Heredity in Protozoa 63 1 



work, for any evidence of it. The question, put as simply as 

 possible, is as follows: 



Is there ever any mechanism or property in virtue. of which, 

 when a structural modification occurs in one part of the body, 

 this will modify another part of the body (not in the same way, but) 

 in such a way that this other part will, at reproduction, start up 

 processes tending to produce a similar structural modification ? 



14 The propositions thus far set forth have had direct refer- 

 ence to the Protozoa; but in the main they apply a fortiori to the 

 Metazoa also. The difference between the two groups as to 

 heredity is not one of principle, but of complexity. The extreme 

 difference in complexity may be put as follows: 



In the Protozoa, when a new inherited character is to appear 

 in the adult, this requires a modification of the adult of the pre- 

 vious generation, of such a character as to change in a definite 

 way only the next fission and processes immediately connected 

 with it. This requirement is sufficiently complex when we come 

 to ask how the numerous locomotor organs of the Hypotricha, in 

 their varied typical patterns, have arisen and become hereditary. 

 But it is not to be compared in complexity with what we have to 

 set forth next. 



In the Metazoa the requirement for the appearance of a heredi- 

 tary new structure in the adult is that the preceding germ cell 

 shall be so modified that at the next fission the reproductive proc- 

 esses shall be changed, but the change shall not yet be of a char- 

 acter to produce the ultimate structures. In the next and the 

 next, and in hundreds of succeeding fissions the processes must all 

 be modified so as to keep in each cell the conditions for the final 

 production of the ultimate new structure. These conditions will 

 necessarily be different in the different cell generations, as differ- 

 entiation occurs, and of course each of the intermediate condi- 

 tions is something quite diverse from the final structure. At the 

 end the new structure is produced, not by a modification in the 

 reproductive processes of one cell, as in the Protozoa, nor by the 

 same modifications in many cells, but by the diverse modifications 

 of thousands and thousands of cells, all so modified as to cooperate 

 in the production of the final structure. The mind refuses the 

 useless attempt to conceive of such complexity of change. 



