This communication presents a discussion of some extensions of the formalism of Verhulst's simple logistics, which may constitute an autonomous growth model of a more general scope.For that purpose, the basis concept of growth diagram or trajectory is called upon, as it affords the graphic representation of the change in the growth variable y, using two relevant kinetic parameters: the instantaneous rate and the instantaneous acceleration. The two possible kinds of trajectories are in relation to the use of absolute (V = dy/dt; r = dV/dt) or relative (or specific) values (R = (1/y)(dy/dt); r R = dR/dt).In the case of simple logistics, the trajectory (V, I') allows 4 growth phases or states to be distinguished. The diagram (R, re.) shows that the deceleration of the specific rate is not monotonous.In the ease of Richards -Nelder's generalized logistics, the qualitative variation of the growth trajectory depends on the value of the dissymmetry parameter (occurrence of a critical value which determines the number of growth states). Blumberg's model is characterized by an analogous property and, moreover, can account for a non monotonous variation of the specific growth rate.

Genetic distances between seven Dutch rare breeds of sheep were established as based upon blood groups and biochemical polymorphisms. A rather close relationship was observed between three breeds of heath sheep (Drente, Veluwe and Kempen) and the Schoonebeker sheep. The Mergelland sheep was less related to this cluster. The largest distances were between Friesian sheep and Black Blaze sheep and all others.

Genetic diversity is the basis for livestock breeding. Diversity within breeds may be reduced by effective breeding methods in commercial breeds; diversity between breeds may be reduced by disappearance of non-commercial breeds. Two means of preserving diversity are discussed: the development of breeding methods that aim at genetic gain yet account for loss of diversity, and the setup of gene banks to preserve current diversity for future use.

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