Page:Popular Science Monthly Volume 61.djvu/42

36 What is the physical basis—the vehicle for the transmission of all the characters large and small, from one generation to the next?

In many minute animals and plants where the individual consists of a single cell it simply divides into two, so that it is impossible to say which of the two is the parent, which the offspring. Heredity is here simply a process of growth and division. If one of these individuals is cut into halves each half will regenerate the part lost. Normally the whole animal or plant is concerned in heredity. In fact, an individual may be divided in any way and each fragment will regenerate the lost part so long as the fragment contains part of the nucleus.

In many other animals, of which we nave a representative in our ponds and streams in Hydra, buds develop at certain regions and grow into a new individual. Here any one group out of a large number of groups of cells may build up a new individual. If a hydra be cut in any one of hundreds of possible ways each part will regenerate the portion lost, and so form a new individual. Every group of cells is here adjusted to reproduce the entire individual if the inhibition exercised by the presence of other cells is removed. The method of budding is the commonest means of transmitting the characters in plants. Those individuals produced by buds are usually exactly, or very nearly, like