ABSTRACT
The Darwinian theory of evolution by natural selection hinges entirely on the notion that favorable variations are preserved and unfavorable ones eliminated, so that a new species eventually arises very gradually, in slow steps, over the course of innumerable generations. Variation is thus the essential basis of evolution. Without variation natural selection would have no material on which to operate. A second essential ingredient of Darwinian evolution is that there must be hereditary mechanisms by which favorable variations can be transmitted to subsequent generations. As we realize now, it is the capacity to develop the favorable variations in the species-typical rearing environment that is transmitted between generations. Thus, whether or not an organism
is capable of a specific variation can only become apparent during the course of individual development. Consequently, genetics and embryology are essential to the understanding of the variations necessary for evolution. In the early 1900s the respective studies of genetics and embryology became dissociated, because the geneticists ignored embryology and the embryologists ignored genetics.
