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 



