MODERN BIOLOGY 419 



the ancient shell-mounds from the Stone Age that are called kjokkenwoddinger 

 (refuse-heaps), and studied them with valuable zoological and ethnographi- 

 cal results. He was, moreover, interested in marine research and he made a 

 discovery in this field that more than anything else ensured to him a place 

 in the history of biology — namely, the alternation of generations. A. von 

 Chamisso, famous as a poet and circumnavigator, had already proved that 

 in the Salpa free individuals and individuals bound together in chains al- 

 ternate with one another by generations, but this discovery had been almost 

 entirely neglected. Other observations of a similar kind had also been made 

 before, as, for instance, by Michael Sars in Norway and Sven Loven in 

 Sweden, and also by Siebold himself in Danzig, but it was reserved for Steen- 

 strup to complete the material for observation and to place it under one 

 common point of view. In 1841 he published his work on the alternation 

 of generations, in which he gives an account of the evolution of the medusas, 

 the Campanularia;, the Salpa, and the Trematoda, and finds the phenomenon 

 common to them all — namely, that there exists in them an alternation be- 

 tween the adolescent stages, which he terms "nurses," because without sex- 

 ual reproduction they develop a new generation, the sexually mature, and 

 this sexual stage, which, in its turn, gives rise by means of ordinary sexual 

 reproduction to new "nurses." Of such asexual generations he sometimes 

 finds many, one after another, particularly in the Trematoda, in which they 

 are often described as one independent genus, the cercarias. In its details this 

 work certainly required both correction and completion, but its service lies 

 in the fact that it laid down a common principle that proved to be indis- 

 pensable if a conception was to be formed of the evolution of a great many 

 of the lower animals. The fact that Steenstrup regarded these phenomena 

 from a strictly romantic-philosophical point of view was only in accordance 

 with the practice of the time; he saw in the alternation of generations a 

 striving on the part of nature after freedom and perfection, and on these 

 grounds he accounted for even the insect communities by the alternation 

 of generations: the sexless workers he considered to be "nurses," which 

 devote to the offspring a care of a more ideal character than that which the 

 Salpa series and the cercarias give to their own progeny. 



Evolution of intestinal ivorms ascertained 

 It was Siebold who now succeeded in making practical use of the alternation- 

 of-generations idea, for he realized its fundamental importance for ascer- 

 taining the evolution of intestinal worms. He at once began to try to find 

 out by experimental means the connexion between a number of parasite 

 formations that had hitherto been regarded as independent of one another. 

 Thus he proved that the parasite which by insinuating itself in the brain 

 of the sheep causes the disease called "gid" or "sturdy," and which had 

 hitherto been described as Coenurus cerebralis, is actually an adolescent stage 



