Species and Varieties, Their Origin by Mutation Part 15

Another corollary of Mendel's law is the following. In each generation two groups return to purity, and one-half remains hybrid. These last will repeat the same phenomenon of splitting in their progeny, and it is easily seen that the same rule will hold good for all succeeding generations. According to Mendel's principle, in each year there is a new hybridization, differing in no respect from the first and original one. If the hybrids only are propagated, each year will show one-fourth of the offspring returning to the specific character, one-fourth a.s.suming the type of the variety and one-half remaining hybrid. I have tested this with a hybrid between the ordinary nightshade with black berries, and its variety, _Solanum nigrum chlorocarpum_, with pale yellow fruits. Eight generations of the hybrids were cultivated, [299]

disregarding always the reverting offspring. At the end I counted the progeny of the sixth and seventh generations and found figures for their three groups of descendants, which exactly correspond to Mendel's formula.

Until now we have limited ourselves to the consideration of single differentiating units. This discussion gives a clear insight into the fundamental phenomena of hybrid fertilization. It at once shows the correctness of the a.s.sumption of unit-characters, and of their pairing in the s.e.xual combinations.

But Mendel's law is not at all restricted to these simple cases. Quite on the contrary, it explains the most intricate questions of hybridization, providing they do not transgress the limits of symmetrical unions. But in this realm nearly all results may be calculated beforehand, on the ground of the principle of probability.

Only one more a.s.sumption need be discussed. The several pairs of antagonistic characters must be independent from, and uninfluenced by, one another. This premise seems to hold good in the vast majority of cases, though rare exceptions seem to be not entirely wanting. Hence the necessity of taking all predictions from Mendel's law only as probabilities, which will prove true in most, but not necessarily in all cases. [300] But here we will limit ourselves to normal cases.

The first example to be considered is obviously the a.s.sumption that the parents of a cross differ from each other in respect to two characters.

A good ill.u.s.trative example is afforded by the thorn-apple. I have crossed the blue flowered th.o.r.n.y form, usually known as _Datura Tatula_, with the white thornless type, designated as _D. Stramonium inermis_.

Thorns and blue pigment are obviously active qualities, as they are dominant in the hybrids. In the second generation both pairs of characters are resolved into their const.i.tuents and paired anew according to Mendel's law. After isolating my hybrids during the period of flowering, I counted among their progeny:

128 individuals with blue flowers and thorns 47 individuals with blue flowers and without thorns 54 individuals with white flowers and thorns 21 individuals with white flowers and without thorns ---- 250

The significance of these numbers may easily be seen, when we calculate what was to be expected on the a.s.sumption that both characters follow Mendel's law, and that both are independent from each other. Then we would have three-fourths blue offspring and one-fourth individuals with white flowers. Each of these [301] two groups would consist of thorn-bearing and thornless plants, in the same numerical relation.

Thus, we come to the four groups observed in our experiment, and are able to calculate their relative size in the following way:

Proportion Blue with thorns 3/4 X 3/4 = 9/16 = 56.25% 9 Blue, unarmed 3/4 X 1/4 = 3/16 = 18.75% 3 White with thorns 1/4 X 3/4 = 3/16 = 18.75% 3 White, unarmed 1/4 X 1/4 = 1/16 = 6.25% 1

In order to compare this inference from Mendel's law and the a.s.sumption of independency, with the results of our experiments, we must calculate the figures of the latter in percentages. In this way we find:

Found Calculated Blue with thorns 128=51% 56.25% Blue unarmed 47=19% 18.75% White with thorns 54=22% 18.75% White unarmed 21= 8% 6.25%

The agreement of the experimental and the theoretical figures is as close as might be expected.

This experiment is to be considered only as an ill.u.s.trative example of a rule of wide application. The rule obviously will hold good in all such cases as comply with the two conditions already premised, viz.: that each character agrees with Mendel's law, and that both are wholly independent of each other. It is clear that our figures show the numerical composition [302] of the hybrid offspring for any single instance, irrespective of the morphological nature of the qualities involved.

Mendel has proved the correctness of these deductions by his experiments with peas, and by combining their color (yellow or green) with the chemical composition (starch or sugar) and other pairs of characters. I will now give two further ill.u.s.trations afforded by crosses of the ordinary campion. I used the red-flowered or day-campion, which is a perennial herb, and a smooth variety of the white evening-campion, which flowers as a rule in the first summer. The combination of flower-color and p.u.b.escence gave the following composition for the second hybrid generation:

Number % Calculation Hairy and red 70 44 56.25% Hairy and white 23 14 18.75% Smooth and red 46 23 18.75% Smooth and white 19 12 6.25%

For the combination of p.u.b.escence and the capacity of flowering in the first year I found:

Number % Calculated Hairy, flowering 286 52 56.25% Hairy, without stem 128 23 18.75% Smooth, flowering 96 17 18.75% Smooth, without stem 42 8 6.25%

Many other cases have been tested by different writers and the general result is the [303] applicability of Mendel's formula to all cases complying with the given conditions.

