THE PROBLEM 13 



physiological condition of the individual and often also 

 with external conditions. In some organisms, for 

 example, many kinds of reproductive processes occur, 

 their character varying with internal and environmental 

 conditions. 



Very generally it is possible to distinguish more or 

 less clearly an orderly character in these reproductive 

 processes; some of them are in fact orderly to a high 

 degree. But they differ so widely in different organisms 

 that attempts to discover common fundamental factors 

 underlying the various forms of the process have not 

 been very successful. 



In addition to those reproductive processes which 

 give rise to whole new organisms there are also those 

 which result in reduplication of more or less complex 

 parts. The repetition of radially arranged parts, such, 

 for example, as tentacles in a sea-anemone, arms in a 

 starfish, a whorl of leaves or the parts of a flower in a 

 plant, and on the other hand the succession of parts 

 along an axis, leaves or branches along the stem of a 

 plant, or the segments in the body of the earthworm, are 

 all reproductive processes and involve processes of indi- 

 viduation. All such reproductive processes must be 

 included in any attempt at a theory of reproduction. 



And finally there remains the process of sexual or 

 gametic reproduction in which the union of two cells, 

 the gametes or their nuclei, is followed by a series of 

 developmental changes. In most cases of gametic re- 

 production the two gametes are sexually differentiated 

 as parts of two different individuals or in different organs 

 of the same individual before they come together. 

 Moreover, they are themselves individuals, and their 



