We are presented, then, with another choice: either to presuppose a spontaneous aptitude in living beings to change their forms (or at least to acquire with succeeding generations a slightly different character from that originally given, so that it will change gradually from one to the next and finally become unrecognizable), or to attribute to them some obscure urge towards a terminal species that will possess the characters of all those that have preceded it, but in a higher degree of complexity and perfection.