Intentionally I have chosen for the last example two pairs of antagonisms, relating to the same pair of plants, and which may be tested in one experiment and combined in one calculation.

For the latter we need only a.s.sume the same conditions as mentioned before, but now for three different qualities. It is easily seen that the third quality would split each of our four groups into two smaller ones in the proportion of 3/4 : 1/4.

We would then get eight groups of the following composition:

9/16 X 3/4 = 27/64 or 42.2% 9/16 X 1/4 = 9/64 " 14.1% 3/16 X 3/4 = 9/64 " 14.1% 3/16 X 1/4 = 3/64 " 4.7% 3/16 X 3/4 = 9/64 " 14.1% 3/16 X 1/4 = 3/64 " 4.7% 1/16 X 3/4 = 3/64 " 4.7% 1/16 X 1/4 = 1/64 " 1.6%

The characters chosen for our experiment include the absence of stem and flowers in the first year, and therefore would require a second year to determine the flower-color on the perennial specimens. Instead of doing so I have taken another character, shown by the teeth of the capsules when opening. These curve outwards [304] in the red campion, but lack this capacity in the evening-campion, diverging only until an upright position is reached. The combination of hairs, colors and teeth gives eight groups, and the counting of their respective numbers of individuals gave the following:

Teeth Hairs Flowers of capsules Number % Calculated

Hairy red curved 91 47 42.2% Hairy red straight 15 7.5 14.1% Hairy white curved 23 12 14.1% Hairy white straight 17 8.5 4.7% Smooth red curved 23 12 14.1% Smooth red straight 9 4.5 4.7% Smooth white curved 5 2.5 4.7% Smooth white straight 12 6 1.6%

The agreement is as comprehensive as might be expected from an experiment with about 200 plants, and there can be no doubt that a repet.i.tion on a larger scale would give still closer agreement.

In the same way we might proceed to crosses with four or more differentiating characters. But each new character will double the number of the groups. Four characters will combine into 16 groups, five into 32, six into 64, seven into 128, etc. Hence it is easily seen that the size of the experiments must be made larger and larger in the same ratio, if we intend to expect numbers equally trustworthy. For [305]

seven differentiating marks 16,384 individuals are required for a complete series. And in this set the group with the seven attributes all in a latent condition would contain only a single individual.

Unfortunately the practical value of these calculations is not very great. They indicate the size of the cultures required to get all the possible combinations, and show that in ordinary cases many thousands of individuals have to be cultivated, in order to exhaust the whole range of possibilities. They further show that among all these thousands, only very few are constant in all their characters; in fact, it may easily be seen that with seven differentiating points among the 16,384 named above, only one individual will have all the seven qualities in pure active, and only one will have them all in a purely dormant condition.

Then there will be some with some attributes active and others latent, but their numbers will also be very small. All others will split up in the succeeding generation in regard to one or more of their apparently active marks. And since only in very rare cases the stable hybrids can be distinguished by external characters from the unstable ones, the stability of each individual bearing a desired combination of characters would have to be established by experiment [306] after pure fertilization. Mendel's law teaches us to predict the difficulties, but hardly shows any way to avoid them. It lays great stress on the old prescript of isolation and pure fertilization, but it will have to be worked out and applied to a large number of practical cases before it will gain a preeminent influence in horticultural practice.

Or, as Bailey states it, we are only beginning to find a pathway through the bewildering maze of hybridization.

This pathway is to be laid out with regard to the following considerations. We are not to cross species or varieties, or even accidental plants. We must cross unit-characters, and consider the plants only as the bearers of these units. We may a.s.sume that these units are represented in the hereditary substance of the cell-nucleus by definite bodies of too small a size to be seen, but const.i.tuting together the chromosomes. We may call these innermost representatives of the unit-characters pangenes, in accordance with Darwin's hypothesis of pangenesis, or give them any other name, or we may even wholly abstain from such theoretical discussion, and limit ourselves to the conception of the visible character-units. These units then may be present, or lacking and in the first case active, or latent.

[307] True elementary species differ from each other in a number of unit-characters, which do not contrast. They have arisen by progressive mutation. One species has one kind of unit, another species has another kind. On combining these, there can be no interchange. Mendelism a.s.sumes such an interchange between units of the same character, but in a different condition. Activity and latency are such conditions, and therefore Mendel's law obviously applies to them. They require pairs of antagonistic qualities, and have no connection whatever with those qualities, which do not find an opponent in the other parent. Now, only pure varieties afford such pure conditions. When undergoing further modifications, some of them may be in the progressive line and others in the retrogressive. Progressive modifications give new units, which are not in contrast with any other, retrograde changes turn active units into the latent condition and so give rise to pairs. Ordinary species generally originate in this way, and hence differ from each other partly in specific, partly in varietal characters. As to the first, they give in their hybrids stable peculiarities, while as to the latter, they split up according to Mendel's law.

