The inheritance of _physical peculiarities_ is so obvious as to need no illustration. Among the ancients the Romans stereotyped its truth by the use of such expressions as the _labiones_, or thick-lipped; the _nasones_, or big-nosed; the _capitones_, or big-headed; and the _buccones_, or swollen-cheeked, etc. In more recent times we read of the Austrian lip and the Bourbon nose. Questions of heredity and variation are cytological ones—that is, questions of the anatomy, physiology, physiological chemistry, and pathology of cells. The most important part of a cell, as far as these questions are concerned, is the =nucleus=. The nucleus is the physical basis of all the heritages of an organism, from the simplest to the most complex. The nuclear threads may, therefore, very appropriately be termed the _hereditary threads_, or, collectively, the hereditary mass; and the physiological units in them the _hereditary units_. The nucleus is of fundamental importance in the reproduction or multiplication of both unicellular and multicellular animals and plants. In unicellular creatures multiplication may take place by fission and by conjugation. Both of these processes can be studied by observation of the infusorians. Maupas’s beautiful investigations on these unicellular animals have demonstrated that multiplication by fission may proceed to a prodigious extent for many generations, but that a time comes when the process fails, and the species will become exhausted and die out unless there is a rejuvenation of it by conjugation of individuals. In conjugation two individual infusoria come in apposition with each other, the nucleus in each undergoes subdivision. They reciprocally exchange part of their nuclear contents so that each infusorian comes to contain hereditary threads of two distinct individuals. From these rejuvenated (or fertilized) individuals multitudes of others may be derived by fission until exhaustion again takes place. Multiplication in multicellular creatures may be accomplished by budding (which is allied to fission), and is exemplified in the plant, hydra, the queen bee (parthenogenesis), etc., and by fertilization (which is allied to conjugation). A knowledge of the phenomena of fertilization of the ovum by the spermatozoid is essential to any understanding of the problems of heredity and variation in mankind. The nuclear threads of the ovum are its hereditary threads—the _groups of maternal hereditary units_; likewise, the nucleus of the spermatozoid contains the _paternal groups of hereditary units_.