In dealing with the great problems of organic development there is probably no department in which so much error and misconception prevails as on the nature and limitations of Heredity. These misconceptions not only pervade most popular writings on the subject of evolution, but even those of men of science and of specialists in biology, and they are the more important and dangerous because their promulgators are able to quote Herbert Spencer, and to a less extent Darwin, as holding similar views. The subject is of special importance here because it involves the question of whether the effects of the environment, including education and training, are in any degree transmitted from the individuals so modified to their progeny—whether they are or are not cumulative. It is, in fact, the much discussed and vitally important problem of the Heredity of Acquired Characters. The effects of use and disuse, another form of the same general phenomenon, were assumed by Lamarck to be inherited, and a large portion of his theory of evolution rested on this assumption; it seemed so probable, and was apparently supported by so many facts, that Darwin, like most other naturalists at the time, accepted it without any special inquiry, and when he worked out his theory of Pangenesis in order to explain the main facts of heredity, his suppositions were adapted to include such phenomena. Let us then first explain what is meant by the "acquired characters" which it was thought that a true theory of heredity must explain. As a rule, the great majority of the peculiarities of any species of animal or plant are constantly reproduced in its offspring. The short tail of the wren, the much longer tail of the long-tailed tit, the crest of the crested tit and of innumerable other birds, always when full-grown exhibit the same characters as in their parents. These are said to be _innate_ characters. In rare cases, however, offspring are born which differ materially from their parents, as when a white blackbird or a six-toed kitten appears, but these are equally _innate_, and are often strongly inherited. All these are subject to variation, and can therefore be modified by selection, whether natural or artificial, and the effects of such selection in the case of domestic animals is often enormous. Such are the pouters and tumblers among pigeons, the bull-dog and the greyhound, the numerous breeds of poultry, all of which are known to have been produced by artificial selections of favourable variations extending over many centuries; and the characters of these varieties are all strongly inherited.