The youngest of the great branches of the living tree of biology is the science we call biological evolution or phylogeny. It came into existence much later, and under much more difficult circumstances, than its natural sister, embryonic evolution or ontogeny. The object of the latter was to attain a knowledge of the mysterious processes by which the individual organism, plant or animal, developed from the egg. Phylogeny has to answer the much more obscure and difficult question: “What is the origin of the different organic species of plants and animals?”

Ontogeny (embryology and metamorphism) could follow the empirical method of direct observation in the solution of its not remote problem; it needed but to follow, day by day and hour by hour, the visible changes which the fœtus experiences during a brief period in the course of its development from the ovum. Much more difficult was the remote problem of phylogeny; for the slow processes of gradual construction, which effect the rise of new species of animals and plants, go on imperceptibly during thousands and even millions of years.