io ASSAYS SCIENTIFIC AND PHILOSOPHICAL. 



be exactly alike, the likeness being still the likeness 

 of identity. Even when the stage is reached in 

 which two unicellular organisms conjugate and 

 coalesce in one, whatever be the immediate advan- 

 tage of such conjugation, there would presumably 

 be the funding of the characters possessed by each, 

 but so long as reproduction takes place by fission, 

 the whole of the parent passes into the offspring, 

 or rather the parent and offspring (if such terms 

 may be improperly applied to the parts of the 

 division) are of one piece. 



When, however, we reach those organisms in 

 which two different sorts of cells are produced, and 

 when sexual reproduction makes its appearance, in 

 place of reproduction by fission, the offspring is no 

 longer identical with either parent, but shares the 

 nature of both, being distinct from either. If we 

 still speak of the child as " a chip of the old block," 

 we are more or less conscious that we are speaking 

 metaphorically. Yet the fact of heredity, whether 

 in plants or brutes or men, is too obvious to escape 

 notice, and is taken as a matter of course long 

 before any attempt is made to explain it. Nor 

 does the real difficulty of the problem present 

 itself to us till we ask, How can a microscopic cell 

 contain in the germ not only the whole body in all 

 its parts, but the special characters of parents or 

 more remote ancestors ? 



It is well known that the fact of heredity plays 



