THE MENDELIAN DISCOVERY.*
THE chief merit of Mr. Darbishire's book is that he makes the apparently complicated facts of heredity, discovered by Mendel, clear to the meanest intelligence. That is high praise in itself, but the work has other merits. Mr. Darbishire is by profession a teacher, but be has himself worked at breeding experiments with peas and mice. Out of a mass of compli- cated detail he has the art of selecting the salient features for the uninstructed reader, He does not hesitate to repeat at thing several times, knowing that the uninstructed reader will have forgotten, or not understood, what was said on a previous page. He manages to write 176 pages. before lie uses the words gamete and zygote, and 246 pages without intro- ducing the word allelomorphe. His illustrations are well chosen and clearly reproduced. A few minutes' study of Mr. Darbishire's diagrammatic frontispiece will impress for ever on the poorest intellect the facts of Mendelian inherit ante in tall and dwarf pea plants. Having grasped and, remembered the clue the reader can apply it and study for himself the inheritance. of colour in mice, sweet peas, or horses, of combs in fowls, horns in sheep, and eye colour or sex in mankind. Having explained the facts concerning Mendel's experiments and observations to what we have yen.: tured to call the uninstructed reader, Mr. Darbishire
tains an attitude of healthy scepticism as to the inferences to be drawn from the facts. It is this part of the book which will prove 'of the greatest interest to what we may call the scientific or instructed reader, to whom much that Mr. Darbi- shire explains so clearly will be familiar. But seeing that the ordinary educated person has either not yet beard of Mendel, or has only the vaguest notion of who he was and what be discovered, it would have been well if Mr. Darbishire had devoted a chapter to Mendel's life and career, and has given some description of the monastery garden where be grew his pea plants.
It will be as well to assume that most readers of the Spectator know nothing of Mendel's discoveries. Gregor Mendel was born in 1822 (not in 1820, as Mr. Darbishire says on p. 193) and died in 1882. He was an Austrian Augustinian monk, and spent much time in breeding hybrids, especially between the various varieties of edible pea, "'taunt ultimo!, Others,, such as John Goss of Hather- leigh, bad tried similar experiments. The genius of Mendel, for the first time, grasped the significance of the results and
• Brooding awl ad Ifiestiaaissa Discovery. Hy A. D. Darhishire, M.A. With Illuataistione is Colour and Meek and White. Leaden r Cassell and Co. [7s..0d. net.) offered an interpretation which is now generally accepted. In our opinion Mendel's success in discovering something about heredity where others failed is due to two things. In the first place ho chose two clearly contrasted characters and devoted the whole of his attention to the manner in which they, and they only, were inherited. The kind of characters he chose in peas were, for instance, tallness and dwarfness, green pod and yellow pod, grey seed-coat and white seed-coat, round seed and wrinkled, seed. The results which he obtained will be described below. In the second place, having got his first generation, Mendel went on breeding front that for several generations, tabulating and counting his results year by year with the utmost care. The strange part of the story is that, though Mendel published his observations in the journal of the local scientific society at Briinn in 1865 and corresponded with Niigeli, his paper remained almost unnoticed until it was raised from obscurity simultaneously by Correns, de 'Vries, and Tschermak in 1900.
Now, after this lengthy preface, let us state in the clearest language and the fewest words possible the facts which Mendel discovered. Mendel crossed a dwarf pea plant with a tall pea plant by brushing the pistil of the one with the pollen of the other. He sowed the seeds produced, and the first hybrid generation of plants came up all tall. He sowed next year the seeds of these tall plants, which were allowed to pollinate their own pistils or breed together. The second hybrid generation, the offspring of nothing but tall plants, produced on an average three tails to one dwarf plant. These tails and dwarfs were allowed again to pollinate their own pistils, and the seeds produced were sown. Then, in the third hybrid generation, came a strange result. The dwarf plants produced nothing but dwarfs, and continued to produce pure dwarfs for mai One out of the three tails produced nothing but tells, and–continued to produce pure falls for ever. But two of the tells for it would be more correct to say fifty per cent, on an average of the generation) produced, like the second hybrid generation, on an average three tails to one dwarf plant. This figure of 3 1 is called the Mendelian ratio ; and, again, twenty-five per cent. of this third genera- tion, being falls, bred pure tells for ever and another twenty- five per cent. of the same generation, being dwarfs, bred pure dwarfs for ever. We will not complicate this simple outline with other factors. If the reader who wishes to understand will draw a genealogical table on a scrap of paper the scheme will be clear.
