880 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



difficult to find a series of forms showing progressively greater and greater 

 deviations from the typical asexual toward the typical sexual method of 

 reproduction, and the existence of such a series is indicative of the derivation 

 of the latter from the former type. 



Origin of Sex, and Theory of Reproduction. It is obvious that the 

 production of new individuals is necessary to the continued existence of any 

 species. It would be interesting to know the origin and significance of the two 

 existing methods of reproduction. Apropos of the asexual process, Leuckart, 

 and especially Herbert Spencer, have pointed out that during the growth of 

 a cell the mass increases as the cube, but the surface only as the square, of 

 the diameter i. e. the quantity of protoplasm increases much more rapidly 

 than the absorptive surface. It follows from this that during the growth of a 

 unicellular organism a size will ultimately be reached beyond which the cell 

 will not be able to absorb sufficient food for the maintenance of the proto- 

 plasm. In order that growth may continue beyond this point, a division of 

 the cell, which ensures a relative increase of surface over mass, is absolutely 

 necessary. Fission is, therefore, a necessary corollary of growth, and, although 

 we are ignorant of the details of its mechanism, it is conceivable that the method 

 of asexual reproduction arose through causes connected with growth. 



The explanation of sexual reproduction is much more difficult, for here, in 

 addition to the budding off of the germ-cells from the parental bodies, which 

 has probably the same fundamental cause as fission in unicellular forms, we 

 must account for the differentiation into sexes, the existence of special sexual 

 cells, and the fusion of the male and the female germinal substance ; in short, 

 we must account for the conception of sexuality itself and all that it implies. 



Eegarding the origin of sexuality itself, as to the question whether sexuality 

 is an original and fundamental attribute of protoplasm or has been acquired, 

 we may say at once that at present we know really nothing. Yet, whatever 

 view is held as to the origin of sexuality, it seems entirely probable that the 

 method of reproduction known as sexual is a derivative of the method known 

 as asexual the latter is primitive, the former has arisen from it. From the 

 wide distribution and prominence of the former among vital phenomena we 

 must believe, with biologists generally, that sexual differentiation and sexual 

 processes have arisen from natural causes, and for the reason that sexual repro- 

 duction is of advantage to living beings and to species. In what way it is of 

 advantage, however, is disputed. Three views, all of which have evidence in 

 their favor and which are not mutually exclusive, are at present engaging the 

 attention of scientific men. The first to be mentioned is the theory advocated 

 by Hensen, Edouard van Beneden, and Butschli, according to whom the fusion 

 of the cells in sexual reproduction exists for the purpose of rejuvenating the 

 living substance. The power possessed by cells of dividing asexually is 

 limited ; in time the protoplasm grows old and degenerates ; its vital powers are 

 weakened, and without help the extinction of the race must follow. But the 

 mingling of another strain with such senescent protoplasm gives it renewed 

 youth and vigor, restores the power of fission, and grants a new lease of life to 



