PULLULATION IN THE GENETIC CYCLE. ]51 



the phenomenon by the term pullulation, in allusion to the 

 sprouting of leaf-shoots in a tree, which is, in fact, merely 

 a special case of a process of this kind. But the expression 

 is here employed simply to denote a continuous succession of 

 gemmae in the same phase of development, without restriction 

 to cases in which they remain in this state of adhesion to each 

 other. No such distinction, indeed, could be well carried out ; 

 for gemmae ordinarily attached sometimes become separate, 

 so as to originate distinct organisms ; and variations in this 

 respect are met with not only in comparing allied species, 

 but even in the same species under altered circumstances. 



On the whole, however, such a general rule as this seems 

 to prevail that in the Vegetable Kingdom, and in the 

 lower divisions of the Animal Kingdom, whose diffuse vita- 

 lity favours such development, the outgrowths do, with 

 some exceptions, remain in adhesion to form compound organ- 

 isms, while from the incompatibility of such structures with 

 the more concentrated vitality of the higher animals, such 

 pullulation is either wholly excluded, as in Vertebrata, or, 

 as in Articulata where it manifests itself exceptionally 

 the gemmae are detached as soon as matured, and 

 assume the guise of independent animals. Such an excep- 

 tional case is that of the Aphides among insects, already 

 referred to. However great the primd facie diversity be- 

 tween the successive swarms of free insects which we here 

 meet with, and the clustered buds of the plant or the zoo- 

 phyte, the physiological identity of their relations has been 

 well demonstrated by Owen, Carpenter, and others, who 

 clearly show that the only difference lies in a character, the 

 variable and accidental nature of which has just been no- 

 ticed viz., the bond of connection between the gemmae, 

 whose presence in the form of a common axis associates in 

 organic union the successive pullulations of the plant or 

 zoophyte, and whose disappearance in the Aphides disso- 

 ciates the derivative zooids, as the snapping of the thread 



