tween discontinuous and continuous variations, but in later works this distinction is given a minor place as compared with the distinction between inherited and non-inherited variations. Inherited variations, whether large or small, are called by de Vries "mutations," whereas non-inherited variations are known as "fluctuations," the former are caused by changes in germinal constitution, the latter by alterations in environmental conditions; the former represents changes in heredity, the latter changes in development.
3. Mutations and Fluctuations.—This clear cut distinction between mutations and fluctuations marks one of the most important advances ever made in the study of development and evolution. Thousands of fluctuations occur which are purely somatic in character and which do not affect the germ cells, for every single mutation or change in the hereditary constitution; and yet only the latter are of significance in heredity and evolution. This distinction between variations due to environment (fluctuations) and those due to hereditary causes (mutations) was recognized by Weismann and many of his followers, but the actual demonstration on a large scale of the importance of this distinction is due largely to de Vries.
All hereditary variations, whether due to new combinations of old characters or to the appearance of actually new characters, whether small and continuous, or large and discontinuous, have their causes in the organization of the germ cells, just as do inherited resemblances. Heredity is not to be contrasted with variation, nor are hereditary likeness and unlikeness due to conflicting principles; both are the results of germinal organization and both are phenomena of heredity.
4. Every Individual Unique.—As a result of the permutations of ancestral characters, the appearance of mutations, and the fluctuations of organisms due to environmental changes, it happens that in all cases offspring differ more or less from their parents and from one another. No two children of the same family are ever exactly alike (except in the case of identical twins which have come from the same oosperm). Every living being appears on careful examination to be the first and last of its identical kind. This is one of the most remarkable peculiarities of living things. The elements of chemistry are constant, and even the compounds fall into definite categories which have constant characteristics. But the individuals of biology are apparently never twice the same. This may be due to the immense complexity of living units as contrasted with chemical ones, indeed lack of constancy is evident in itself of lack of analysis into real elements or of lack of uniform conditions, but whatever its cause the extraordinary fact remains that every living being appears to be unique.
There seems to be no reason to doubt that all the extraordinary dif-
Reproduction is the generation of unique beings that are, on the average, more like their kind than like anything else (Brooks).