Unpaired or unbalanced characters lie side by side with paired or balanced qualities, and they [308] do so in nearly all the crosses made for practical purposes, and in very many scientific experiments. Even Mendel's peas were not pure in this respect, much less do the campions noted above differ only in Mendelian characters.

Comparative and systematic studies must be made to ascertain the true nature of every unit in every single plant, and crossing experiments must be based on these distinctions in order to decide what laws are applicable in any case.

[309]

D. EVER-SPORTING VARIETIES

LECTURE XI

STRIPED FLOWERS

Terminology is an awkward thing. It is as disagreeable to be compelled to make new names, as to be constrained to use the old faulty ones.

Different readers may a.s.sociate different ideas with the same terms, and unfortunately this is the case with much of the terminology of the science of heredity and variability. What are species and what are varieties? How many different conceptions are conveyed by the terms constancy and variability? We are compelled to use them, but we are not at all sure that we are rightly understood when we do so.

Gradually new terms arise and make their way. They have a more limited applicability than the old ones, and are more narrowly circ.u.mscribed.

They are not to supplant the older terms, but permit their use in a more general way.

[310] One of these doubtful terms is the word _sport_. It often means bud-variation, while in other cases it conveys the same idea as the old botanical term of mutation. But then all sorts of seemingly sudden variations are occasionally designated by the same term by one writer or another, and even accidental anomalies, such as teratological ascidia, are often said to arise by sports.

If we compare all these different conceptions, we will find that their most general feature is the suddenness and the rarity of the phenomenon.

They convey the idea of something unexpected, something not always or not regularly occurring. But even this demarcation is not universal, and there are processes that are regularly repeated and nevertheless are called sports. These at least should be designated by another name.

In order to avoid confusion as far as possible, with the least change in existing terminology, I shall use the term "ever-sporting varieties" for such forms as are regularly propagated by seed, and of pure and not hybrid origin, but which sport in nearly every generation. The term is a new one, but the facts are for the most part new, and require to be considered in a new light. Its meaning will become clearer at once when the ill.u.s.trations afforded by [311] striped flowers are introduced. In the following discussion it will be found most convenient to give a summary of what is known concerning them, and follow this by a consideration of the detailed evidence obtained experimentally, which supports the usage cited.

The striped variety of the larkspur of our gardens is known to produce monochromatic flowers, in addition to striped ones. They may be borne by the same racemes, or on different branches, or some seedlings from the same parent-plant may bear monochromatic flowers while others may be striped. Such deviations are usually called sports. But they occur yearly and regularly and may be observed invariably when the cultures are large enough. Such a variety I shall call "ever-sporting."

The striped larkspur is one of the oldest garden varieties. It has kept its capacity of sporting through centuries, and therefore may in some sense be said to be quite stable. Its changes are limited to a rather narrow circle, and this circle is as constant as the peculiarities of any other constant species or variety. But within this circle it is always changing from small stripes to broad streaks, and from them to pure colors. Here the variability is a thing of absolute constancy, while the constancy consists in eternal changes. Such apparent [312]

contradictions are unavoidable, when we apply the old term to such unusual though not at all new cases. Combining the stability and the qualities of sports in one word, we may evidently best express it by the new term of eversporting variety.

We will now discuss the exact nature of such varieties, and of the laws of heredity which govern them. But before doing so, I might point out, that this new type is a very common one. It embraces most of the so-called variable types in horticulture, and besides these a wide range of anomalies.

Every ever-sporting variety has at least two different types, around and between which it varies in numerous grades, but to which it is absolutely limited. Variegated leaves fluctuate between green and white, or green and yellow, and display these colors in nearly all possible patterns. But there variability ends, and even the patterns are ordinarily narrowly prescribed in the single varieties. Double flowers afford a similar instance. On one side the single type, on the other the nearly wholly double model are the extreme limits, between which the variability is confined. So it is also with monstrosities. The race consists of anomalous and normal individuals, and displays between them all possible combinations of normal and monstrous [313] parts. But its variability is restricted to this group. And large as the group may seem on first inspection, it is in reality very narrow. Many monstrosities, such as fasciated branches, pitchers, split leaves, peloric flowers, and others const.i.tute such ever-sporting varieties, repeating their anomalies year by year and generation after generation, changing as much as possible, but remaining absolutely true within their limits as long as the variety exists.

It must be a very curious combination of the unit-characters which causes such a state of continuous variability. The pure quality of the species must be combined with the peculiarity of the variety in such a way, that the one excludes the other, or modifies it to some extent, although both never fully display themselves in the same part of the same plant. A corolla cannot be at once monochromatic and striped, nor can the same part of a stem be twisted and straight. But neighboring organs may show the opposite attributes side by side.