CLASSIFICATION OF MEDUS. 83 
3d. The case of Coryne, Se. 
a. The zoophyte produces medusze by gemmation. 
b. The medusz produce eggs. 
c. The eggs produce infusoria. 
d. The infusoria fix and become zoophytes. 
4th. The case of Lizxia and Sarsia. 
a. The medusa produces medusz by gemmation. 
(The remaining stages as yet unobserved, but probahly)— 
b. The medusee produce eggs. 
c. The eggs produce infusoria. 
d. The infusoria fix as polypes, and produce medusz. 
With such facts—unquestioned facts—before us, it seems to me that we have no choice 
between theories, and that we must admit the idea of “ Alternation of Generations” to be true. 
Steenstrup was assuredly the first naturalist who announced that idea as a general fact 
dependent on a law. “The special subject of this Essay’—I quote from the author’s preface 
to the German version of his celebrated work, as translated by Mr. Busk—“ is the funda- 
mental idea expressed by the words ‘4lternation of Generations, or the remarkable, and 
till now inexplicable, natural phenomenon of an animal producing an offspring, which at no 
time resembles its parent, but which, on the other hand, itself brings forth a progeny, which 
"returns in its form and nature to the parent animal, so that the maternal animal does not meet 
with its resemblance in its own brood, but in its descendants of the second, third, or fourth 
degree of generation; and this always takes place in the different animals which exhibit the 
phenomena in a determinate generation, or with the intervention of a determinate number of 
generations. This remarkable precedence of one or more generations, whose function it is, as 
it were, to prepare the way for the later succeeding generation of animals destined to attain 
a higher degree of perfection, and which are developed into the form of the mother, and 
propagate the species by means of ova, can, I believe, be demonstrated in not a few instances 
in the animal kingdom.” 
The main position thus stated appears to me sound and true: the assumption of a definite 
regularity in the alternations is a secondary and non-essential one, and true probably when 
disturbing conditions are not at work. But numerous observations, especially those of 
Dalyell,* Reid,t and Price,{ show that under peculiar circumstances, in what may be termed 
unnatural situations, the polype generations may go on continually producing polype genera- 
tions ; and those of Sars and myself, on the other hand, that a Medusa generation may go on 
producing Medusa generations; although, under normal conditions in each instance, there is 
every reason to suppose that zoophytic and Medusoid forms would have regularly alternated. 
I am anxious to bear testimony to the value of the idea enunciated by Steenstrup, 
because I believe it has given a strong impulse in a right direction to Invertebrate Zoology. 
* Remarkable Animals of Scotland. 
+ Annals of Nat. Hist., Second Series, vol. i. 
t British Association Report, 1846, p, 86. 
