142 . ERNST HAECKEL. 



The Gastraea-Theory, the Phylogenetic Classification 

 of the Animal Kingdom and the Homology of the 

 Germ-Lamellje. By Ernst Haeckel. (Translated by 

 E. Perceval Wright, F.L.S., Sec. R.I.A., Professor 

 of Botany, Trin. Coll., Dublin. With PI. VII.) 



I. — The Causal Significance of Phylogenesis in On- 

 togenesis. 



The history of the development of organisms has in 

 these latter times entered upon a new period, in that 

 it has raised itself from the mere empirical inquiry into 

 facts requiring to be investigated to a philosophical interro- 

 gation into their natural causes. Indeed, the thoughtful 

 inquirer in the domain of biogenesis has been troubled 

 for more now than half a century in attempting to dive 

 beneath the mere knowledge of biogenetic appearances to a 

 deeper intelligence of their meaning, or to search after " the 

 laws of organic development," by the too intimate connection 

 of empirical observation and philosophical reflection. But so 

 long as the subject of the development of the organic indi- 

 vidual was exclusively pursued, so long could not even 

 such praiseworthy efforts aim at a causal knowledge. Indeed, 

 this satisfying of the necessities of scientific causality is only 

 possible since we have begun within the last decade to 

 investigate the natural development of organic species, and 

 by the story of the descent of organic species to explain the 

 story of the first appearance of the organic individual. 



After Caspar Friedrich Wolff*, in the year 1759, by his 

 * Theoria generationis' had built up epigenesis on the im- 

 movable foundations of a history of common development, 

 and after that with this strong foundation-stone for more than 

 half a hundred years overlooked, Christian Pander had 

 sketched out, in 1817, the first outlines of the germ-lamella 

 theory, Carl Ernst Baer, in 1828, in his ' Entwickelungs- 

 geschichte der Thiere,' determined the direction, and defined 

 the path along which all subsequent embryological re- 

 search must move. In this classical work the quite new 

 science of the individual development of animals has been 

 laid down by the happy combination of most careful obser- 

 vation with philosophical reflection, as well as by the blending 

 together of the researches of the embryologist, the comparative 

 anatomist, and the systematic zoologistic, as the starting-]>oint 

 of all scientific zoology, and has become the focus around 

 which all the different laws of this science must eventually 



