The ethical values may be limited and primitive, such as unquestioning loyalty to a tribe, or high and universal, like those which Jesus first introduced into the affairs of the world: the point is that only in the human sector do they become a part of the mechanism of change and evolution, These broad trends are not universal. In the biological sector, for instance, stability may under certain conditions be the rule instead of change, or change may be restricted to the quite minor alterations involved in producing new species or genera of an already existing general type. Even when broad trends exist, they need not be desirable from the long-term point of view. Thus most trends observed in life, like that seen in the horse or elephant stock, are only specializations. These, after tens of millions of years of one-sided improvement for a particular way of life, lead inevitably to an evolutionary dead end, after which no further major change is possible. However, a few trends do occur which promote an all-round improvement of organisation, such as the evolution of early mammals from reptiles, or early man from mammals. These do not close the door on further major change, as was demonstrated by the large-scale evolution of mammals in the Tertiary, or of man’s societies since the Ice Age ; they are thus the only changes which are, from the longest-range point of view, desirable, the only trends which deserve to be called progressive.