Changes in size can be ascertained for mammalian brains. These changes must be valued as functional adaptations. The most important biological mechanisms are almost certainly the limited variability of brain size within species caused by genetic diversity and changes of selection factors when conquest of new ecological niches takes place. During phylogenetic evolution such changes only occur very slowly. Thus very small changes in brain size per time unit during the radiation of species must be expected. During domestication a similar process took place on the species level only much more accelerated and in other directions. Decrease of brain size characterises the change from wild living ancestors to domesticated mammals. In connection herewith significantly stronger changes in brain size per time unit could be proved for the initial phase of domestication. Furthermore, according to hitherto existing results on the brain, Dollo's law of the irreversible development of species characters during phylogenetic evolution also seems to prove right during domestication as the feralisation of domestic mammals doesn't lead to increase in brain size. Unusual decreases in brain size during ontogenetic development occur in certain basal mammals. Those are significant and they occur within extreme short periods of time. Although the biological importance of this phenomenon still remains unsolved it nevertheless is evident that brain ontogenesis in mammals may run different ways.