250 THE AMERICAN NATURALIST. [Vol. XXXIX. 



tilization" of the female cell of the Uredinales. Indeed, we 

 may expect to find that the actual fusion of paternal and mater- 

 nal chromatin does not take place in the higher plants until the 

 end of the sporophyte generation in the spore mother cell, as 

 zoologists have concluded that such union occurs just previous to 

 gametogenesis in animals. But is Blackman justified in regard- 

 ing the phenomenon substituted for the activities of ancestral 

 sexual organs in Phragmidium, now functionless, as a sexual act 

 and is it desirable to apply the term fertilization to the phe- 

 nomenon ? 



Blackman (:04 b, p. 153) speaks of the introduction of a 

 nucleus into the fertile cell of the Uredinales and the phenome- 

 non in the apogamous development of the fern after the account 

 of Farmer, Moore, and Digby (: 03) as " reduced forms of ferti- 

 lization." It may be questioned whether the use of the term 

 fertilization is fully justified by the events under discussion. We 

 are all likely to agree with these authors that the physiological 

 aspects of the phenomena in the cases under consideration are 

 similar to sexual acts. But, by the writer, the act of fertiliza- 

 tion is always considered in phylogenetic relations and strictly 

 limited to the union of sexually differentiated cells, which are 

 defined by their morphology through principles of homology. 

 Whenever one or both of the gametes are suppressed in a life 

 history and a succeeding generation develops of the sort that 

 normally follows a sexual act, then such a development is apoga- 

 mous and the phenomena always introduce features which are 

 foreign to the processes of normal fertilization and the funda- 

 mental principles of sexuality. 



Perhaps the most important characteristic of sexuality from 

 an evolutionary standpoint is the fusion of gametes of unrelated 

 parentage, for in the mingling of diverse protoplasm lie two 

 factors : (i) a physiological stimulus to development, and (2) an 

 increased probability of inherited variation which in new combi- 

 nations will appear to the advantage of the species. Blackman's 

 " reduced forms of fertilization" which I should prefer to con- 

 sider apart from normal fertilization as examples of apogamy, 

 and have so classed in this treatment, do satisfy the physiologi- 

 cal requirements of a sexual act in that a form of nuclear fusion 



