1.6 VARIATION 



It has been assumed that the ultimate unit of organized 

 beings is the cell. This is true in a structural sense only, for 

 there is positive evidence that the cell is itself made up of 

 various and distinct elements, capable of somewhat independent 

 action in both cell division and growth. The content of a cell 

 is not to be regarded as a mass of amorphous protoplasm to be 

 halved or quartered by chance, but on the contrary it is an 

 organized body with a distinct difference between the nucleus 

 with its definite number of chromosomes (the supposed seat of 

 the physiological units that give character to its activities) and 

 its surrounding cytoplasm or food material. 



Again, a whole group of similar cells may constitute a special 

 organ (liver, kidney, or heart), discharging a highly specialized 

 function quite different from that of any other portion of the 

 body. This colony of many cells discharging the same function 

 appears to move together, thus constituting a kind of functional 

 unit larger than and quite distinct from the ultimate physio- 

 logical units that must reside within the cell. 



Correlated variation. Still again, it is found in practice that 

 occasionally whole groups of characters seem to be so correlated 

 as to move together, so that having found one we may reason- 

 ably expect to discover the other. Familiar examples of this 

 'are found in nearly all cases of reversion. For instance, a white 

 calf among Devon cattle will almost certainly show black or 

 brown points (ears, nose, and legs), while a white shorthorn will 

 not. The one is a case of reversion to the ancestral color of the 

 breed, the wild cattle of Britain; the other is simply the 

 appearance of one of the normal color characters of the race. 

 In increase of numbers of parts there is some tendency to repeat 

 a whole group, as in cases of " double hand." l 



The same tendency for many distinct characters to move 

 together in groups is found in cases of so-called " sports," in 

 which we instinctively recognize something more than ordinary 

 variation. 



These instances of grouping of characters normally independ- 

 ent in such a way as afterward to move together is to be dis- 

 tinguished from such variation as is involved in extreme milk 



1 See under Meristic Variation. 



