30 THE PHYSICAL BASIS OF INHERITANCE 



The reasons for lingering over the modes of reproduction — 

 which it is confessedly difficult to arrange in a perfectly clear 

 scheme — are (i) that our general view of the hereditary relation 

 must be one which is applicable to all cases and not merely to 

 the most frequent, and (2) that some of the simplest cases shed 

 light upon the more complex. It is also important that we 

 should make clear that the common phrases, " asexual repro- 

 duction " and " sexual reproduction," are somewhat ambiguous, 

 since attention has to be directed to two distinct points — 

 (a) whether there are specialised reproductive elements, and 

 (5) whether there is any form of amphimixis. 



§ 3. The Hereditary Relation in Unicellular Organisms 



At what is called " the limit of growth," when the cell has 

 attained to as much volume as its surface can adequately supply 

 with food and oxygen, and so on, a unicellular organism normally 

 divides into two, obviating the difficulties which would ensue 

 if volume increased out of proportion to surface. The halves 

 separate and grow. Two more or less exact replicas of the 

 original unit result. It has been demonstrated that the division 

 is often preceded by that intricate and orderly process of nuclear 

 division, known as karyokinesis, which results in an equal 

 partition of the nuclear constituents between the two daughter- 

 cells. As each of the halves is in the strictest sense half of the 

 organisation of the parent unit, we are not surprised that each 

 should in appropriate environment grow into an almost exact 

 image of the original whole. In most cases we have no methods 

 subtle enough to detect any difference. There is complete here- 

 ditary resemblance, and it would be puzzling if it were otherwise. 

 Even when the unit divides into many units (as in spore-forma- 

 tion), there is no puzzle in the fact that each reproduces the 

 likeness of the original whole, except the puzzle of growth — of 



