542 THE NATURAL HISTORY OF 



in reality make themselves apparent until a series of inde- 

 pendent successive generations has been gone through. 



3. The individuals of each such generation are capable 

 of propagating themselves in new generations. The 

 individuals of the second generation, are among them- 

 selves, speaking generally, of equal value; as respects the 

 individuals of the parent generation, they are sometimes 

 of equal value with them, sometimes not. 



4. If the secondary cells are not of equal value to 

 their parent-cells, a series of successive generations must 

 precede the last generation, the individuals of which are 

 again equivalent to the first mother-cell. The number of 

 these generations does not appear to be determinate. 



Let us assume that a parent-cell (A) has produced a 

 number of secondary cells (B) which are of unequal value 

 to their parent. The individuals of this second genera- 

 tion propagate a third generation equivalent to their 

 parent-cells (B) or not equivalent (c.) 



In the first case there may also be a fourth generation 

 (B"), a fifth (B'"), and more, which are all equal among 

 themselves, and to their parents, but not equal to the 

 parent-cell of the first generation (A) ; until at last a 

 generation is produced which is not equivalent to its own 

 parent. Now this is either equivalent (A') to the first 

 generation, and the cycle closes with it, or it is still not 

 equivalent to it (E). In that case, it either propagates 

 again a number of equivalent generations, (c'. c". . .) or 

 non-equivalent (D, E . . .) until at last one appears, A', 

 which is equivalent to the first generation, and thus the 

 cycle closes. By equivalent, the author means such indi- 

 viduals or generations as correspond with each other in 

 their essential, physiological, and organological relations, 

 although they may differ in unessential properties, such 

 as colour, size, internal consistence, &c. Non-equivalent, 

 are those generations which in their structure and vital 

 relations exhibit essential differences, such as " still" and 

 "motile" cells, and among these, again, their various 

 forms ; but particularly those which are derived from a 

 different mode of propagation. 



