LIFE CYCLES, SEXUALITY, AND SEXUAL MECHANISMS 73 



sexual organs conipctelv and the ability of all vegetative cells to par- 

 ticipate in sexual fusions (somatic copulation). The developmental 

 histories of sexual aspects per se, furthermore, show a marked tend- 

 ency toward simplification, probably tiirough reduction, in most of 

 the more highly evolved groups. 



\\'har, then, can rationally be said of the probable origin of the 

 array of sexually different types that exist in the fungi at the present 

 time? Any attempt to rationalize six different life cycles, homothal- 

 lism, six distinct types of heterothallism, and four basic sexual appara- 

 tuses in the particular combinations in which they exist rapidly runs 

 afoul of difficulties that appear to be insurmountable. This can be 

 illustrated by testing two antithetical propositions. To start from the 

 assumption, as many have, that homothallic forms having gametic 

 copulation represent the primitive type from which all else has been 

 derived must totally ignore the random distribution of homothallic 

 and heterothallic species in every group of the fungi. The various 

 patterns of heterothallism would necessarily have been independently 

 evolved, at the appropriate levels, from the main stem of homothallic 

 forms. How, then, is it possible to account for homothallic, bipolar, 

 and tetrapolar species in a single genus, such as Copmms, the members 

 of which are obviously closely related phylogenetically, except that 

 heterothallism, of two very precise types common throughout the 

 much larger group to which it belongs, the Hymenomycetes, be in- 

 dependently evolved in this genus? Essentially the same situation 

 obtains in every phylogenetic grouping and in toto constitutes a 

 compelling argument against the derivation of heterothallism from 

 homothallism. The alternative proposition, that the variously ex- 

 pressed forms of homothallism were derived from heterothallic ances- 

 tral forms, encounters equally serious difficulties. Most important of 

 these difficulties are the twin necessities of (1) the origin of the 

 various types of heterothallism from some one primitive heterothallic 

 type and (2) the independent origin of homothallism in each homo- 

 thallic species. The latter might conceivably occur in sufficient fre- 

 quency to account for the large number of homothallic forms, but 

 it would appear most unlikely. More serious is the difficulty of the 

 evolution of the various types of heterothallism from other heter- 

 othallic types; the various types would appear to be much more 

 closely related to the corresponding homothallic types within any 

 phylogenetic grouping than to each other. Nor would espousal of 



