8 MARINE BIOLOGICAL LABORATORY. 



of the colony. Neither class can dispense with the 

 services of the other ; and this dependence of part upon 

 part gives the colony a certain physiological unity. But 

 the integration of the colony is of such a simple order, 

 that w^ might conceive it splitting up into as many 

 independent colonies as it contains times the least 

 number of cells of both sorts necessary to maintain the 

 physiological connexus. Although in practice, we could 

 not carry the division so far, still we know that artifi- 

 cial, if not spontaneous, division would be possible with- 

 out destroying the physiological unity necessary to the 

 continued existence of the severed parts. 



One feature of labor-division in Volvox deserves men- 

 tion here, chiefly as foreshadowing more complex condi- 

 tions seen in hisfher forms. It is the alternation of 

 agamic with gamic generation. The agamic reproduc- 

 tive cells are all alike, and correspond to parthenogenetic 

 ova ; while the gamic generation is represented by two 

 distinct kinds of cells answering to ova and spermatozoa, 

 and conjugation is necessary to development. 



This alternation of parthenogenesis with hermaphro- 

 ditic gamogenesis — is not, we may be sure, an acquisi- 

 tion of the colony ; it is rather to be regarded as a 

 combination of features that originated separately and 

 successively among the unicellular ancestors of the col- 

 ony. Parthenogenesis must have been the primitive 

 mode of reproduction ; gamogenesis undoubtedly origi- 

 nated secondarily in adaptation to infusorial conditions 

 'of life. 



This sequence of generations is common enough 

 among the unicellar Protozoa ; and the colonial forms 

 exhibit it as an inheritance of their component cells. 



