5IO THE HISTORY OF BIOLOGY 



however, has proved unsuccessful and has failed to gain the acceptance of 

 more recent systematists; in his own later systematic works he himself uses 

 the old traditional terms of genus and species, in spite of all the assurances 

 of Darwinism. 



On the medusa 

 A THIRD subject in which Haeckel worked as a systematist is the medusa; 

 here, too, he summarized his results in a monograph of huge dimensions, 

 entitled Das System der Medusen (1879), containing a large number of newly- 

 described forms and a system of classification that in part is of some value. 

 In particular, the two main groups that he classifies in it, the Craspedota 

 and the Acraspeda, have been retained by later systematists. On the other 

 hand, it has been found upon examination that some of the diagnoses of 

 species are full of serious mistakes, which is explained by the fact that Haeckel 

 has in general a far keener eye for the demarcation of the large groups in the 

 system than for genera and species; careful detailed examination was never 

 his strong point. 



There is still another group of life -forms which engaged Haeckel 's in- 

 terest and which perhaps appealed more to him than any other — namely, 

 the order Monera. To this order he refers single-celled organisms without 

 nucleus — that is, those formed of only a homogeneous mass. He has de- 

 scribed a great number of these — generally amoeboid organisms — many of 

 them with systematic validity. Nevertheless, the improved microscopy of 

 modern times has actually discovered in the majority of these a nuclear sub- 

 stance, either in the form of a single nucleus or divided into minute parti- 

 cles, and modern biology, which has learnt by experience to count the 

 nuclear substance among the essential components in a cell capable of life, 

 has in general presupposed the existence of the nucleus even in cells in which, 

 owing to its minimal dimensions or indistinct cell-content, it has not been 

 possible to confirm its existence. Haeckel, however, stubbornly held to his 

 non-nuclear Monera, the existence of which he regarded as an essential quali- 

 fication of that spontaneous generation by which he believed life to have 

 arisen, and which he looked upon as "a logical postulate for philosophical 

 natural science." This brings us to Haeckel's natural-philosophical specula- 

 tions — that part of his activities which, far more than his specialized re- 

 search-work, brought him both fame and ill fame. 



The essentials of his opinion 

 As has already been mentioned, Haeckel declared his adherence to Darwinism 

 in his work on the Radiolaria. At a scientific congress in 1863 he expounded 

 Darwin's theory in a manner that considerably enhanced its success in Ger- 

 many. The lecture really comprised a brief summary of the Origin of Species — 

 of the doctrine of selection and the struggle for existence. In its essentials the 

 argumentation is Darwin's own, taken from the theory of domestic animals, 



