GEMMATION OF MOLLUSCA AND VERTEBRATA. 557 
body, but from a kind of stolon within it; and they 
differ from their parent-stock in haying organs of attach¬ 
ment to each other, whereby they hold together in long 
Fig. 299.—Aggregate SALPiB. Fig. 300.—Solitary Salpa. 
chains (fig. 299). These, in their turn, being furnished with 
true generative organs, give origin to the solitary Salpse (fig. 
300) by ordinary sexual reproduction ; whilst the solitary 
Salpa never comes to possess any sexual apparatus, but merely 
continues the race by gemmation from its internal stolon. 
729. In the Vertebrated classes, as in the higher Mollusca, 
we lose all trace of propagation by gemmation as an ordinary 
method of multiplication. Yet there is evidence that this 
power is not altogether extinguished, even in Man. For we 
not unfrequently hear of “ monstrosities by excess,” that is, 
of cases in which the body possesses a superfluity of some of 
its parts ; the simplest cases being those of double thumbs or 
of six fingers on each hand, and the gradation being so conti¬ 
nuous from these to cases like that of the Siamese twins (in 
which there are two complete bodies united only by a cross 
band), as to make it evident that they are all referable to one 
common principle. And although it has been commonly 
believed that monsters with two heads and one body, or with 
two bodies and one head, or with supernumerary legs or arms, 
are results of the partial “ fusion ” of two distinct germs at an 
early period, yet there is now far stronger reason to believe that 
they proceed from a kind of attempt at multiplication by 
fission or gemmation, that is sometimes made by a single germ 
at a time when its grade of development corresponds with 
that of a Hydra or Planaria; which attempt, under peculiarly 
favourable circumstances, may proceed to the full length of 
production of two complete bodies. (See § 390.) 
Sexual Reproduction , or Generation . 
730. We now have to notice the most important features 
of the proper Generative process, which differs from the 
