Page:Popular Science Monthly Volume 65.djvu/209

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THE theory of evolution has influenced human thought in the most various ways during the past half century. In the sphere of biological science, where Darwin sowed the seeds which have grown up with such unexpected luxuriance, there has been a continuous process of fermentation which shows no signs of subsiding.

Still it has often seemed as if the immense mass of facts, which Darwin collected and arranged with so much care and skill, were provisionally looked upon as sufficient, and as if actual experiment were no longer a primary necessity. Whenever a new observation happened to be made, it was in danger of being drowned in pailfuls of theoretical considerations. Genealogical trees were planted, grafted, transplanted and finally often committed to the flames. Statistics were brought together to demonstrate the importance of natural selection, not only in the struggle between individual organisms, but also within the organism between the elements of which it was composed. Thus Roux wrote in 1881 his 'Kampf der Theile im Organismus,' Weismann only a few years ago (1896) his 'Germinal Selection.' In a series of very remarkable publications the Freiburg professor of zoology has thrown light on a series of difficult problems and has shown himself to be not only a faithful pupil of Darwin, but one who on several occasions has been more ultra-Darwinian than perhaps Darwin himself would have been.

Those who consult the very voluminous literature of the subject will soon be convinced that the number of biologists who have preferred patient experimentation to theoretical speculation is very limited indeed. Experimentation on this subject demands a great deal of time, of patience, of devotion, and is liable to meet with many pitfalls. Yet for those who came after Darwin this should have been the task that lay closest to their heart: to test the two great groups of facts on which descent and selection are founded, by means of new and more detailed experiments.

These two groups of facts are the phenomena cf heredity and of