|Between parts of the same cell.|
|Between cells in the same organism.|
|Between organisms of the same species.|
|Between organisms of different species.|
I shall say nothing about the first except that it has been very fully and clearly dealt with in Professor Katz' inaugural lecture at University College.* The second class of communications consists of those as a result of which an organism is an organism and not a mere aggregate of cells. Again my colleagues Professors Brown, Young and Katz could describe them better than I can.
Apart from intracellular communication the simplest form known to us is communication between Protozoa, one-celled organisms. They are too small to have eyes or ears, so communication between them not involving contact is chemical communication. To take one example, there are at least six different biochemical types of the ciliate Euplotes patella, each producing either one or two of three different chemical substances, when sexually mature. Any of these substances induces____________________