of ancestry will explain the facts not only in regard to dominance and its fluctuations but in regard to the purity of the germ-cells. He does not apply this suggestion in detail, for its error would be immediately exposed. In every strictly Mendelian case the _ancestry_ of the pure extracted recessives or dominants, arising from the breeding of first crosses, is identical with that of the impure dominants [or impure recessives in cases where they exist]. Yet the posterity of each is wholly different. The pure extracted forms, in these simplest cases, are no more likely to produce the form with which they have been crossed than was their pure grandparent; while the impure forms break up again into both grand-parental forms.

Ancestry does not touch these facts in the least. They and others like them have been a stumbling-block to all naturalists. Of such paradoxical phenomena Mendel now gives us the complete and final account. Will Professor Weldon indicate how he proposes to regard them?

* * * * *

Let me here call the reader’s particular attention to that section of Mendel’s experiments to which Professor Weldon does not so much as allude. Not only did Mendel study the results of allowing his cross-breds (_DR_’s) to fertilise themselves, giving the memorable ratio

1 _DD_ : 2 _DR_ : 1 _RR_,

but he fertilised those cross-breds (_DR_’s) both with the pure dominant (_D_) and with the pure recessive (_R_) varieties reciprocally, obtaining in the former case the ratio

1 _DD_ : 1 _DR_

and in the latter the ratio

1 _DR_ : 1 _RR_.

The _DD_ group and the _RR_ group thus produced giving on self-fertilisation pure _D_ offspring and pure _R_ offspring respectively, while the _DR_ groups gave again

1 _DD_ : 2 _DR_ : 1 _RR_.

How does Professor Weldon propose to deal with these results, and by what reasoning can he suggest that considerations of ancestry are to be applied to them? If I may venture to suggest what was in Mendel’s mind when he applied this further test to his principles it was perhaps some such considerations as the following. Knowing that the cross-breds on self-fertilisation give

1 _DD_ : 2 _DR_ : 1 _RR_

three explanations are possible:

(_a_) These cross-breds may produce pure _D_ germs of both sexes and
pure _R_ germs of both sexes on an average in equal numbers.

(_b_) _Either_ the female, _or_ the male, gametes may be _alone_
differentiated according to the allelomorphs, into pure _D_’s, pure
_R_’s, and crosses _DR_ or _RD_, the gametes of the other sex being
homogeneous and neutral in regard to those allelomorphs.

(_c_) There may be some neutralisation or cancelling between
characters in _fertilisation_ occurring in such a way that the
well-known ratios resulted. The absence of and inability to transmit
the _D_ character in the _RR_’s, for instance, might have been due
not to the original purity of the germs constituting them, but to
some condition incidental to or connected with fertilisation.

It is clear that Mendel realized (_b_) as a possibility, for he says _DR_ was fertilised with the pure forms to test the composition of its egg-cells, but the reciprocal crosses were made to test the composition of the pollen of the hybrids. Readers familiar with the literature will know that both Gärtner and Wichura had in many instances shown that the offspring of crosses in the form (_a_ × _b_) ♀ × _c_ ♂ were less variable than those of crosses in the form _a_ ♀ × (_b_ × _c_) ♂, &c. This important fact in many cases is observed, and points to differentiation of characters occurring frequently among the male gametes when it does not occur or is much less marked among the maternal gametes. Mendel of course knew this, and proceeded to test for such a possibility, finding by the result that differentiation was the same in the gametes of both sexes[155].

[155] See Wichura (46), pp. 55–6.

Of hypotheses (_b_) and (_c_) the results of recrossing with the two pure forms dispose; and we can suggest no hypothesis but (_a_) which gives an acceptable account of the facts.

It is the purity of the “extracted” recessives and the “extracted” dominants--primarily the former, as being easier to recognize--that constitutes the real proof of the validity of Mendel’s principle.

Using this principle we reach immediately results of the most far-reaching character. These theoretical deductions cannot be further treated here--but of the practical use of the principle a word may be said. Where-ever there is marked dominance of one character the breeder can at once get an indication of the amount of trouble he will have in getting his cross-bred true to either dominant or recessive character. He can only thus forecast the future of the race in regard to each such pair of characters taken severally, but this is an immeasurable advance on anything we knew before. More than this, it is certain that in some cases he will be able to detect the “mule” or heterozygous forms by the statistical frequency of their occurrence or by their structure, especially when dominance is absent, and sometimes even in cases where there is distinct dominance. With peas, the practical seedsman cares, as it happens, little or nothing for those simple characters of seed-structure, &c. that Mendel dealt with. He is concerned with size, fertility, flavour, and numerous similar characters. It is to these that Laxton (invoked by Professor Weldon) primarily refers, when he speaks of the elaborate selections which are needed to fix his novelties.

We may now point tentatively to the way in which some even of these complex cases may be elucidated by an extension of Mendel’s principle, though we cannot forget that there are other undetected factors at work.

_The value of the appeal to Ancestry._

But it may be said that Professor Weldon’s appeal to ancestry calls for more specific treatment. When he suggests ancestry as “one great reason” for the different properties displayed by different races or individuals, and as providing an account of other special phenomena of heredity, he is perhaps not to be taken to mean any definite ancestry, known or hypothetical. He may, in fact, be using the term “ancestry” merely as a brief equivalent signifying the previous history of the race or individual in question. But if such a plea be put forward, the real utility and value of the appeal to ancestry is even less evident than before.

