Professor Stapledon, working with Cocksfoot, has shown that ecoty-pes offer favourable material on which to commence breeding strains suitable for a particular habitat. that material and simple selection considerable progress may be made without a rational understanding of the genetic fundamentals underlying the results, The object of this paper is to discuss some genetical data relative to Cocksfoot breeding and to include information emphasising the importance of a knowledge of the genetic make up of the individual.