42 THE DOCTRINE OF DESCENT, 
animal produces one or more gemmules similar to itself, 
capable in their turn of producing similar gemmules. Of 
this, every collection of corals gives numerous examples, 
and shows how the diversified appearance of the several 
genera of coral depends merely on minor modifications 
of this mode of reproduction. Yet single corals exist 
in which, on careful comparison, not only may accidental 
deviations be already discerned, but regularly recurring 
variations between parent and progeny, as Semper has 
recently shown in Madrepores and Fungiform corals. 
This brings us to the highly-important phenomenon of 
Alternate Generation, which we must elucidate by a 
few examples before entering upon the nature of sexual 
reproduction. 
Figure 3 shows in A a polype-shaped being with 
cruciform tentacles, on which its discoverer, Dujardin, 
bestowed the generic name of Cross-polype, or Stauri- 
dium. This animal, growing like a polype upon a stalk, 
forms above its lower cross, gemmules which make their 
appearance as spherical balls, gradually assume a bell-like 
shape, and detach themselves on attaining the structure 
and form of a Medusa or sea-nettle. The Medusa (termed 
Cladonema Radiatum, Fig. 3 B) is thus the offspring of 
its utterly dissimilar parent, the Stauridium ; it repro- 
duces itself in the sexual method, and from its eggs 
proceed Stauridia. The two generations thus alter- 
nate; the cross-polype is an intermediate generation in 
the development of the Medusa, so that the sexual genera- 
tion never originates directly from its egg. 
In the tape-worm, we have an illustration of the same 
process, only in a somewhat more complicated form. 
It is known that from the intestinal canal of individuals 
