5l8 THE HISTORY OF BIOLOGY 



this stage the primal forms of the higher animals and man have also passed 

 through, and during that period they have resembled such cell-colonies, as, 

 for instance, the Vol vox. Out of the simple cell-mass there evolves in the 

 sponges, by means of invagination, a stage of development v^ith double 

 walls, a gastrula, which corresponds to the simplest form of an animal pos- 

 sessing an intestinal canal; the original form of the higher animals must 

 likewise have passed through this stage. This original form common to all 

 higher animals is called gastrasa. From each of the walls of the gastrula 

 there splits off through segmentation a fresh layer; these two secondary 

 layers combine and form the mesoderm, which gives rise to the muscula- 

 ture and various other organs in the higher animals. This process has also 

 taken place at some time or other in the primal form of the higher animals, 

 and therefore all these three layers and their derivatives are homologous 

 throughout the entire animal kingdom. 



Importance of Haeckel's bio genet ical principle 

 This evolutional theory is undeniably Haeckel's most brilliant and most 

 important contribution to the history of biology. O. Hertwig was right in 

 saying that for fifty years biological literature was under the influence of 

 this idea; the abundant facts that were amassed on the subject of embryology 

 during this period were mostly intended to confirm the biogenetical principle 

 or the "recapitulation" theory, as it has also been called, and biologist;, 

 strained every effort to apply it to every detail in the development of th; 

 embryo. And the application was "strained" in the fullest sense of the word. 

 Haeckel knew from the outset that the gastrula stage of the mammals is not 

 formed through invagination, as the theory claimed, but through delamina- 

 tion, or splitting off; he consoled himself, however, with the thought that 

 in the lancet-fish invagination generally takes place, and from this primi- 

 tive animal he derives the Mammalia, with the assertion that their gastrula 

 form is due to later adaptation — to the "falsification" of documents, of 

 which Fritz Miiller had spoken. He also explains a number of other facts 

 of a similar kind according to the same method. Matters became still worse 

 w^hen the embryologist His came forward with an attempt to explain the en- 

 tire cause of embryonic development on purely mechanical grounds. Haeckel 

 was furious and replied with a shower of abuse, quite forgetting all his 

 own utterances, in which he insisted upon a mechanical explanation of na- 

 ture. In reality this mechanical, or, in other words, physiological, side of 

 embryonic development is of very great importance, though Haeckel quite 

 overlooked the fact in his anxiety to explain natural creation; later on, how- 

 ever, it received all the greater attention. But, even apart from this, time has 

 dealt hardly with Haeckel's ontogenetical theories. The gastrula forma- 

 tion by means of invagination has proved far less general than Haeckel 

 believed — inter alia, it is lacking in most of the Coelenterata — and the 



