206 RELATIONS OF OVA AND GEMMiE. 



patible mth its continuance, in the majority of cases, 

 unless re-invigorated by the access of the spermatic element. 



§ 9. This doctrine of an essential community of nature 

 between ova and gemmae has an obvious bearing on our 

 views regarding the modes of propagation in which they 

 are respectively concerned. If the nascent bodies approxi- 

 mate so much in their real nature as to pass into each 

 other, the processes of development can hardly be so widely 

 diiferent in the two cases as is held by some. 



The prevailing view in this direction seems to be that 

 gemmation perpetuates the individual rather than the 

 species, the successive zooids or ph}i:oids preserving, more 

 completely than the progeny of embryonic origin, the cha- 

 racters of the parent stock ; and it has been thought, too, 

 that there is a tendency for the plastic power to wear out 

 in process of time, so that a recurrence of sexual generation 

 at intervals is necessary to preserve the pristine vigour of 

 the species. The more highly organized the species, the 

 more dependent it is supposed to be on the frequent recur- 

 rence of sexual reproduction in the genetic cycle. In the 

 higher animals we meet with no obvious phenomena at all 

 of the nature of gemmation, while in the lowest there may 

 be a very prolonged pullulation of gemmae, the sexual act 

 recurring only at distant intervals. Still it is held that 

 even in these it must recur from time to time, to give a 

 fresh start, as it were, to the organizing process. It is true 

 there are species in which we have no positive knowledge 

 of any sexual act occurring at all — such is the case with 

 most of the Protozoa. There are species, too, in which no 

 males have yet been detected — so it is with some Insects 

 and Entomostraca.* But such negative evidence is not 

 held of sufficient weight to counterbalance the argument 



* Certain species of Cynips, Apus, Limno.diaj and Polyphemus are in 

 this case. Siebold's Parthenogenesis, 105. 