Two terms invented by Mendel must now be explained. When he first crossed tails and dwarfs the result was nothing but tails. He therefore called tallness a " dominant "character and dwarfness "recessive." The dwarfness was only hidden, as we have seen, because in the next generation dwarfs were produced from tails. If we ask why one character is dominant and what recessiveness implies, Mr. Darbishire tells us that no one knows. These are names for observed facts. He is always unwilling to pretend that he has explained any problem by restating it in a different form. In all parts of the world zoologists and botanists are making Mendelian breeding ex- periments, and no doubt much light on the problems will be obtained when speculation and hypothesis are tested by observ- ing facts. A great many pairs of characters in plants and animals have been found to follow the inheritance of tallness and dwarfness in peas. Duplex, or brown, eyes are domi- nant, and simplex, or blue, eyes are recessive in mankind. It follows that two pure blue-eyed parents can never have but blue-eyed children, though two brown-eyed parents may hare blue-eyed children, since blue may be masked by brown. Neither Mendel nor any one else has explained the phenomenon of dominance ; but he offered an explanation of the strange proportion of 3 : 1 and the appearance of pure tall and pure dwarf strains. His explanation was that the pairs of characters, such as tallness and dwarfness, always became segregated, or separated, in the sexual cella of hybrids. The hybrid tall peas of the first hybrid generation will produce half of its ovules with the character of tallness in them and half with the character of dwarfness. It will be the same with the pollen grains. These mate by chance, and according to the law of chances the Mendelian proportion of three to one will result, as it does result. It must be remembered that when a tall mates with a dwarf the offspring is tall; but dwarfness is masked by tallness. When two tells or two dwarfs meet we have a pure strain. A similar result, so far as chance goes, would be produced by drawing pairs of: counters from a bag containing equal numbers of red audi white counters.
Mr. Darbishire dwells almost too shortly on the value which must accrue to breeders from a study of Mendelian phenomena.
of heredity. This, of course; applies only to certain pairs of. characters. Some characters appear to blend in hybrids. Desirable characters can be crossed and pure strains obtained- combining for ever the two characters. A strain of. Nvbeat has been obtained which is immene from the disease of rust. Some strains of hybrids such as the blue Andalusian fowl can never, by a thousand generations of selection, be got to breed pure. It is also possible, knowing the pedigrees of horses, to predict the colour of their offspring. Until the matter haa, been further studied by experimental, breeding, or by collect,. ing human pedigrees, it would be unwise to predict what limits may be set to the breeder's power. Mr. Darbishire cautious :- " Very little—it may be said almost nothing—is known of the causes which determine the origin of new characters, eij,her int the domestic state or in a state of nature. A.t any rate there is not enough known to base a scientific practice of breeding upon. But the art of breeding by the recombination of already existing characters—and it is a question whether in its ultimate analysis this may not be all that the breeder can do—has been brought to a point of extreme precision by the discovery made by Mendel, in 1865."
To the scientific biologist Mendelian phenomena are, perhaps, among the most interesting discoveries' ever ,made. That the facts apply to mankind there is no doubt, though to what characters they apply is still a matter for investigation. We shall perhaps some day picture a human being as a bundle of physical and mental unit characters which exist in paira but are inherited alternatively. So far as the general doctrine of evolution goes, Mendel's discovery and Professor de Vries' theory of the origin of species by mutations must be thought over together. The biologist may be forgiven for deploring that the mind of Darwin never had an opportunity of con. sidering the facts discovered by Mendel.
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