HISTORY OF RESEARCH. 13 



In 1838 Dr. Johnston published his 'History of Britisli Zoophytes."' The "zoophytes" of 

 Johnston inchicle not only the plant-like hydroids, but the Jclif/ozoa and Polpoa. He does not, 

 however, confound the natural boundaries of these groups, and proposes for the hydroid forms the 

 name of Hydroida, which thus coincides with the Hydroida of the present Monograph in ail 

 respects, except in the fact of its not including the hydroid Medusaj, whose relation to tlic plant- 

 like trophosomes had not yet been definitely recognised. 



Without the originality of Ellis's classical ' Essay,' Johnston's ' History of British Zoophytes' 

 is still a work of great utility for the student. The descriptions of the species are very good, 

 and are accompanied by a copious and valuable synonomy ; and the figures, though mostly drawn 

 from the dried hydrosome, and certainly not equal in artistic feeling to those of Ellis, are often 

 excellent, and always of great use in aiding in the determination of the species. The value of the 

 ' British Zoophytes,' however, lies in its character as a descriptive work, and with its publication 

 we may date a new impulse to the study of the Hydroida, similar to that which nearly a century 

 before, Ellis's 'Natural History of Corallines' had exercised in the same direction. In 1847 a 

 second edition of the ' British Zoophytes ' made its appearance. A great number of additional 

 species are described in it, and many new plates are added to those which were contained in the first. 



The observations of Cavolini" in the last century, and afterwards those of Wagner' and 

 Loven,* had already made us acquainted with certain facts which show that the hydroid tropho- 

 some may give rise to buds presenting a close resemblance to Medusae; while the beautiful 

 researches of Sars had shown that among the Discojjhora phenomena occur which in many points 

 resemble this budding of Medusae among the Hydroida, and have an intimate relation with it.^ 

 The true significance of these observations, however, was but imperfectly appreciated when 

 Steenstrup, in 1842, combining them with analogous ones in other groups of animals, correlated 

 with great skill all the known facts, and generalised the phenomena under the name of " al- 

 ternation of generations," an expression already employed by Chamisso when describing the 

 gemmation and generation of Salpa!^ 



Though in the terms in which Steenstrup enunciates his law of alternation of generations a 

 false conception of the phenomena may appear to be involved, it is evident that his own view of 

 them is a correct one, and the modification which Steenstrup's expression of the law has since 

 undergone can never deprive it of its value in opening up more philosophic views of the 

 morphology of the invertebrate animals, and marking out a new era in their study. 



^ George Johnston, 'A History of the Britisli Zoophytes,' Edinburgh, 1838. 



^ Op. cit., 1785. 



' Rudolf Wagner, ' Neue im Adriatischen Meere gefundene Art von nacktem Armpolypen,' 

 Isis, 1833, iii, p. 256. 



* S. L. Loven, " Beitrag zur Kenntni.ss der Gatungen Campaiiuhiriu und Syncoryne," ' Muller's 

 Archiv,' 1837. 



" Martin Sars, ' Beskrivelser og Jagttagelser over nogle raoerkelige eller nye i Havet ved den 

 Bergenske Kyst levende Dyr,' &c., Bergen, 1835; and ' Ueber die Entwickclung der Medusa aurita 

 und der Cyanea capillata,' in 'Wiegm. Arch.,' 1841. 



° Joh. Japetus Steenstrup, ' Ueber den Generationswechsel oder die Fortpflanzung und Eut- 

 wickelung durch abwechselnde Generationeu, eine eigenthiimliche Form der Brutpflege in den niederu 

 Thierclassen. Uebers. von Lorenzen,' Kopenh., 1842. Also translated for the Ray Society, by Busk, 

 Loudon, 1815. 



