flashed upon him all the possible effects of the struggle for existence. In two days the entire draft of the paper showing the application of the views to organic species in general was sketched and posted to Darwin, who had been working upon the verification of the same idea for twenty years. Darwin had actually written out a sketch of his theory in 1842; and in 1844 this sketch was enlarged to 230 folio pages, giving a complete presentation of the arguments afterwards set forth in the Origin of Species.

When Darwin received Wallace's paper he brought the communication before Sir Charles Lyell and Sir Joseph Hooker, and urged that it should be printed at once. Upon their advice, however, he consented to let an extract from the sketch of 1844 be presented to the Linnean Society with Wallace's paper on July 1st, 1858. "The one great result which I claim for my paper of 1858," said Wallace, "is that it compelled Darwin to write and publish his Origin of Species without further delay."

For evolution to proceed in any particular direction, it is necessary for variations in that direction to be encouraged continually, and for the trend of tendency of the parent to be inherited by the offspring. Variation and heredity are thus complementary to each other; and until the laws by which they are determined are understood, no evolutionary theory can be said to be complete. The rule of the road of heredity was discovered in 1866 by Gregor Johann Mendel, an Augustinian monk, Abbot of the old monastery at Brünn, Austria; but the biological world remained unaware of it for thirty years, and Darwin died without knowing of the epoch-making work of his contemporary. "Had Mendel's work,' says Prof. W. Bateson, the leading authority upon it,

come

7

Photo. L. Kuhn & Co., Paris.

V. THE THINKER.

Statue by Auguste Rodin, (1840-...), Pantheon, Paris.

into the hands of Darwin, it is not too much to say that the history of the development of evolutionary philosophy would have been very different from that which we have witnessed."

For eight years Mendel carried on experiments on the hybridisation of peas and other plants in the large garden of the cloister of which he was Abbot or Prälat. He described his experiments in the Proceedings of the Natural History Society of Brünn, in 1866 and 1869, and they remained unknown outside the circle of that local society until the year 1900, when attention was directed to them. He was in the habit of saying Meine Zeit wird schon kommen-my time will no doubt come― and his confidence has been fully justified. Since the re-discovery of his two papers, the principles they teach have been applied to hundreds of different plants and animals, and Mendel's law has guided most studies of heredity, while Mendelism has become a clearly-defined branch of science. The white marble statue erected to his memory in Brünn, in 1910, represents the esteem in which scientific men of all nations now hold the Abbot of the Königskloster in that city.

It would be out of place here to describe in detail the nature and consequences of Mendel's great biological discovery, but we may sketch its main principle. Working with garden peas, Mendel found that different characters, such as wrinkled or smooth seed, colour of the seed-coats, form of the pods, length of the stem, and so on, could be used to distinguish them. He observed that when two plants differing by a given feature were crossed, the hybrid offspring invariably exhibited one of the parental characters to the entire or partial exclusion of the other. Thus when tall varieties of peas were crossed with dwarf varieties, the offspring were all

tall. This character of tallness was called by Mendel "dominant," while the character of shortness, which does not appear in the first generation of the hybrid plants, was described as "recessive."

The tall hybrids were then allowed to fertilise themselves, and it might be supposed that they would breed true; but that was not the case. After self-fertilisation each hybrid produced offspring in which the two original parental characters-dominant and recessive-were exhibited in the ratio, on the average, of three to one, there being in the case of tall and dwarf peas three tall plants to one short. From the dwarf plants, when self-fertilised, or fertilised from the original short stock, only dwarfs can be obtained henceforth; in other words the recessive minority of the second generation breeds true. Of the three tall plants belonging to the same generation, one possesses the dominant character alone and will produce only pure-bred or tall plants, when self-fertilised; but the two remaining plants are crossbreds and these on self-fertilisation again give the mixed generation of three plants possessing the dominant character of tallness to one plant having the recessive character of shortness.

The same simple rule was found by Mendel to be true when plants differing in several characters instead of one were cultivated; and thousands of experiments made by biologists since 1900 have shown that the results of hybridisation discovered by him hold good for specific characters exhibited by plants and animals of many kinds. The work carried on by Mendel in the seclusion of the cloister at Brünn has, indeed, put the whole subject of heredity upon an entirely new footing, and indicated a mode by which the isolation and perpetuation of definite characters can be assured.