Ancestry, as used in the method of Galton and Pearson, means a definite thing. The whole merit of that method lies in the fact that by it a definite accord could be proved to exist between the observed characters and behaviour of specified descendants and the ascertained composition of their pedigree. Professor Weldon in now attributing the observed peculiarities of _Telephone_ &c. to conjectural peculiarities of pedigree--if this be his meaning--renounces all that had positive value in the reference to ancestry. His is simply an appeal to ignorance. The introduction of the word “ancestry” in this sense contributes nothing. The suggestion that ancestry might explain peculiarities means no more than “we do not know how peculiarities are to be explained.” So Professor Weldon’s phrase “peas of probably similar ancestral history[156]” means “peas probably similar”; when he speaks of Mendel having obtained his results with “a few pairs of plants of known ancestry[157],” he means “a few pairs of known plants” and no more; when he writes that “the law of segregation, like the law of dominance appears to hold only for races of particular ancestry[158],” the statement loses nothing if we write simply “for particular races.” We all know--the Mendelian, best of all--that particular races and particular individuals may, even though indistinguishable by any other test, exhibit peculiarities in heredity.

[156] See above, p. 192.

[157] See above, p. 187.

[158] See above, p. 184.

But though on analysis those introductions of the word “ancestry” are found to add nothing, yet we can feel that as used by Professor Weldon they are intended to mean a great deal. Though the appeal may be confessedly to ignorance, the suggestion is implied that if we did know the pedigrees of these various forms we should then have some real light on their present structure or their present behaviour in breeding. Unfortunately there is not the smallest ground for even this hope.

As Professor Weldon himself tells us[159], conclusions from pedigree must be based on the conditions of the several ancestors; and even more categorically (p. 244), “_The degree to which a parental character affects offspring depends not only upon its development in the individual parent, but on its degree of development in the ancestors of that parent._” [My italics.] Having rehearsed this profession of an older faith Professor Weldon proceeds to stultify it in his very next paragraph. For there he once again reminds us that _Telephone_, the mongrel pea of recent origin, which does not breed true to seed characters, has yet manifested the peculiar power of stamping the recessive characters on its cross-bred offspring, though pure and stable varieties that have exhibited the same characters in a high degree for generations have _not_ that power. As we now know, the presence or absence of a character in a progenitor _may_ be no indication whatever as to the probable presence of the character in the offspring; for the characters of the latter depend on gametic and not on zygotic differentiation.

[159] See above, p. 186.

The problem is of a different order of complexity from that which Professor Weldon suggests, and facts like these justify the affirmation that if we could at this moment bring together the whole series of individuals forming the pedigree of _Telephone_, or of any other plant or animal known to be aberrant as regards heredity, we should have no more knowledge of the nature of these aberrations; no more prescience of the moment at which they would begin, or of their probable modes of manifestation; no more criterion in fact as to the behaviour such an individual would exhibit in crossing[160], or solid ground from which to forecast its posterity, than we have already. We should learn then--what we know already--that at some particular point of time its peculiar constitution was created, and that its peculiar properties then manifested themselves: how or why this came about, we should no more comprehend with the full ancestral series before us, than we can in ignorance of the ancestry. Some cross-breds follow Mendelian segregation; others do not. In some, palpable dominance appears; in others it is absent.

[160] Beyond an indication as to the homogeneity or “purity” of its
gametes at a given time.

If there were no ancestry, there would be no posterity. But to answer the question _why_ certain of the posterity depart from the rule which others follow, we must know, not the ancestry, but how it came about _either_ that at a certain moment a certain gamete divided from its fellows in a special and unwonted fashion; _or_, though the words are in part tautological, the reason why the union of two particular gametes in fertilisation took place in such a way that gametes having new specific properties resulted[161]. No one yet knows how to use the facts of ancestry for the elucidation of these questions, or how to get from them a truth more precise than that contained in the statement that a diversity of specific consequences (in heredity) may follow an apparently single specific disturbance. Rarely even can we see so much. The appeal to ancestry, as introduced by Professor Weldon, masks the difficulty he dare not face.

[161] May there be a connection between the extraordinary fertility
and success of the _Telephone_ group of peas, and the peculiar
frequency of a blended or mosaic condition of their allelomorphs?
The conjecture may be wild, but it is not impossible that the two
phenomena may be interdependent.

In other words, it is the _cause of variation_ we are here seeking. To attack that problem no one has yet shown the way. Knowledge of a different order is wanted for that task; and a compilation of ancestry, valuable as the exercise may be, does not provide that particular kind of knowledge.

Of course when once we have discovered by experiment that--say, _Telephone_--manifests a peculiar behaviour in heredity, we can perhaps make certain forecasts regarding it with fair correctness; but that any given race or individual will behave in such a way, is a fact not deducible from its ancestry, for the simple reason that organisms of identical ancestry may behave in wholly distinct, though often definite, ways.

It is from this hitherto hopeless paradox that Mendel has begun at last to deliver us. The appeal to ancestry is a substitution of darkness for light.