The development of the life sciences may be said to have effected a gradual transition from a more or less intuitive prescientific approach based on crude observation, via a more refined type of observation to experimentation and hence to the level of formal theories. Quantitative methods are introduced at the second level; they comprise: (a) quantitative design of experiments; (b) regrouping of experimental results; (c) evalua­ tion of results by means of mathematical or special statistical techniques. The last step implies the introduction of theoretical concepts, but we are not justified in speaking of theoretical science unless true theoretical con­ siderations-models or hypotheses-precede experiment, and this is then followed by an attempt to link results with theory so as to verify the theory. Biology at present seems to lie somewhere between the second and third level mentioned above, not yet having achieved the status of a theoretical science in all its branches. Thus, though the need for quantification and mathematical formulation is widely recognized, many biologists still believe that e.g. general systems theory is too abstract to be of use in handling concrete problems. Those, however, who look critically at the present state of affairs cannot adopt this attitude.