33 o GENERAL BIOLOGY 



period of adult life may be long, as in elephants, or short, as 

 in mayflies; it may be reached by a gradual and regular 

 development, or by a series of abrupt form changes; but 

 when it arrives with full maturity of powers, the reproduc- 

 tive process takes the ascendency. Primarily it is the 

 period of making provision for descendants. Whether 

 such provision consist merely in mating and depositing 

 eggs, or whether in addition to this, the substance of the 

 body be transformed into food for the young, or whether, 

 still further, the physical powers of the body be devoted 

 to the care of the young, or whether, finally, as in human 

 society, with long years for individual activity, the labors 

 of life be devoted to securing for posterity the betterment 

 of those conditions that hinder its best development, it is 

 all the same ; the primary concern of adult life is provision 

 for the future of the race. 



The regularly progressive life cycle is sufficiently familiar 

 and needs no further illustration. But this undergoes 

 some remarkable changes under natural conditions, and 

 other alterations of it may be caused artificially. Some 

 of the more typical of these phenomena will now be con- 

 sidered, under the following headings: i) Alternation of 

 generations, 2) Special methods of asexual reproduction, 

 3) Change of form with alternation of hosts, 4) Meta- 

 morphosis, 5) Artificial division and combination of or- 

 ganisms. 



I. ALTERNATION OF GENERATIONS. 



This phenomenon consists, as we have already seen, in 

 the establishment of two segregated phases within the life 

 cycle, one sexually, and the other asexually reproducing. 

 We have already traced the development of it in gameto- 

 phyte and sporophyte of the higher green plants. An 

 equally good example of it is found among animals in the 

 group of marine hydroids. Medusa and hydranth are 



