GENESIS, HEREDITY, AND VARIATION. 279 



in many higher molluscs, the ova and spermatozoa are ma- 

 tured at different times ; or, as in annelids, they are prevented 

 by their relative positions from coming in contact. 



Remembering the fact that among the higher classes of 

 organisms, fertilization is always effected by combining the 

 sperm-cell of one individual with the germ-cell of another ; 

 and joining with it the fact that among hermaphrodite organ- 

 isms, the germ-cells developed in any individual, are usually 

 not fertilized by sperm-cells developed in the same individual ; 

 we see reason for thinking that the essential thing in fertiliz- 

 ation, is the union of specially-fitted portions of different or- 

 ganisms. If fertilization depended on the peculiar properties 

 of sperm-cell and germ-cell, as such ; then, in hermaphrodite 

 organisms, it would be a matter of indifference whether the 

 united sperm-cells and germ-cells were those of the same in- 

 dividual, or those of different individuals. But the circum- 

 stance that there exist in such organisms, elaborate ap- 

 pliances for mutual fertilization, shows that unlikeness of 

 derivation in the united reproductive centres, is the deside- 

 ratum. Now this is just what the foregoing hypothesis 

 implies. If, as was concluded, fertilization has for its object 

 the disturbance of that approximate equilibrium existing 

 among the physiological units separated from an adult organ- 

 ism ; and if, as we saw reason to think, this object is effected 

 by mixture with the slightly-different physiological units of 

 another organism ; then, we at the same time see reason to 

 think, that this object will not be effected by mixture with 

 physiological units belonging to the same organism. Thus, 

 the hypothesis leads us to expect such provisions as we find 

 exist. 



§ 94. But here a difficulty presents itself. These proposi- 

 tions seem to involve the conclusion, that self-fertilization is 

 impossible. It apparently follows from them, that a group of 

 physiological units from one part of an organism, ought to 

 have no power of altering the state of approaching balance jj 



