220 RAMBLES OF A NATURALIST. 



stranger facts than these. For a long time, zoolo- 

 gists, who were guided solely by external characters, 

 admitted into the great division of the Radiata two 

 distinct classes, one of which includes the Acalephae, 

 and the other the Polypes. The former of these 

 embraces the extensive family of the Medusas, of 

 which we have already spoken. Among Polypes the 

 family of the Hydroida comprises animals which are 

 almost always fixed, grouped into colonies, united by 

 one common part ; in some cases like the stem of a 

 creeping plant; in others, ramified like little shrubs; 

 in others, again, expanded into a sort of plate or 

 



we are speaking consists in this, that in certain animals, neuter 

 individuals produce, by budding, sexual individuals, which in their 

 turn propagate by the ordinary methods ; from whence it results that 

 in these animals the offspring, instead of resembling their parents, 

 resemble their grand-parents. Steenstrup has given the name of 

 Ammen (Nurses) to those neuter individuals which produce sexual 

 individuals. Although the work of Steenstrup is composed in a 

 spirit at once absolute and contracted, it is not the less valuable for 

 the great services which it has rendered to science, by showing 

 that a number of phenomena, which had previously been regarded 

 as isolated, are, in fact, connected together by the most unexpected 

 links. In a remarkable work, to which the prize of the Academy 

 of Sciences has been awarded, Van Beneden has made a broad and 

 most felicitous application of these views to the history of intestinal 

 worms, whilst he has at the same time developed and enlarged 

 Steenstrup's theory by regarding alternate generation as a special 

 case of a more general phenomenon, which he terms Digenesis, in 

 opposition to the phenomenon of Monogenesis. According to this 

 author we observe in nature two general modes of reproduction. 

 The one is effected by the intervention of sexual individuals, the 

 other without any such intervention. Every animal in which 

 these two methods occur, either simultaneously or successively, is 

 termed digenetic ; every animal in which one of these methods only 

 occurs is termed monogenetic. 



