As here used, the term inheritance denotes something more than a mere recurrence of parental characters in the offspring. It signifies a process of genuine transmission from generation to generation. Strictly speaking, it is not the _characters_, such as coloration, shape, size, chemical composition, structural type, and functional specificity, that are “inherited,” but rather the hereditary _factors_ or chromosomal _genes_, which are actually transmitted, and of which the characters are but an external expression or manifestation. Hence, it is scarcely accurate to speak of “inherited,” as distinguished from “acquired,” characters. As a matter of fact, all somatic characters are joint products of the interaction of germinal and environmental factors. Consequently, the external character would be affected no less by a change in the environmental factors than by a change in the germinal factors. In a word, somatic characters are not the exclusive expression of the genetic factors, but are equally dependent upon environmental influence, and hence it is only to the extent that these characters are indicative of the specific constitution of the germ plasm that we may speak of them as “inherited,” remembering that what is really transmitted to the offspring is a complex of genes or germinal factors, and not the characters themselves. The sense is, therefore, that “inherited” characters are manifestative of what is contained in the germ plasm, whereas “acquired” characters have no specific germinal basis, but are a resultant of the interaction between the somatic cells and the environment. In modern terminology, as we have seen, the aggregate of germinal factors transmitted in the process of reproduction is called the genotype, while the aggregate of somatic characters which manifest these germinal factors externally is spoken of as the phenotype. Only the genotype is transmitted, the phenotype being the subsequent product of the interplay of genetic factors and environmental stimuli, dependent upon, and expressive of, both. Variation, therefore, may be based upon a change in the germ plasm, or in the environment, or in both. If it rests exclusively upon an extraordinary change in the environmental conditions, the resulting modification is non-inheritable, and will disappear so soon as the exceptional environmental stimulus that evoked it is withdrawn. If, on the contrary, it is based upon a germinal change, it will manifest itself, even under ordinary, i.e. unchanged or uniform environmental influence. In this case, the modification is inheritable in the sense that it is the specific effect of a transmissible germinal factor, which has undergone alteration.