Organisms are not made up merely by the few characters enumerated by the systematist; an infinite number of differing relations of parts might be formulated. Evolutionary divergence is not confined to external adult characters, but may appear in any structure, function or instinct, and at any time in the life history. Species very different as adults may have closely similar young, or larvæ may be much more diverse than the mature insects. Only the inadequacy of our notions of the vital structure and activities has led us to expect that reproductive cells will be found to contain special 'hereditary mechanisms' for the predetermination of the characteristics of adults. The largest and most complex individuals are still groups of cells, and no adequate reason has been shown for believing that particular cells or links of the organic sequence are more hereditary or more determinant than the others. Characters are to be thought of as lines of biological motion, not as structures or entities of reproductive cells. The predetermination of the infinity of structural and morphological characters and positional relations of the millions of cells of the adult by a working model resulting from the conjugation of sexual elements may be dismissed as a crudely anthropomorphic notion of biological processes, as unsupported by facts as it is illogical in conception. Cells have their functions and organs, but evolution is not confined to these; it is also a supercellular or organic process. Cytology is a very interesting branch of descriptive biology, but it enjoys no special evolutionary facilities.
Polycellular organisms grow by the division of cells; but instead of proving that all cells divide in the same way cytologists have found that the same result may be accomplished by a great variety of protoplasmic organs and processes. Unicellular organisms are known to be extremely diverse cytologically, and the cells of compound organisms are, if possible, more so. We know also that the diversity of organisms is not due so much to differences of the individual cells as to differences of number and arrangement in the cell-colonies of which they are constituted.
Heredity is the unknown means by which successive generations of organisms are able to construct themselves in similar, though not identical, forms; it is, in short, an organic memory, and is responsible, not alone for the repetition of the structural type, but also for vast numbers of involuntary functional coordinations and instinctive acts, whether of unicellular or of compound organisms, or of whole colonies of organisms. A colony of social termites is as truly an evolutionary unit as a tree with its many branches, and the cooperative instincts which pervade the individual insects are as truly a hereditary phenomenon as the peculiar arrangement of branches which we term a 'character' of the tree.
