mode of aggregation have been thus transmitted: a mode of transmission which, with his prevailing tendency to invent long and learned names for everything, he calls the ‘Perigenesis of plastids.’ In any case, however, these must be taken not as solutions of the problem, but as guesses at the truth which show that its solution is not impossible. The opposite principle to heredity, that of variation, is equally important and universal. It is apparent in the fact, that although every individual of every species reproduces qualities of parents and ancestors, no two individuals do so in precisely the same manner; no two are exactly alike. This difference, or individuality, becomes more marked as the organism is higher. Thus, sheep and hounds differ from one another by slight differences which require the practised eye of the shepherd or huntsman to detect; while human beings are so unlike, that of the many millions existing in each generation no two exactly resemble one another. The reason of this is apparent if we consider that the higher the organism the more complex does it become, and the less the chance of the whole complicated relations of parent and ancestral organisms being transmitted by single cells so solidly and completely as to overpower and remain uninfluenced by external influences. Variation evidently depends mainly on the varying influences of environment. If the exterior layer of molecules of a lump of protoplasm become differentiated from the interior ones and form a cell-wall, it is because they are in more immediate contact with the air or other surrounding medium. Internal changes depend on conditions such as temperature and nutrition. In the case of cultivated plants and domestic animals we can see most clearly how varieties are produced by adaptation to changes of environment. These variations, however, would not proceed very far, were it not for the interaction of the opposing forces of variation and heredity, by which latter the variations appearing in individuals are fixed and accumulated in descendants, until they become wide and permanent divergencies. This is done in the case of cultivated plants and domestic animals by man’s artificial selection in pairing individuals who show the same variations; and in nature by the struggle for existence, giving victory and survival to those forms, and in the long run to those forms only, whose variations, slight as they may be in each generation, tend to bring individuals into better adaptation to their environment.