314 SEA-SIDE STUDIES. 



same species [Myriophyllum), dredged at the same 

 time and from the same place, I found the capsules 

 containing eggs and also Medusce ; and others — but 

 not on the same Polypidom — containing eggs and 

 Polypes, i.e. the ciliated gemmules which we know to 

 be the infusorial stage of the Polype. 



I have seen this so often that the whole history of 

 evolution thus presents itself : Taking the medullary 

 substance of the Polype as the analogue of the cellular 

 basis of the plant, we may trace a somewhat similar 

 course of evolution in each : the cellular basis becomes 

 differentiated into leaves, stamens, pistils, germ-cells, 

 and sperm-cells ; the medullary substance becomes dif- 

 ferentiated into nucleated cells — these cells into germ- 

 cells and sperm-cells, or into germ-cells alone, from 

 which are developed, 



1st, Under one set of conditions, probably of tem- 

 perature and food, Polypes ; 



2d, Under another set of conditions, Medusae. Just 

 as a leaf-bud is developed under one set of conditions, 

 and a flower under another set ; or as only germ-cells 

 are developed in one plant, sperm-cells in another, or 

 both on the same plant. 



Of great importance as regards the theory of Par- 

 thenogenesis are two of the facts just indicated — 

 namely, that the Polype produces ova which become 

 Medusae ; and that these ova may indifferently become 

 either Polypes or Medusae. The latter fact ceases to be 

 so marvellous, when we consider that Agassiz has demon- 

 strated the identity in structure of Polype and Medusa. 



