222 CASES SIMULATING ALTERNATION OF GENERATIONS. 



In many of the lower algae we find a similar alternation 

 of free and attached gemmae, in the form of zoospores, and 

 of cells cohering to form confervoid filaments. And much in 

 the same way we have in the Tolyzoa, besides the attached 

 Polype-bud, a peculiar form of deciduous gemma the stato- 

 blast. 



In the cases cited, the two forms of gemmae which 

 thus take part in the propagation of the species differ in 

 the matter of attachment to the parent stock as well as in 

 their own character, but cases occur in which both are fixed, 

 as well as others in which both are free. Thus, in some 

 species of Sertularia we may have a frond of seaweed 

 covered with a miniature forest of small polypidoms, each 

 with its own polype-buds, but all at the same time organi- 

 cally connected by stolons proceeding from radical gemmae, 

 which creep in a network over the surface of the frond. 

 Two forms of free gemmae again appear to co-exist in 

 some Algae, for there is reason to believe that the- stato- 

 spore, which is often associated with the zoospore, and is in- 

 tended for the reproduction of the species after a consider- 

 able lapse of time, is not in every case the result of impreg- 

 nation, but is sometimes rather a true gemma, modified to 

 meet the peculiarities of the case. 



3. As an alternation of different forms may thus occur 

 in a succession of gemmations, without the intervention of 

 any sexual process, so it has been thought that a periodic 

 recurrence of certain peculiarities may be observed in the 

 course of generations, even where the only mode of propa- 

 gation is by the formation and impregnation of ova. In 

 our own species something of this kind has been recognized 

 under the term atavism, by which is meant the reappear- 

 ance of particular features, tempers, morbid diatheses, &c., 

 in the progeny of the second generation, passing over the 

 immediate offspring. Facts of this kind are of frequent 

 observation a man's resemblance to his grandfather is a 



