CONNECTION BETWEEN PHYLOGENY AND ONTOGENY. 9 



of gaps. We are, however, able to bridge over the greater 

 part of these gaps satisfactorily by the help of Compa- 

 rative Anatomy, though not to fill them up directly by 

 ontoo-enetic research. It is therefore all the more im- 

 portant that we are acquainted with a considerable number 

 of lower animal forms which still find place in the history of 

 the individual evolution of man. In such cases, from the 

 nature of the transient individual form, we may quite safely 

 infer the nature of the ancestral animal form. 



For example, from the fact that the human egg is a 

 simple cell, we may at once infer that there has been at a 

 very remote time a unicellular ancestor of the human race 

 resembling an Amoeba. Again, from the fact that the 

 human embryo originally consists merely of two simple 

 germ-layers, we may at once safely infer that a very ancient 

 ancestral form is represented by the two-layered Gastrsea. A 

 later embryonic form of the human being points with equal 

 certainty to a primitive worm-like ancestral form which is 

 related to the sea-squirts or Ascidians of the present day. 

 But the low animal forms which constitute the ancestral 

 line between the unicellular amoeba and the gastrsea, and 

 further between the gastrsea and the ascidian form, can only 

 be approximately conjectured with the aid of Comparative 

 Anatomy and Ontogeny. On account of a shortened process 

 of Heredity, various ontogenetic intermediate forms, which 

 must have existed phylogenetically, or in the ancestral 

 lineage, have in the coui'se of historic evolution gradually 

 dropped out from these gaps. But notwithstanding these 

 numerous and sometimes very considerable gaps, there is, on 

 the whole, complete agreement between the two series of 

 evolution. Indeed, it will be one of my principal objects to 



