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 repro- 

 duction. 



Figure 3 shows in A a polype-shaped being with 

 cruciform tentacles, on which the 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 oflfspring of 

 its utterly dissimilar parent, the Stauridium; it repro- 

 duces itself in the sexual method, and from its eggs pro- 

 ceed Stauridia. The two generations thus alternate; the 

 cross-polype is an intermediate generation in the develop- 

 ment of the Medusa, so that the sexual generation never 

 originates directly from its tgg. 



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 



