GENESIS, HEREDITY, AND VARIATION. 341 



stamens of one flower to the pistil of another. Simi- 



larly, it has been found that among the lower animals, herma- 

 phrodism does not usually involve the production of fertile 

 ova by the union of sperm-cells and germ-cells developed in 

 the same individual; but that the reproductive centres of 

 one individual are united with those of another to produce 

 fertile ova. Either, as in Pyrosoma, Perophora, and in many 

 higher molluscs, the ova and spermatozoa are matured at 

 different times ; or, as in annelids, they are prevented by their 

 relative positions from coming in contact. 



Eemembering 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 above fact that among hermaphrodite 

 organisms, 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 fertilization, is the union of specially-fitted portions 

 of different organisms. 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 indiffer- 

 ence whether the united sperm-cells and germ-cells were 

 those of the same individual or those of different individuals. 

 But the circumstance that there exist in such organisms 

 elaborate appliances for mutual fertilization, shows that un- 

 likeness of derivation in the united reproductive centres, is 

 the desideratum. Now this is just what the foregoing 



hypothesis implies. If, as was concluded, fertilization has 

 for its object the disturbance of that approaching equilibrium 

 existing among the physiological units separated from an 

 adult organism ; and if, as we saw reason to think, this object 

 is effected by mixture with the slightly-different physiologi- 

 cal units of another organism ; then, we at the same time see 

 that this object will not be effected by mixture with physio- 

 logical units belonging to the same organism. Thus, the hypo- 

 thesis leads us to expect such provisions as we find. 



