I. Haeckel and His Opponents

The key to this explanation Haeckel found by seeking out the evidences in the individual development of the higher organisms—in their ontogeny—showing that they really originated from lower forms of life. When one follows out the form-development of one of the higher organisms from the earliest germ up to its fully developed condition, the different stages are found to present configurations corresponding to the forms of lower organisms.[^2] At the outset of his individual existence man and every other animal is a simple cell. This cell divides itself, and from it arises a germinal vesicle consisting of many cells. From that develops the so-called “cup-germ,” the two-layered gastrula, which has the shape of a cup- or jug-like body. Now, the lower plant-animals (sponges, polyps, and so on) remain throughout their entire existence on a level of development which is equivalent to this cup-germ. Haeckel remarks thereupon: “This fact is of extraordinary importance. For we see that man, and generally every vertebrate, runs rapidly, in passing, through a two-leaved stage of formation, which in these lowest plant-animals is maintained throughout life” ( Anthropogenesis). Such a parallelism between the developmental stages of the higher organisms and the developed lower forms may be followed out through the entire evolutionary history. Haeckel clothes this fact in the words: “The brief ontogenesis or development of the individual is a rapid and abbreviated repetition, a condensed recapitulation of the prolonged phylogenesis or development of the species.” This sentence gives expression to the so-called fundamental biogenetic law. Why then do the higher organisms in the course of their development come to forms which resemble lower ones? The natural explanation is that the former have developed themselves out of the latter; that therefore every organism in its individual development shows us one after another the forms which have clung to it as heirlooms from its lower ancestors.