It must be the object of further experiments to ascertain whether the law of development discovered for _Pisum_ applies also to the hybrids of other plants. To this end several experiments were recently commenced. Two minor experiments with species of _Phaseolus_ have been completed, and may be here mentioned.
An experiment with _Phaseolus vulgaris_ and _Phaseolus nanus_ gave results in perfect agreement. _Ph. nanus_ had together with the dwarf axis simply inflated green pods. _Ph. vulgaris_ had, on the other hand, an axis 10 feet to 12 feet high, and yellow coloured pods, constricted when ripe. The ratios of the numbers in which the different forms appeared in the separate generations were the same as with _Pisum_. Also the development of the constant combinations resulted according to the law of simple combination of characters, exactly as in the case of _Pisum_. There were obtained--
Constant Axis Colour of Form of combinations the unripe pods. the ripe pods.
1 long green inflated 2 " " constricted 3 " yellow inflated 4 " " constricted 5 short green inflated 6 " " constricted 7 " yellow inflated 8 " " constricted
The green colour of the pod, the inflated forms, and the long axis were, as in _Pisum_, dominant characters.
Another experiment with two very different species of _Phaseolus_ had only a partial result. _Phaseolus nanus_, L., served as seed parent, a perfectly constant species, with white flowers in short racemes and small white seeds in straight, inflated, smooth pods; as pollen parent was used _Ph. multiflorus_, W., with tall winding stem, purple-red flowers in very long racemes, rough, sickle-shaped crooked pods, and large seeds which bore black flecks and splashes on a peach-blood-red ground.
The hybrids had the greatest similarity to the pollen parent, but the flowers appeared less intensely coloured. Their fertility was very limited; from seventeen plants, which together developed many hundreds of flowers, only forty-nine seeds in all were obtained. These were of medium size, and were flecked and splashed similarly to those of _Ph. multiflorus_, while the ground colour was not materially different. The next year forty-four plants were raised from these seeds, of which only thirty-one reached the flowering stage. The characters of _Ph. nanus_, which had been altogether latent in the hybrids, reappeared in various combinations; their ratio, however, with relation to the dominant characters was necessarily very fluctuating owing to the small number of trial plants. With certain characters, as in those of the axis and the form of pod, it was, however, as in the case of _Pisum_, almost exactly 1 : 3.
Insignificant as the results of this experiment may be as regards the determination of the relative numbers in which the various forms appeared, it presents, on the other hand, the phenomenon of a remarkable change of colour in the flowers and seed of the hybrids. In _Pisum_ it is known that the characters of the flower- and seed-colour present themselves unchanged in the first and further generations, and that the offspring of the hybrids display exclusively the one or the other of the characters of the original stocks[43]. It is otherwise in the experiment we are considering. The white flowers and the seed-colour of _Ph. nanus_ appeared, it is true, at once in the first generation [_from_ the hybrids] in one fairly fertile example, but the remaining thirty plants developed flower colours which were of various grades of purple-red to pale violet. The colouring of the seed-coat was no less varied than that of the flowers. No plant could rank as fully fertile; many produced no fruit at all; others only yielded fruits from the flowers last produced, which did not ripen. From fifteen plants only were well-developed seeds obtained. The greatest disposition to infertility was seen in the forms with preponderantly red flowers, since out of sixteen of these only four yielded ripe seed. Three of these had a similar seed pattern to _Ph. multiflorus_, but with a more or less pale ground colour; the fourth plant yielded only one seed of plain brown tint. The forms with preponderantly violet coloured flowers had dark brown, black-brown, and quite black seeds.
[43] [This is the only passage where Mendel can be construed as asserting universal dominance for _Pisum_; and even here, having regard to the rest of the paper, it is clearly unfair to represent him as predicating more than he had seen in his own experiments. Moreover in flower and seed-coat colour (which is here meant), using his characters dominance must be almost universal, if not quite.]
The experiment was continued through two more generations under similar unfavourable circumstances, since even among the offspring of fairly fertile plants there were still some which were less fertile or even quite sterile. Other flower- and seed-colours than those cited did not subsequently present themselves. The forms which in the first generation [bred from the hybrids] contained one or more of the recessive characters remained, as regards these, constant without exception. Also of those plants which possessed violet flowers and brown or black seed, some did not vary again in these respects in the next generation; the majority, however, yielded, together with offspring exactly like themselves, some which displayed white flowers and white seed-coats. The red flowering plants remained so slightly fertile that nothing can be said with certainty as regards their further development.
Despite the many disturbing factors with which the observations had to contend, it is nevertheless seen by this experiment that the development of the hybrids, with regard to those characters which concern the form of the plants, follows the same laws as does _Pisum_. With regard to the colour characters, it certainly appears difficult to perceive a substantial agreement. Apart from the fact that from the union of a white and a purple-red colouring a whole series of colours results, from purple to pale violet and white, the circumstance is a striking one that among thirty-one flowering plants only one received the recessive character of the white colour, while in _Pisum_ this occurs on the average in every fourth plant.
Even these enigmatical results, however, might probably be explained by the law governing _Pisum_ if we might assume that the colour of the flowers and seeds of _Ph. multiflorus_ is a combination of two or more entirely independent colours, which individually act like any other constant character in the plant. If the flower colour A were a combination of the individual characters _A_{1} + _A_{2} + ... which produce the total impression of a purple colouration, then by fertilisation with the differentiating character, white colour, _a_, there would be produced the hybrid unions _A_{1}_a_ + _A_{2}_a_ + ... and so would it be with the corresponding colouring of the seed-coats[44]. According to the above assumption, each of these hybrid colour unions would be independent, and would consequently develop quite independently from the others. It is then easily seen that from the combination of the separate developmental series a perfect colour-series must result. If, for instance, _A_ = _A_{1} + _A_{2}, then the hybrids _A_{1}_a_ and _A_{2}_a_ form the developmental series--
_A_{1} + 2_A_{1}_a_ + _a_ _A_{2} + 2_A_{2}_a_ + _a_.
