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While laying great stress on the conditions of existence as the great cause of the modification of species, and as one of the main causes of change in the organs of reproduction, and also on "natural selection" as a modifier of the egg, the seed, the young, as well as the adult, Darwin finds in what he calls his provisional hypothesis of "pangenesis" the solution of the problem and the immediate means by which the change is effected. In his own words, this hypothesis "implies that the whole organization in the sense of every atom or unit reproduces itself. Hence ovules and pollen-grains — the fertilized seed or egg, as well as buds — include and consist of a multitude of germs thrown off from each separate atom of the organism." It is, in the words of a writer in the "Quarterly Journal of Science" for July, 1868, the application of the atomic theory to living forms, and is in perfect conformity with all the teachings of correlation between vital and physical forces. — "The author believes that all changes in the various organisms which result from the contact of the spermatozoon and ovum, as well as those which are derived from gemmation or budding, have the origin in the nature of the cells which constitute the elements or materials in operation. The cells or units which constitute all living bodies, from the simplest to the most complex, are themselves organized, and consist of lesser cells or atoms having various natures, and, according to the author, they give off those constituent atoms as 'gemmules,' and the nature of those atoms or gemmules fixes the future character of the organism into which they enter. In this manner he seeks to account for the first variation in living types; for the transmission of inherited peculiarities from a grandfather, say, through a daughter to a grandchild; for hybridism."

Darwin says, at the conclusion of his chapter on pangenesis:

"Finally, the power of propagation possessed by each separate cell, using the term in its largest sense, determines the reproduction, the variability, the development, and renovation of each living organism. Each living creature must be looked at as a microcosm, — a little universe formed of a host of self-propagating organisms, inconceivably minute, and as numerous as the stars in heaven."

For a further development of the theory the reader is referred

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to Darwin's work on "The Variation of Animals and Plants under Domestication," and to the review above quoted.