278 HAECKEL 



zoology. The difficulty of it, on the other hand, 

 lay in the infinite modifications of the embryonic 

 processes in detail that had been brought about 

 by cenogenesis ; almost everywhere this had more 

 or less obscured the original features. On the 

 whole it gave rise to the greatest and most 

 far-reaching discussion that has taken place in 

 zoology for the last thirty years, apart from the 

 Darwinian theory itself. To-day, at the close 

 of these three decades, there are only two alterna- 

 tives. One is that there is still an absolutely 

 mysterious and hidden law of ontogeny, that 

 compels countless animals over and over again 

 to pass through these embryonic forms and 

 assume a likeness to the gastraea. After all the 

 eagerness with which the whole school of embryo- 

 logists opposed to Haeckel have sought, up to 

 our own day, to establish such a direct law, we 

 have not yet got the shadow of a clear formulation 

 of it. The other alternative is that Haeckel is 

 right in believing that he has discovered the 

 correct formula in his phylogenetic interpretation 

 of embryonic processes in accordance with the 

 biogenetic law. If that is so, the gastraea-theory 

 is the crown of all his labours in technical zoology 

 proper. Let us wait another thirty years. 



The scientific controversy over the gastrasa- 

 theory was in full swing when Haeckel entered 

 upon another bold experiment in the direction 

 of the biogenetic law. He thought it would be 

 useful, instead of framing wider hypotheses, to 

 take one single instance of one of the highest 



