Every one who is familiar with comparative anatomy and the history of development will find as little difficulty about the origin of completely new organs as about the utter disappearance of rudimentary organs. The disappearance of the latter, viewed by itself, is the converse of the origin of the former. Both processes are particular phenomena of differentiation, which, like all others, can be explained quite simply and mechanically by the action of natural selection in the struggle for life. The infinitely important study of rudimentary organs and their origin, the comparison of their palæontological and embryological development, now naturally leads us to the consideration of one of the most important and instructive of all biological phenomena, namely, the parallelism which the phenomena of progress and divergence present to us in three different series. When, in the last chapter, we spoke of perfecting and division of labour, we understood by those words progress and separation, and those changes effected by them, which in the long and slow course of the earth’s history have led to a continual variation of the flora and fauna, to the origin of new and to the disappearance of ancient species of animals and plants. Now, if we follow the origin, the development, and the life of every single organic individual, we meet with exactly the same phenomena of progress and differentiation. The individual development, or the _ontogenesis_ of every single organism, from the egg to the complete form is nothing but a growth attended by a series of diverging and progressive changes. This applies equally to animals, plants, and protista. If, for example, we consider the ontogeny of any mammal, of man, of an ape, or of a pouched animal, or if we follow the individual development of any other vertebrate animal of another class, we everywhere find essentially the same phenomena. Every one of these animals develops itself originally out of a single cell, the egg. This cell increases by self-division, and forms a number of cells, and by the growth of this accumulation of cells, by the divergent development of originally identical cells, by the division of labour among them, and by their perfecting, there arises the perfect organism, the complicated composition of which excites our admiration. It seems to me here indispensable to draw attention more closely to those infinitely important and interesting processes which accompany _ontogenesis, or the individual development of organisms_, and especially to that of vertebrate animals, man included. I wish especially to recommend these exceedingly remarkable and instructive phenomena to the reader’s most careful consideration, first, because they are among the strongest supports of the Theory of Descent, and secondly, because, considering their immense general importance, they have hitherto been properly considered only by a few privileged persons.