1890.] [Ryder. 



cases, become a flat thallus if grown upon a solid, moist substratum. And 

 doubtless, corresponding modifications may be otherwise induced in the 

 further development of its sexual offspring, but of this I know of no direct 

 proof. 



That it should have been assumed that sexuality provides for variability 

 is not strange. If one considers the problem of variability from morpho- 

 logical and physiological points of view, the evidence is wholly in favor 

 of the conclusion that increased complexity would favor variability. That 

 sexuality has increased the complication of its attendant processes there 

 cannot be the slightest doubt. If the results have become more complex 

 as viviparously developed germs were evolved, not only would the capac- 

 ity of those germs to vary be increased in virtue merely of such increased 

 complexity, but the offspring of two parent individuals, differing even 

 very slightly, would also have to be added as a factor favorable to varia- 

 tion. 



Unfavorable to some forms of the doctrine of rejuvenescence or that 

 view which regards sexuality as a means of rejuvenating certain cells by 

 means of conjugation or the act of fertilization, are the facts which prove 

 that, in the vegetable world at least, growth may go on indefinitely without 

 the recurrence of sexuality, and with increased, rather than with dimin- 

 ished, vigor. I need only to cite the Banana which has been asexually 

 propagated by cuttings for centuries. The significant and persistent vigor 

 through twenty centuries of a Dracaena, or Dragon's blood tree, is also of 

 interest in this connection. The persistent growth of the asexual genera- 

 tions of tree ferns in the present age and of the gigantic LepTdodendrons 

 and Equisetums of the carboniferous period, shows that conditions of .life 

 have much to do in maintaining the vigor of such asexual generations. 



Senility, or impairment of vigor, does not then seem to result from con- 

 tinued growth, as is shown by these facts, and this conclusion is equally 

 well established by the facts which are known in relation to the reproduc- 

 tion of the Cyanophycese, Schizomycetes and the yeast plant. 



This unimpaired vigor seems to be associated with the continuous pro- 

 duction of new axes in the higher plants, or with continuous fission of 

 cell-units in the lower ones. In animals, on the other hand, this vigor 

 shows itself most pronounced in the colonial forms (cormi), or in such as 

 are specially nourished, as the Queen Bee or ant-queen of Termites, 

 amongst Anthropods, and amongst which these animals are also the long- 

 est lived, and where it finds expression partly, at least, in parthenogenesis. 

 The astonishing vigor of the fertile parents of these forms is largely de- 

 termined by their abundant nutriment. 



The genesis of sexuality, upon final analysis, will probably be found to 

 be a purely physiological question, in the discussion of which the energies 

 represented by the cytoplasm of the egg on the one hand, and its nucleus 

 and that of the spermatozoan on the other, will have to be considered. 

 This will, however, represent only the germinal or embryological side of 

 the problem, which takes no cognizance of the preembryonic history of 



PROC. AMER. PHILOS. SOC. XXVIII. 132. O. PRINTED MAY 24, 1890. 