[44] [It appears to me clear that this expression is incorrectly given, and the argument regarding compound characters is consequently not legitimately developed. The original compound character should be represented as _A_{1}_A_{2}_A_{3} ... which when fertilised by _a_{1} gives _A_{1}_A_{2}_A_{3} ... a as the hybrid of the first generation. Mendel practically tells us these were all alike, and there is nothing to suggest that they were diverse. When on self-fertilisation, they break up, they will produce the gametes he specifies; but they may also produce _A_{1}_A_{1} and _A_{2}_A_{2}, _A_{1}_A_{2}_a_, &c., thereby introducing terms of a nature different from any indicated by him. That this point is one of the highest significance, both practical and theoretical, is evident at once.]
The members of this series can enter into nine different combinations, and each of these denotes another colour[45]--
1 _A_{1}A_{2}_ 2 _A_{1}aA_{2}_ 1 _A_{2}a_ 2 _A_{1}A_{2}a_ 4 _A_{1}aA_{2}a_ 2 _A_{2}aa_ 1 _A_{1}a_ 2 _A_{1}aa_ 1 _aa_.
[45] [It seems very doubtful if the zygotes are correctly represented by the terms _A_{1}aA_{2}a_, _A_{2}aa_, _A_{1}aa_; for in the hybrids _A_{1}a_, &c. the allelomorphs _A_{1}_ and _a_, &c. should by hypothesis be separated in the gametes.]
The figures prescribed for the separate combinations also indicate how many plants with the corresponding colouring belong to the series. Since the total is sixteen, the whole of the colours are on the average distributed over each sixteen plants, but, as the series itself indicates, in unequal proportions.
Should the colour development really happen in this way, we could offer an explanation of the case above described, viz. that the white flowers and seed-coat colour only appeared once among thirty-one plants of the first generation. This colouring appears only once in the series, and could therefore also only be developed once in the average in each sixteen, and with three colour characters only once even in sixty-four plants.
It must, however, not be forgotten that the explanation here attempted is based on a mere hypothesis, only supported by the very imperfect result of the experiment just described. It would, however, be well worth while to follow up the development of colour in hybrids by similar experiments, since it is probable that in this way we might learn the significance of the extraordinary variety in the colouring of our ornamental flowers.
So far, little at present is known with certainty beyond the fact that the colour of the flowers in most ornamental plants is an extremely variable character. The opinion has often been expressed that the stability of the species is greatly disturbed or entirely upset by cultivation, and consequently there is an inclination to regard the development of cultivated forms as a matter of chance devoid of rules; the colouring of ornamental plants is indeed usually cited as an example of great instability. It is, however, not clear why the simple transference into garden soil should result in such a thorough and persistent revolution in the plant organism. No one will seriously maintain that in the open country the development of plants is ruled by other laws than in the garden bed. Here, as there, changes of type must take place if the conditions of life be altered, and the species possesses the capacity of fitting itself to its new environment. It is willingly granted that by cultivation the origination of new varieties is favoured, and that by man’s labour many varieties are acquired which, under natural conditions, would be lost; but nothing justifies the assumption that the tendency to the formation of varieties is so extraordinarily increased that the species speedily lose all stability, and their offspring diverge into an endless series of extremely variable forms. Were the change in the conditions of vegetation the sole cause of variability we might expect that those cultivated plants which are grown for centuries under almost identical conditions would again attain constancy. That, as is well known, is not the case, since it is precisely under such circumstances that not only the most varied but also the most variable forms are found. It is only the _Leguminosæ_, like _Pisum_, _Phaseolus_, _Lens_, whose organs of fertilisation are protected by the keel, which constitute a noteworthy exception. Even here there have arisen numerous varieties during a cultural period of more than 1000 years; these maintain, however, under unchanging environments a stability as great as that of species growing wild.
It is more than probable that as regards the variability of cultivated plants there exists a factor which so far has received little attention. Various experiments force us to the conclusion that our cultivated plants, with few exceptions, are _members of various hybrid series_, whose further development in conformity with law is changed and hindered by frequent crossings _inter se_. The circumstance must not be overlooked that cultivated plants are mostly grown in great numbers and close together, affording the most favourable conditions for reciprocal fertilisation between the varieties present and the species itself. The probability of this is supported by the fact that among the great array of variable forms solitary examples are always found, which in one character or another remain constant, if only foreign influence be carefully excluded. These forms develop precisely as do those which are known to be members of the compound hybrid series. Also with the most susceptible of all characters, that of colour, it cannot escape the careful observer that in the separate forms the inclination to vary is displayed in very different degrees. Among plants which arise from _one_ spontaneous fertilisation there are often some whose offspring vary widely in the constitution and arrangement of the colours, while others furnish forms of little deviation, and among a greater number solitary examples occur which transmit the colour of the flowers unchanged to their offspring. The cultivated species of _Dianthus_ afford an instructive example of this. A white-flowered example of _Dianthus caryophyllus_, which itself was derived from a white-flowered variety, was shut up during its blooming period in a greenhouse; the numerous seeds obtained therefrom yielded plants entirely white-flowered like itself. A similar result was obtained from a subspecies, with red flowers somewhat flushed with violet, and one with flowers white, striped with red. Many others, on the other hand, which were similarly protected, yielded progeny which were more or less variously coloured and marked.
Whoever studies the colouration which results in ornamental plants from similar fertilisation can hardly escape the conviction that here also the development follows a definite law which possibly finds its expression _in the combination of several independent colour characters_.
